Dr. Jeff Masters' WunderBlog

Invest 98L fails to develop, as predicted

By: JeffMasters, 04:49 GMT le 31 juillet 2008

The tropical wave that recently moved off the coast of Africa has died down and failed to develop, just as I predicted yesterday. I know that some of you were getting a bit excited at the prospect of tracking another system, but it is rare to see development this close to Africa. Convection is all but gone from Invest 98L at this time. However, we will continue to monitor the system for any signs of development, however unlikely they may be. The circulation remains relatively healthy for an Africa wave, but conditions are not favorable for convective development. The window has basically passed on this system as it is now encountering increasingly cooler waters.

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Invest 98L - Infrared satellite imagery

There are no other signs of development in the Atlantic at this time. We are still about a month or so away from the peak of the season so there is likely plenty of 'excitement' to come.

Cheers,
Bryan Woods

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Atlantic still quiet, new wave moves off Africa

By: JeffMasters, 19:08 GMT le 29 juillet 2008

After a busy last few weeks, it seems that things have quieted down in the Atlantic. The only 'excitement' to be had is a new wave that has just moved off the coast of Africa. Convection in the wave is limited and certainly not organized. Development is not expected, but we will continue to monitor the system if only for a lack of anything else to watch.

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Infrared Channel 4

Looking for something to pass the time while you wait for the next storm? Check out this "xkcd" comic:

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Click for full comic

Cheers,
Bryan Woods

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Invest 97L quiets down, nothing else in sight

By: JeffMasters, 15:49 GMT le 27 juillet 2008

The only game in town, Invest 97L, has quieted down and the Atlantic is pretty dull. Convection with Invest 97L has subsided, which is no real surprise considering that it had been on the wrong side of the wave if you're looking for development. The wave still lacks any organization and there is nothing else in sight.



Bryan Woods

Updated: 18:32 GMT le 28 juillet 2008

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Tropics quiet except for disturbance 97L

By: JeffMasters, 17:24 GMT le 26 juillet 2008

A tropical disturbance (97L) in the mid-Atlantic ocean consists of a closed circulation with one spot of heavy thunderstorm activity to the northeast. The disturbance is under about 20 knots of wind shear and is over waters of about 26.5°C, and there is some dry air to the west. These are very marginal conditions for tropical storm formation. Conditions will gradually become more favorable for development over the few days, but none of the computer models are developing this system. 97L will probably not affect any land areas, although Bermuda should keep an eye on it.

Jeff Masters

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Dolly's rains continue to inundate Texas and Mexico

By: JeffMasters, 14:41 GMT le 24 juillet 2008

Tropical Storm Dolly continues plowing west over South Texas, dumping huge quantities of rain. Radar-estimated rainfall amounts as high as 25 inches (Figure 1) have already been reported, and Dolly will probably rank as one of the the ten rainiest tropical cyclones to affect Texas. At its peak, Dolly delivered five inches of rain per hour to the coast at landfall. AIR Worldwide insurance company estimates that the total insured damage from Dolly will run $300 million - $1.2 billion, mostly due to flooding and wind damage (total damage is typically estimated as double the insured damage, and would thus be $600 million - $2.4 billion). One bright spot to Dolly's rampage--the storm's rains brought an end to the moderate to severe drought gripping extreme South Texas, and will help with drought conditions along coastal Texas northwards to Corpus Christi.

Dolly's total damage is difficult to estimate at this point, because much of it has yet to occur. Dolly is dumping prodigious rains, thanks to its very slow forward speed of about 7 mph. The rains from Dolly may rival those of Hurricane Beulah of 1967. Beulah, a huge and powerful Category 3 hurricane, dumped up to 27 inches of rain inland, triggering major flooding throughout South Texas and Northeast Mexico. Beulah did over $1 billion in damage to Texas, due in part to failure of levees on the Rio Grande River. Considerable damaging flooding will affect South Texas over the next few days, as Dolly's rains continue to pound the state. However, the levees on the Rio Grande River are expected to hold.


Figure 1. Radar estimated rainfall from Dolly.


Figure 2. Rainfall measured during Hurricane Beulah of 1967. Beulah caused over $1 billion in damage to South Texas, mostly due to serious flooding from rains that totaled up to 27 inches. Image credit: NOAA.

Elsewhere in the tropics
The tropical wave (97L) off the coast of Africa, a few hundred miles west of the Cape Verde Islands, is now over cool water of 25°C. The wave still has a large circulation, but has lost all of its heavy thunderstorm activity. Until 97L can find some warmer water (which should happen by Saturday), there is little chance of it developing. Wind shear is expected to be around 15 knots on Saturday, which is marginal for development, and there is plenty of stable, dry air for it to contend with. None of the reliable computer models show development of this system.

The four reliable computer models are not predicting development anywhere else in the Atlantic for the next 7 days.

Last blog until August 6
With the tropics relatively quiet now, I'm going to take some time off and do some hiking in Colorado (I've spent the past 3 days blogging from the back seat of a heavily loaded rental car, driving cross-county!) The peak part of hurricane season is almost a month away, and if the behavior of the tropics in July is any indication, it will be a long and severe hurricane season. So, I'm going to get some R&R in, and let Bryan Woods blog for me while I'm gone. I will be keeping an eye on things, and can jump back in if anything nasty pops up.

Jeff Masters

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Hurricane Dolly smashes ashore

By: JeffMasters, 19:55 GMT le 23 juillet 2008

Hurricane Dolly smashed ashore on the South Texas coast on Padre Island early this afternoon, bringing 100 mph winds and a storm surge of 6-8 feet to the coast. The southern edge of the eyewall is now battering Harlingen, where sustained winds of 56 mph, gusting to 74 mph, have been observed. Reports from the Hurricane Hunters show that Dolly's pressure is now rising, and radar imagery out of Brownsville, Texas confirms that the storm is starting to weaken. Visible satellite loops show the eye is starting to deteriorate, though Dolly still looks plenty impressive.


Figure 1. Hurricane Dolly at landfall.

Links to follow:
Brownsville, TX long range radar
Brownsville, TX weather
Harlingen, Texas weather
Corpus Christi, TX weather

Damage expected from Dolly
The southern portion of Dolly's eyewall passed over the town of South Padre Island, located on the coast on a barrier island. Wind damage from Dolly will be heaviest here. The sister cities of Brownsville, Texas, and Matamoros, in the Mexican state of Tamaulipas, missed getting the eyewall, but did get gusts near hurricane force. Damage to roofs and mobile homes has already been reported in these regions. Harlingen, located 25 miles inland and 20 miles north of Brownsville, is getting a portion of the southern eyewall, and will suffer more damage than Brownsville. There are a few tiny towns on Laguna Madre, the sound behind Padre Island, that received the full force of Dolly. These towns, Port Mansfield and Arroyo City, will receive heavy wind damage and some storm surge damage. Dolly has already spawned two tornadoes, but neither of these did damage. More tornadoes are expected.


Figure 2. Radar estimated precipitation of Dolly. Amount in excess of 14 inches have already fallen near the coast.

Floods remains a huge concern from Dolly. Rainfall amounts of five inches per hour were observed along the coast, with total rainfall amounts in excess of 14 inches. The real concern is how much rain will fall inland over the Rio Grande River watershed. In 1967, Hurricane Beulah, a huge and powerful Category 3 hurricane, dumped up to 27 inches of rain inland, triggering major flooding throughout South Texas and Northeast Mexico. Beulah did over $1 billion in damage to Texas, according to Wikipedia.

Elsewhere in the tropics
The tropical wave (97L) off the coast of Africa, just west of the Cape Verde Islands, is now over cool water of 25°C. The wave still has a large circulation, but has lost all of its heavy thunderstorm activity. Until this disturbance can find some warmer water (which should happen by Saturday), there is little chance of it developing. None of the reliable computer models show development of this system.

The GFS model is predicting development of a new tropical wave, due to move off the coast of Africa 4-5 days from now.

I'll have an update Thursday.

Jeff Masters

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Dolly intensifying rapidly as it approaches landfall

By: JeffMasters, 13:18 GMT le 23 juillet 2008

Hurricane Dolly is putting on a impressive burst of rapid intensification as it approaches landfall on the Texas coast near Brownsville. Reports from the Hurricane Hunters show that Dolly's pressure is dropping rapidly, down 12 mb in just five hours, to 964 mb (as of the 9:17 am EDT Hurricane Hunter eye report). Dolly's central pressure dropped 15 mb in the 18 hours previous to that, so this is an impressive sudden drop this morning. Radar imagery out of Brownsville, Texas shows an well-organized hurricane, with excellent spiral banding and a 20-mile diameter eye. Visible satellite loops show an impressive eye, excellent upper-level outflow, and good symmetry. It's a good thing Dolly does not have another 24 hours over water, or it would have become a major hurricane. It takes a while for the winds of a hurricane to respond to a rapid pressure fall, and Dolly's winds do not yet reflect the recent big drop in pressure. The peak surface winds measured by the SFMR instrument on the latest Hurricane Hunter mission were 70 knots (81 mph), at 9:17 am EDT. Dolly's winds should rise above 100 mph in the next few hours.


Figure 1. Radar image of Dolly at 6:04 am CDT, several hours before landfall. During a rapid intensification phase as Dolly neared landfall, small "mesovorticies" formed in the eyewall. In the image above, you can see that the eye is distorted by the presence of four of these small vorticies embedded in the eyewall. These vorticies are several kilometers across, and have concentrated areas of very high winds capable of doing much heavier damage than the rest of the eyewall's winds.

The intensity forecast
Dolly should keep intensifying right up until landfall, now just hours away. A Category 2 hurricane is likely at landfall. Once inland, Dolly will decay to tropical storm intensity within six hours.

Links to follow:
Brownsville, TX long range radar
Texas marine forecasts and observations
Brownsville, TX weather
Corpus Christi, TX weather


Figure 2. Evacuation zones for Category 1,2,3,4,5 hurricanes. The city of Brownsville needs to evacuate only for a Category 5 hurricane. Image credit: Texas division of emergency management.

Damage expected from Dolly
It appears portions of Dolly's eyewall will pass directly over the sister cities of Brownsville, Texas, and Matamoros, in the Mexican state of Tamaulipas. Storm surge damage will be limited, due to the fact that these cities are 20-25 miles inland from the coast (Figure 2). Dolly's expected storm surge of 4-6 feet will mostly affect uninhabited sections of Padre Island, on the right side of where the 20-mile diameter eye is coming ashore. Wind damage is the primary threat from Dolly, along with flash flooding from heavy rains. Hurricane Bret spawned two damaging tornadoes in the region in 1999, and we can expect Dolly to spawn a few tornadoes as well. I expect considerable wind damage from Dolly, exceeding $100 million.

I'll have an update this afternoon, with a look at the rest of the tropics.

Jeff Masters

Updated: 14:17 GMT le 23 juillet 2008

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Hurricane Dolly closes in on Texas/Mexico

By: JeffMasters, 21:25 GMT le 22 juillet 2008

Hurricane Dolly has become the second hurricane of the 2008 hurricane season. Dolly is barely a hurricane, and is still struggling to build a complete eyewall. Visible satellite loops show an eye developing, and heavy thunderstorm activity continues to increase near the core of the storm. Dolly has good upper-level outflow to the west and north, but restricted on the south side, where an upper level low pressure system is still interfering. Maximum surface winds measured by the SFMR instrument on the current Hurricane Hunter aircraft inside Dolly were 74 mph (65 kt), measured at 4:17 pm EDT. Brownsville, Texas long-range radar shows the eyewall is complete on Dolly's west side, but is struggling to get established on the east side. Radar estimated rainfall amounts of 1/10 of an inch have fallen on the Texas/Mexico coast so far, thanks to the outermost spiral bands of Dolly.


Figure 1. Latest long-range radar image from Brownsville, Texas.

The intensity forecast
The intensification potential for Dolly remains high, but until Dolly can form a full eyewall, it won't be able to take full advantage of the favorable environment. Wind shear over Dolly is about five knots, and is expected to remain below ten knots over the next two days. An upper level high pressure system is moving into place over the storm, which should enhance Dolly's upper-level outflow and allow more rapid intensification. Dolly is over waters of 29°C. The waters cool to about 28°C by midnight tonight as Dolly approaches the coast and passes over a cool ocean eddy. The depth of the warm waters Dolly is over has decreased, and the Tropical Cyclone Heat Potential has fallen from about 40 to 20 kJ/cm**2. This decreases the potential of rapid intensification. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 hurricane. A Category 2 hurricane still a possibility, though. I put the chances of Dolly reaching major hurricane status (Category 3 or higher) at 1%.

The track forecast
The track forecast has changed little since this morning, with the various computer models predicting pretty much the same behavior as they did 12 hours ago. Four models are predicting a landfall in northeastern Mexico--the UKMET, ECMWF, NOGAPS, and GFS--and two models are predicting a Texas landfall--the GFDL (just north of Brownsville) and the HWRF (near Corpus Christi). Dolly could come ashore anywhere within the cone of uncertainty, and one should not assume the storm will track down the "skinny black line" NHC has drawn through their official forecast. The timing of Dolly's landfall, as predicted by the computer models, will be anywhere from 8 am - 10 pm Wednesday. However, the GFS and ECMWF hint that Dolly may stall out right by the coast Wednesday, and some slow and erratic motion is possible tomorrow before the storm finally comes ashore.

Links to follow:
Brownsville, TX long range radar
Texas marine forecasts and observations
Brownsville, TX weather
Corpus Christi, TX weather

Damage expected from Dolly
I posed the question in this morning's blog, Since hurricanes began getting names in 1950, only one major hurricane that hit the U.S. did not get its name retired. Which one was it? A hint: the track was similar to Dolly's expected track. The answer is, Hurricane Bret, which hit Padre Island National Seashore, just north of Brownsville, Texas, as a Category 3 hurricane on August 22, 1999. The park is a long barrier island with few structures and no commercial developments, and the fury of Bret's 115 mph winds were confined to desolate unpopulated seashore. Bret did $60 million in damage, mostly in the small towns just inland from the National Seashore. Bret dumped 14 inches of rain on northern Mexico, and brought an 8-10 foot storm surge to Padre Island. Several new channels were cut through the barrier island, connecting the ocean to the Laguna Madre sound behind Padre Island.


Figure 2. Evacuation zones for Category 1,2,3,4,5 hurricanes. The city of Brownsville needs to evacuate only for a Category 5 hurricane. Image credit: Texas division of emergency management.

If Dolly follows a similar path, we can expect relatively minor damage. Even if Dolly makes a direct hit on the sister cities of Brownsville, Texas, and Matamoros, in the Mexican state of Tamaulipas, damage will be limited, due to the fact that these cities are 20-25 miles inland from the coast (Figure 2). Dolly's expected storm surge of 4-6 feet will probably not do much damage, since there are few structures near the coast in the region. Wind damage will be the primary threat from Dolly, along with flash flooding from heavy rains. Hurricane Bret spawned two damaging tornadoes in 1999, and we can expect Dolly to spawn a few tornadoes as well.

I'll have an update in the morning.
Jeff Masters

Updated: 21:27 GMT le 22 juillet 2008

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Dolly nears hurricane strength

By: JeffMasters, 13:12 GMT le 22 juillet 2008

Tropical Storm Dolly is building an eyewall and is near hurricane strength, as it approaches a Wednesday landfall in Texas or northern Mexico. Visible satellite loops show that heavy thunderstorm activity continues to increase near the core of the storm, with good upper-level outflow to the west and north, but restricted on the south side, where an upper level low pressure system is interfering. Maximum surface winds measured by the SFMR instrument on the current Hurricane Hunter aircraft inside Dolly were 74 mph (65 kt), measured at 8:39 am EDT. These are bottom end Category 1 hurricane winds, so Dolly is very close to becoming a hurricane. However, the winds at the flight level of 10,000 feet have been considerably less than 74 mph, so don't expect NHC to upgrade Dolly based on this measurement. Water vapor satellite loops still show some dry air on the south side of Dolly, but the storm has insulated itself from this dry air by building an eyewall that was 50% complete as of 4 am EDT (Figure 1). Dolly's outer spiral bands are approaching the Texas/Mexico coast, and the center of the storm is now visible on Brownsville, Texas long-range radar


Figure 1. Microwave satellite image of Dolly at 4:08 am EDT Tue July 22, 2008. Note the partial formation of an eyewall on the north and east sides. The eyewall was about 50% complete at this time. Image credit: Navy Research Lab, Monterey.

The intensity forecast
Wind shear over Dolly is less than five knots, and is expected to remain below ten knots over the next two days. An upper level high pressure system is moving into place over the storm, which should enhance Dolly's upper-level outflow and allow more rapid intensification. Dolly is over waters of 29°C. The waters cool to about 28°C as Dolly approaches the coast and passes over a cool ocean eddy. The warm waters Dolly is currently over extend to a moderate depth, with a Tropical Cyclone Heat Potential of about 40-60 kJ/cm**2. This is below the value of 80 usually associated with rapid intensification, but still high enough to allow Dolly to put on a burst of rapid intensification prior to landfall. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 hurricane. A Category 2 hurricane still a possibility, but this would require Dolly staying over water a longer period of time than forecast. Even if Dolly does slow down and stay over water until Thursday, the very slow motion of the storm would cause it to upwell cold water from the ocean depths to the surface, putting a damper on intensification. I put the chances of Dolly reaching major hurricane status (Category 3 or higher) at 2%.

The track forecast
A trough of low pressure approaching the Midwest U.S. tonight will weaken the steering currents driving Dolly west-northwest, and probably impart more of a northwesterly motion to the storm tonight and Wednesday. The exact influence this feature will have on Dolly is uncertain, with four models predicting a landfall in northeastern Mexico--the UKMET, ECMWF, NOGAPS, and GFS--and two models predicting a Texas landfall--the GFDL (near Brownsville) and the HWRF (near Corpus Christi). Dolly could come ashore anywhere within the cone of uncertainty, and one should not assume the storm will track down the "skinny black line" NHC has drawn through their official forecast. The timing of Dolly's landfall, as predicted by the computer models, will be anywhere from 10 am - 10 pm Wednesday. However, the GFS and ECMWF hint that Dolly may stall out right by the coast Wednesday, and some slow and erratic motion is possible tomorrow before the storm finally comes ashore.

Links to follow:
Brownsville, TX long range radar
Texas marine forecasts and observations
Brownsville, TX weather
Corpus Christi, TX weather


Figure 2. Visible satellite image from 1130 GMT (7:30 am EDT) Tue July 22, 2008, showing the new tropical low pressure system that moved off the coast of Africa. Note the stratocumulus clouds to the west of the low, indicating cooler waters that will slow down development of the system. Image credit: Navy Research Lab, Monterey.

Elsewhere in the tropics
Tropical Storm Cristobal continues to head out to sea, and is not expected to hit land. Nova Scotia and Newfoundland may feel the fringes of the storm as it races to the east today and transitions into an extratropical storm.

A large low pressure system (97L) moved off the coast of Africa last night, and is generating some heavy thunderstorm activity over the ocean waters near the Cape Verde Islands. This morning's 2:53 am EDT QuikSCAT pass shows a closed circulation near 17N 19W, with winds in the 10-20 mph range. Sea Surface Temperatures (SSTs) are about 27°C, which is one degree above the 26°C threshold needed for tropical storm formation. SSTs will cool to 25-26°C on Wednesday as the low moves westward, which will slow any development. Wind shear is low enough for development, 5-10 knots, and is forecast to remain low enough for development over the next few days. Once the low reaches the mid-Atlantic late this week, SSTs will warm once again above 26°C, and we may get yet another July tropical depression. However, systems that come off the coast of Africa this far north rarely affect the Caribbean or U.S., and Bermuda is probably the only location that needs to be concerned.

Quiz question: Since hurricane began getting names in 1950, only one major hurricane that hit the U.S. did not get its name retired. Which one was it? A hint: the track was similar to Dolly's expected track. I'll have the answer in my update on Dolly this afternoon.

Jeff Masters

Updated: 14:06 GMT le 22 juillet 2008

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Dolly slowly organizing, having trouble with dry air

By: JeffMasters, 23:48 GMT le 21 juillet 2008

Tropical Storm Dolly is slowly organizing over the Gulf of Mexico as it approaches a Wednesday landfall in Texas or northern Mexico. Visible satellite loops show that heavy thunderstorm activity is increasing near the core of the storm, with good upper-level outflow to the west and north. The surface circulation is now well formed, and Dolly will soon be forming an eyewall as the inner core continues to organize and consolidate. Until this process gets further along, the winds and pressure will remain fairly steady. Maximum surface winds measured by the SFMR instrument on the current Hurricane Hunter aircraft inside Dolly were 50 mph (45 kt), measured at 6:25 pm EDT. There are indications on water vapor satellite loops that a pocket of dry air on the south side of Dolly is interfering with the organization of the storm. This dry air was sucked in from the Yucatan Peninsula, and created strong downdrafts in the thunderstorms on Dolly's south side that sapped moisture and energy from the storm (Figure 1). If you watch the visible loop carefully, you can see evidence of these downdrafts in the form of surface lines of cumulus clouds propagating away from Dolly's center on the south side of the storm.


Figure 1. Visible satellite image of Dolly at 6:41 pm EDT. Note the band of cumulus clouds that mark the boundary of outward flowing air along the surface, generated when dry air from the Yucatan Peninsula got ingested into thunderstorms on the south side of Dolly, creating strong downdrafts that spread out in all directions upon reaching the surface.

The intensity forecast
The upper-level low that interfered with Dolly for so long has now moved away and weakened, and is no longer an issue. Wind shear over Dolly is essentially zero, and expected to remain below five knots over the next two days. Dolly is having trouble with dry air sucked in from the Yucatan, but this will grow less of an issue by Tuesday as the storm draws away from the dry air source. Dolly will be over waters of 28-29°C. These SSTs are slightly below average for this time of year, but plenty warm enough to support Dolly intensifying into a hurricane before landfall. These warm waters extend to a moderate depth, with a Tropical Cyclone Heat Potential of about 40 kJ/cm**2. This is below the value of 80 usually associated with rapid intensification, but still high enough to allow Dolly to strengthen into a major hurricane, if it has enough time to do so. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 hurricane. A Category 2 hurricane now looks unlikely, and I put the chances of Dolly reaching major hurricane status (Category 3 or higher) at 2%.

The track forecast
The models are pretty united in forecasting a track towards the Texas/Mexico border over the next day, with a sharp decrease in forward speed Tuesday. Significant uncertainty creeps into the forecast when this deceleration occurs and the steering currents weaken. The models have differing solutions on the orientation and strength of the ridge of high pressure steering Dolly, and the storm could come ashore anywhere within the cone of uncertainty--from northern Mexico to Corpus Christi, Texas. Tropical storm force winds will impact a 200-mile long stretch of coast.

Links to follow:
Brownsville, TX long range radar
Texas marine forecasts and observations
Brownsville, TX weather
Corpus Christi, TX weather

I'll have an update Tuesday morning by 9 am EDT.

Jeff Masters

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Dolly a significant threat to Texas

By: JeffMasters, 13:54 GMT le 21 juillet 2008

Tropical Storm Dolly has emerged over the Gulf of Mexico after a short passage over Mexico's Yucatan Peninsula, and is already beginning to organize. Dolly will potentially be a dangerous hurricane for Texas and northern Mexico. Satellite loops show a large and expanding circulation, with good upper-level outflow developing to the west and north. The surface circulation is not well formed yet. Dolly's intense thunderstorms lie mainly to the north of the center, and these thunderstorms brought heavy rains of 4-6" to the Cayman Islands yesterday. Dolly's heaviest rains passed through the Yucatan Channel last night and this morning. Rainfall amounts were less than four inches over Cancun, Cozumel, and Western Cuba. Winds were only 40-50 mph over this region, so there should have been little damage.


Figure 1. Current satellite image of Dolly.

The intensity forecast
The upper-level low that interfered with Dolly for so long has now moved away and weakened, and is no longer an issue. Wind shear over Dolly is essentially zero, and expected to remain below five knots over the next 2-3 days. Dolly will be over waters of 28-29°C. These SSTs are slightly below average for this time of year, but plenty warm enough to support Dolly intensifying into a hurricane before landfall. These warm waters extend to a moderate depth, with a Tropical Cyclone Heat Potential of about 40 kJ/cm**2. This is below the value of 80 usually associated with rapid intensification, but still high enough to allow Dolly to strengthen into a major hurricane, if it has enough time to do so. Our skill in making intensity forecasts is poor, but it currently appears that Dolly only has enough time to intensify into a Category 1 or 2 hurricane. I give a 10% chance that Dolly will go through a burst of rapid intensification shortly before landfall and achieve major hurricane status at landfall.

The track forecast
The surface center of Dolly is reforming a bit further to the north of the storm, near the heaviest thunderstorm activity. This will require a northward adjustment to the track forecast, increasing the danger to Texas. The models are pretty united in forecasting a track towards the Texas/Mexico border over the next two days, with a decrease in forward speed. Some significant uncertainty creeps into the forecast for Wednesday, when steering currents weaken and the models have differing solutions on the orientation and strength of the ridge of high pressure steering Dolly. The latest GFDL and HWRF models runs from 2 am EDT this morning have shifted significantly north, bringing Dolly ashore near Corpus Christi, Texas. The GFDL shows a very large borderline Category 1/2 hurricane, with tropical storm force winds extending from Galveston to Brownsville at landfall. Dolly will probably grow quite large and affect a 200-mile stretch of coast with tropical storm-force winds. Like her namesake, actress Dolly Parton, Dolly the tropical cyclone appears destined to become a media star.

Links to follow:
Cancun, Mexico radar
Texas marine forecasts and observations
Brownsville, TX weather

Cristobal
Tropical Storm Cristobal continues to chug along the U.S. East Coast after dropping welcome rains over the eastern portion of North Carolina yesterday. rainfall amounts of 1-2 inches were common, with a few isolated areas of four inches (Figure 2). Cristobal did little or no damage to the state, and the rains and wind of Cristobal have left North Carolina.

Cristobal will continue to the northeast today, and it currently appears that the rains of the storm will affect only the right side of its path, bringing little rain to Nantucket Island, southeastern Massachusetts, and coastal Maine. The storm could bring tropical storm force winds and heavy rains in excess of four inches to Nova Scotia. The combination of cooler waters, moderate wind shear, and dry air should prevent Cristobal from ever reaching hurricane strength. Cristobal will transition to an extratropical storm as it approaches Canada, and some of the models suggest that this system will rotate around the Bermuda High and head southward in the Mid-Atlantic Ocean late this week. There is a small chance that Cristobal would get born again as a tropical storm if it makes a complete circuit and passes over warmer water again.

Links to follow:
Nantucket Island, Massachusetts


Figure 2. Radar estimated rainfall over North Carolina from Tropical Storm Cristobal.

2008 Hurricane season notables
July 19, 2008, marked the first time on record that three July tropical storms were active on the same day in the Atlantic. Bertha, Cristobal, and Dolly were all tropical storms yesterday. Thanks to Margie Kieper for checking on this stat for me.

July 2008 already ranks 5th all-time for number of July named storms (three), and 4th all-time for the amount of Accumulated Cyclone Energy (ACE)--30 ACE units. Only 2005 (60.4 ACE units), 1916 (48.5 ACE units) and 1933 (34.0 ACE units) had more. All three of these years had at least five major hurricanes and were exceptionally active. Thanks go to Dr. Phil Klotzbach of Colorado State for compiling this stat.

Bertha: longest-lived July hurricane on record (and the longest-lived hurricane so early in the season). Bertha was a hurricane 7.75 days, which eclipses the previous record of 7 days held by Hurricane Emily of 2005.

Bertha: longest-lived tropical storm on record for July (and for so early in the season), 17.25 Named Storm Days (NSD). Bertha was the fourth longest lived named storm since 1950 to have formed in any month (trailing Ginger (1971) - 21.25 NSD, Carrie (1957) - 19.50 NSD, and Alberto (2000) - 19.25 NSD).

Bertha: farthest east forming tropical storm and hurricane for so early in the season.

Elsewhere in the tropics
There's no sign that this year's unusually early and active hurricane season is going to slow down. Three of our four reliable computer models continue to predict that a strong tropical wave about to emerge off the coast of Africa tonight will develop into a tropical depression later this week. This wave, located over Senegal (15N 15W), has a very strong rotation and a large circulation. Once it moves over the Atlantic, water temperatures that are up to 2°C above average for this time of year should allow this wave to develop.

I'll have an update later today.

Jeff Masters

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Hello, Dolly; so long, Bertha; Cristobal keeps on chugging along

By: JeffMasters, 17:17 GMT le 20 juillet 2008

Tropical Storm Cristobal continues to chug along the coast of North Carolina, dropping welcome rains over the eastern portion of the state. Morehead City radar shows a well-organized storm with plenty of spiral bands of thunderstorms. These bands have only moderate amounts of rainfall, which is fortunate, since Cristobal is moving so slowly. Rainfall amounts from Cristobal have generally been below two inches over eastern North Carolina, with one isolated pocket of four inches just north of Wilmington, NC. Cristobal is over waters of about 27°C, a degree above the threshold of 26°C needed to sustain a tropical storm. Wind shear is about 10 knots over the storm, and is forecast to fall below 5 knots by Monday. There is some dry air at mid and high levels that is interfering with Cristobal, contributing to the storm's relatively low rainfall amounts.

The expected decrease in wind shear may allow Cristobal to intensify to a 60 mph tropical storm by Monday, but conditions do not appear favorable enough to allow the storm to reach hurricane status. None of the models forecast that Cristobal will intensify beyond a 60 mph tropical storm. Cristobal will likely cause minimal or no damage to North Carolina. Cristobal will likely bring winds below 25 mph to Southeast Massachusetts and Nantucket Island on Monday night. However, these regions are likely to receive 2-4 inches of rain from the storm as it zooms by. The HWRF model predicts heavy rain in excess of four inches along the coast of Maine from Bar Harbor to the Canadian border from Cristobal's remnants on Tuesday. Lesser rainfall amounts of 2-4 inches are expected over Nova Scotia.

Links to follow:
Morehead City, NC radar
Southeast U.S. Marine observations and forecasts
Cape Hatteras, NC weather


Figure 1. Visible satellite image of Dolly and Cristobal at 1:02 pm EDT Sun July 20, 2008. Image credit: NASA GSFC.

Hello, Dolly!
Wind shear finally relaxed enough over the Western Caribbean today to allow Tropical Storm Dolly to form. Satellite loops show a large area of intense thunderstorms lies to the north of >Dolly's center, and these thunderstorms will bring heavy rains of 4-8" today and tomorrow to the Cayman Islands, Western Cuba, and the northern Yucatan Peninsula, including Cancun and Cozumel. This morning's QuikSCAT pass shows a large are of tropical storm force winds affecting the Western Caribbean. There was no closed circulation at the time of the QuikSCAT pass, but the Hurricane Hunters have since found a closed circulation, which one can also see on visible satellite loops. Heavy thunderstorm activity is now starting to build near the center of Dolly, which is the hallmark of an intensifying tropical cyclone.

The upper-level low that prevented Dolly from developing a closed circulation has weakened considerably over the past day, and will continue to weaken and slide southwestward out of Dolly's way today. This will create a low-shear environment for Dolly to intensify in. With very warm waters of 28.5°C ahead of it extending to great depth, Dolly is expected to intensify right up until landfall tonight over the Yucatan Peninsula. It is unlikely Dolly has time to reach hurricane status today, but a 60 mph tropical storm is possible tonight at landfall. Passage over the Yucatan will disrupt Dolly, which should need a day or so to recover once it emerges over the Gulf of Mexico. Steering currents will weaken once it does emerge, and Dolly will probably have at least two days over the warm waters of the Gulf before making landfall near the Texas/Mexico border Wednesday night or Thursday morning. This may allow Dolly to intensify into a Category 1 hurricane over the Gulf of Mexico, as predicted by the GFDL model. Rainfall amounts at landfall in Texas and northern Mexico are predicted to be in the 2-8" range, according to the HWRF and GFDL models.

Links to follow:
Cozumel, Mexico weather
Buoy 42056

So long, Bertha
Bertha is finally dead, and is now a powerful extratropical storm over the North Atlantic. The remnants of Bertha are expected to deliver tropical storm force winds to Iceland on Monday. Bertha set the record for the longest-lived July hurricane on record (and the longest-lived hurricane so early in the season). Bertha was a hurricane 7.75 days, which eclipses the previous record of 7 days held by Hurricane Emily of 2005. Bertha was also the longest-lived tropical storm on record for July (and for so early in the season), 17.25 days. Bertha was also the farthest east forming tropical storm and hurricane for so early in the season.

Elsewhere in the tropics
Three of our four reliable computer models for are predicting a new tropical depression will develop off the coast of Africa 3-5 days from now.

I'm not sure, but today might mark the first time on record that we've have three named storms on the same day in July in the Atlantic. The year 2005 came close, when Dennis and Emily existed on July 11, and Cindy's extratropical remnants were over New England.

I'll have a full update Monday morning.

Jeff Masters

Updated: 17:42 GMT le 20 juillet 2008

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Carolina tropical depression strengthening, Caribbean disturbance disorganized

By: JeffMasters, 15:43 GMT le 19 juillet 2008

Tropical Depression Three continues to organize off the coast of South Carolina. Wilmington, NC long range radar shows the steady development of solid spiral bands of thunderstorms on all sides of the center, and these bands have begun bringing heavy rains to the North Carolina coast. Rainfall amounts as high as 3 inches have fallen just north of Wilmington, NC, as estimated by weather radar (Figure 1). TD 3 is over waters of marginal warmth, about 27°C, a degree above the threshold of 26°C needed to sustain a tropical storm. The warm waters are very shallow, and the Tropical Cyclone Heat Potential is near zero, meaning we're unlikely to see rapid intensification of TD 3. Wind shear is about 15 knots over the storm, and is forecast to fall below 10 knots by Sunday. There is some dry air over the Southeast U.S. that may get entrained into TD 3's circulation. TD 3 should be able to intensify into a weak tropical storm, if the center manages to remain over water. None of the models forecast that TD 3 will intensify beyond a 50 mph tropical storm, due to the relatively cool water temperatures, moderate wind shear, and the presence of dry air nearby. I think TD 3 is organizing a bit more than the models are expecting, and has a 50% chance of becoming a 50 mph tropical storm by Monday. Coastal areas of North Carolina are likely to get heavy rains from TD 3, but it remains to be seen if these rains will penetrate far enough inland to significantly alleviate moderate drought conditions that exist over eastern portions of the state. Heavy rains may also develop over South Carolina, but the main rain will probably remain offshore as the storm passes that state. TD 3 is unlikely to affect any other states, with the possible exception of Cape Cod and Nantucket, Massachusetts Monday/Tuesday.

Links to follow:
Wilmington, NC long range radar
Southeast U.S. Marine observations and forecasts
Wilmington, NC weather


Figure 1. Latest radar estimated rainfall from TD 3.

Caribbean disturbance 94L
A well-organized disturbance in the Central Caribbean (94L) is bringing heavy rain and gusty winds to Haiti and Jamaica today. Kingston, Jamaica recorded sustained winds of 31 mph at 7 am local time, but the winds have since died down. Winds at the Central Caribbean buoy south of Jamaica have been sustained at 34 mph this morning. Satellite loops show no surface circulation, and the cloud pattern is disorganized. This morning's QuikSCAT pass missed 94L. An upper-level cold low centered over Cuba is bringing about 20 knots of wind shear to the northern portion of 94L, but the southern portion of the disturbance is under only about 10 knots of wind shear. A Hurricane Hunter flight will examine 94L this afternoon.

Links to follow:
Kingston, Jamaica weather
Central Caribbean buoy

The upper-level low bringing high wind shear to 94L has weakened considerably in the past six hours, and is forecast to slide westward and continue to weaken over the next two days. Very low wind shear of 5 knots or less should be over the disturbance Sunday. The low shear combined with the warm (28.5°C) water of the Western Caribbean should allow 94L to finally organize into a tropical depression by Sunday. NHC is giving 94L a high (>50% chance) of developing into a tropical depression by Monday morning. The disturbance has time to potentially strengthen into a tropical storm with 50 mph winds before coming ashore on the Yucatan Peninsula sometime between Sunday night and Monday afternoon. Passage over the Yucatan will no doubt significantly disrupt what should be a relatively weak system, and it is unclear what threat, if any, the storm will pose to the Gulf of Mexico coast. Once it does cross into the Gulf, wind shear should be low enough to permit development. The models favor a second Mexican landfall just south of Brownsville, Texas. There is a excellent chance that 94L will bring welcome drought relief to the parched regions of Southern Texas and northern Mexico along its path, regardless of whether the system develops into a tropical storm.

Bertha sets a new record
Hurricane Bertha is now the longest-lived July hurricane on record (and the longest-lived hurricane so early in the season). Bertha has been a hurricane 7.5 days, which eclipses the previous record of 7 days held by Hurricane Emily of 2005. Bertha is also the longest-lived tropical storm on record for July (and for so early in the season), as well as the farthest east forming tropical storm and hurricane for so early in the season. I find it amazing we've had a July storm that has lasted nearly 17 days! Bertha's days are numbered, though. Bertha will reach cold water less than 70°F by Sunday, which should finally kill it.

Elsewhere in the tropics
Three of our four reliable computer models for are predicting a new tropical depression will develop off the coast of Africa 5-7 days from now. The level of tropical activity the past week has been quite remarkable, and is more typical of September than July. It's a very good thing that sea surface temperatures are more than 1°C cooler than during the record-breaking Hurricane Season of 2005. As it is, the nature of this unusually high early season activity likely means we are in for a busier than average main portion of hurricane season, August-October.

I'll have a full update Sunday morning, and perhaps a short update later today if we get a new named storm.

Jeff Masters

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Tropical Depression 3 forms near South Carolina

By: JeffMasters, 03:27 GMT le 19 juillet 2008

Tropical Depression Three has formed off the Southeast U.S. coast. Charleston, SC long range radar shows the slow development of a solid spiral band of thunderstorms to the southeast of the center, away from land. The storm is over waters of marginal warmth, about 27°C, a degree above the threshold of 26°C needed to sustain a tropical storm. The warm waters are very shallow, and the Tropical Cyclone Heat Potential is near zero, meaning we're unlikely to see rapid intensification of TD 3. Wind shear is about 15 knots over the storm, and is forecast to remain 10-15 knots over the next two days. There is some dry air over the Southeast U.S. that may get entrained into TD 3's circulation. TD 3 could intensify into a weak tropical storm, if the center manages to remain over water. None of the models forecast that TD 3 will intensify beyond a 50 mph tropical storm, and this is a reasonable forecast, due to the relatively cool water temperatures, moderate wind shear, and the presence of dry air nearby. Coastal areas of North Carolina are likely to get heavy rains from TD 3, but it remains to be seen if these rains will penetrate far enough inland to significantly alleviate drought conditions over the state. Heavy rains may also develop over South Carolina, but the main rain will probably remain offshore as the storm passes that state. TD 3 is unlikely to bring heavy rain to any other states, with the possible exception of Cape Cod and Nantucket, Massachusetts Monday/Tuesday.

Links to follow:
Charleston, SC long range radar
Southeast U.S. Marine observations and forecasts


Figure 1. Latest satellite image of TD 3.

Caribbean disturbance 94L
A well-organized disturbance in the Central Caribbean (94L) is bringing heavy rain and gusty winds to Haiti and Jamaica tonight. This afternoon's Hurricane Hunter flight was unable to find a closed surface circulation, and this evening's 6:51 pm EDT QuikSCAT pass also shows no surface circulation. An upper-level cold low centered over Cuba is bringing about 20 knots of wind shear to the northern portion of 94L, but the southern portion of the disturbance is under only about 10 knots of wind shear.

The upper-level low is forecast to slide westward and weaken over the next two days, bringing low wind shear of 5-10 knots over the disturbance Saturday and Sunday. The low shear combined with the warm (28.5°C) water of the Western Caribbean should allow 94L to finally organize into a tropical depression as early as Saturday afternoon. NHC is giving 94L a high (>50% chance) of developing into a tropical depression by Sunday morning. Another Hurricane Hunter mission is scheduled to fly into 94L at 2 pm EDT Saturday. The disturbance has time to potentially strengthen into a tropical storm with 50 mph winds before coming ashore on the Yucatan Peninsula sometime between Sunday night and Monday afternoon. Passage over the Yucatan will no doubt significantly disrupt what should be a relatively weak system, and it is unclear what threat, if any, the storm will pose to the Gulf of Mexico coast. Once it does cross into the Gulf, wind shear should be low enough to permit development.

Bertha
Hurricane Bertha is still out there. I find it amazing we've had a July storm that has lasted 17 days and counting! Bertha's days are numbered, though. Bertha will reach cold water less than 70°F by Sunday, which should finally kill it.

I'll have a full update on the tropics Saturday morning. Boy, it sure looks like more like September than July in the tropics!

Jeff Masters

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Area off of GA/SC looking healthier, Invest 94L still sputtering, Bertha revived

By: JeffMasters, 21:24 GMT le 18 juillet 2008

Things still remain busy in the tropics (at least for July). The area that was over the western Caribbean yesterday -- Invest 95L for those keeping track -- is now over Central America and no longer poses a significant threat of development. Now in the central Caribbean, Invest 94L is continuing to play games with us by first looking like it wants to develop and then fading away. Closer to home, Invest 96L is looking more organized and could develop in a tropical depression or storm by tomorrow. Bertha likewise has again reintensified and is again a hurricane.

Currently Bertha may just be reaching the peak of another intensification trend as she has reformed a nearly complete eyewall. Her convection looks healthy and satellite based intensity estimates now rank Bertha as a hurricane. In addition to strong convection, there is also healthy outflow exhausting from the storm and no signs of significant shear. While this reintensification may be interesting to watch for now, it is not likely to last.

20080718.1945.goes12.x.ir1km.02LBERTHA.55kts-995mb-355N-530W.100pc.jpg
Bertha - Infrared satellite

Currently Bertha is now packing winds of 75 mph, which is minimal hurricane force, with her central pressure is down to 989 mb. Bertha is now picking up steam to the northeast and is headed safely out to sea. She is expected to remain a hurricane overnight, but not much longer.


Bertha - Watches and Warnings

The dynamical models have done a good job forecasting Bertha's movement and development. However, now that we are facing extratropical transition, confidence is limited. Cyclone phase diagrams indicate that over the next two days Bertha will gradually transition to an extratropical storm. HWRF fields indicate that we should first see Bertha grow assymetric, and then transition to a cold core system.

Bertha.phase1.png
Bertha - HWRF phase space

Model forecast tracks all shoot Bertha off to the northeast and continually weaken the storm. Bertha's transition should be complete by Sunday, by which time we may have our eyes turned to other areas.

Click for full size model imagery:


Bertha - Track Forecast



Bertha - Wind Speed Forecast




Another hurricane hunter visited Invest 94L today, but once again was unable to locate a closed circulation. Is anyone noticing a trend here? The eastern Caribbean is typically devoid of tropical development and this just goes to reinforce the rule of thumb. Now that the Invest is in the central Caribbean, climatology suggests that we could see additional development. The warm pool of the western Caribbean is an incredibly powerful fuel for nascent tropical cyclones -- just remember how Wilma went from a tropical storm to the strongest hurricane ever recorded in the Atlantic... in a mere 24 hours. With that said, Invest 94L is nowhere near ready to rapidly intensify; we would first need a closed circulation and we all know how that has gone.

20080718.1945.goes12.x.ir1km.94LINVEST.30kts-1008mb-137N-710W.100pc.jpg
Invest 94L - Infrared satellite

Invest 94L should continue its march across the Caribbean and we will monitor it the entire (painstaking) way. If this system does develop, there is no telling where it will go as both Central America and the Gulf of Mexico look like reasonable realms. Guidance forecast of systems that have yet to develop are rarely accurate as a fixed center of circulation does no exist to fix, and the strength of the system will dictate would it interacts with its environment.

Click for full size model imagery:

Invest 94L - Track Forecast

The area of disturbed weather off of the Georgia and South Carolina coasts has begun to look better organized. The convection is moderate in strength, but the curved structure of the storms suggest that the system is developing vorticity. The convective outflow looks healthy, and the proximity of the system to land is a natural cause for concern. A hurricane hunter is currently scheduled to investigate the system tomorrow.

20080718.1945.goes12.x.ir1km.96LINVEST.25kts-1012mb-311N-805W.100pc.jpg
Invest 96L - Infrared Satellite

The system is expected to stay very close to the coast and accordingly warrants careful attention. Local weather service offices should already be issuing statements on the storm, so check in with them. The water in this area is very warm as it is fed by the Gulf Stream. Development looks likely so check back soon.

Click for full size model imagery:

Invest 96L - Track Forecast

For those of you who miss him, Jeff Masters will return for you tomorrow and Sunday, but you'll be stuck with me again next week. ;-) Have a fun weekend!

Cheers,
Bryan Woods

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Four areas of interest to watch in the Atlantic

By: JeffMasters, 20:32 GMT le 17 juillet 2008

We're now up to a total of four areas in the Atlantic that we're keeping an eye on. Where should I start? I guess the most logical place is the only named storm. Before I get there, below you'll find a satellite shot with all four systems marked. I know you guys have been waiting for this post judging by the emails that I've gotten, so here we go:

two_atl.gif
Current active systems of interest

Bertha is still hanging around the central Atlantic and at this point is becoming one of the longest-lived tropical storms on record. She is already the longest-lived July storm on record. The over longest? Hurricane Inga in 1969. The big question on how long Bertha last is how long it takes her to fully extratropically transition.

A note of interest, Bertha brought 4.76 inches of rain to Bermuda, which was right on with forecasts. There was a bit of a surprise to the intensity of her southern bands as a hurricane hunter found near hurricane force winds there. Fortunately, the Bermuda Weather Service was able to post a hurricane watch. The maximum recorded wind in Bermuda was a 79 knot gust, and the eyewall never actually cleared the west end of the island and left that area under the more severe southwest eyewall.

Currently Bertha is still packing winds of 60 mph, which may be generous, with a central pressure of 997 mb. After taking a swing to the southeast which was well forecast, Bertha has now turned to the east at 9 mph and should soon begin moving to the northeast.


Bertha - Watches and Warnings

Bertha has been looking ragged for a long time, but there is still relatively healthy convection to one side of the storm. If not for this burst of convection, Bertha would be weakening much more quickly. Right now there are no clear signs of extratropical transition visible on satellite, but this will be something to watch in the new few days.

20080717.1845.goes12.x.ir1km.02LBERTHA.50kts-997mb-339N-578W.100pc.jpg
Bertha - Infrared satellite

The dynamical models have done a good job forecasting Bertha's movement. It is largely for this reason that confidence is high in their forecast over the next 48 hours. As Bertha moves toward the northeast, there is some question of how she will transition. Extratropical transition is not well grasped by any forecast model. Accordingly, confidence in the forecast beyond 48 hours is low, especially when it comes to intensity. I expect Bertha to be with us for at least the next few days.

Click for full size model imagery:

Bertha - Track Forecast


Bertha - Wind Speed Forecast

Alright, now moving on to our first area of interest, Invest 94L. This system showed a great deal of potential earlier in the week, but the circulation never developed. At this point the proximity of the convection to the land of South America combined with the 'dead zone' for tropical cyclone genesis that seems to exist over the eastern Caribbean will likely preclude development. The warm water is fairly shallow in this area and subsidence from storms over South America also tend to act to cap convection. We will continue to monitor the system, but at this point conditions are not favorable. While convection looks healthy at this time, I expect that it will soon quiet down again.

20080717.1845.goes12.x.ir1km.94LINVEST.30kts-1009mb-126N-642W.100pc.jpg
Invest 94L - Infrared satellite

Invest 94L should continue its march across the Caribbean and we will monitor it the entire way. There is also a hurricane hunter scheduled to investigate the storm today, but I doubt it will find much if it flies.

Click for full size model imagery:

Invest 94L - Track Forecast

In the western Caribbean there is another area of organized thunderstorms approaching the coast of Guatemala. This system looks like it is not going to have enough time to develop before reaching land, but it will bring heavy rains and gusty winds to portions of Central America.

20080717.1845.goes12.x.ir1km.95LINVEST.25kts-1009mb-136N-825W.100pc.jpg
Invest 95L - Infrared Satellite

Of more immediate interests to those of us in the states, the area of thunderstorms that was in the Gulf yesterday has moved across the Florida peninsula and is now floating off the Georgia coast. The convection is not all that impressive, but its proximity to land warrants close observation over the next couple of days.

20080717.1845.goes12.x.ir1km.96LINVEST.20kts-1013mb-305N-803W.100pc.jpg
Invest 96L - Infrared Satellite

The system is expected to slowly move northeast along the coast, following the warm waters of the Gulf Stream. This is another reason while we'll keep a close eye out. I'm still actually awaiting the latest model runs but they should populate to this page automatically... (fingers crossed)

Click for full size model imagery:

Invest 96L - Track Forecast

Cheers,
Bryan Woods

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Bertha turns to the east, Invest 94L still there

By: JeffMasters, 11:33 GMT le 16 juillet 2008

Tropical Storm Bertha has finally turned toward the east at 6 mph and is now packing maximum sustained winds of 65 mph and her central pressure has ticked up to 996 mb. A general track to the east is expected to continue for the next few days with slight strengthening possible.


Bertha - Watches and Warnings

Bertha is starting to look pretty ragged on satellite imagery. Her circulation is becoming elongated due to synoptic steering patterns, but shear over the center of circulation remains low. Deep convection is limited but sufficient to maintain intensity at this time.


Bertha - Infrared satellite

As Bertha moves to the east, large meanders are expected in her track. Additionally, there is slightly warmer water ahead of the storm so slight strengthening is possible. Bertha is still not expected to be a threat to land of the next week. Shipping interests should still monitor the storm as there is a huge amount of divergence in track forecasts beyond 72 hours.

Click for full size model imagery:


Bertha - Track Forecast



Bertha - Wind Speed Forecast




The wave that has been traveling across the Atlantic still does not look that well organized, but convection has increased from yesterday. Right now I give the system a roughly 40% chance of development. Convection has finally started to build near the center of circulation, but has not been sustained enough to create a closed circulation. We will continue to monitor the system for signs of development.

20080716.1015.goes12.x.ir1km.94LINVEST.25kts-1011mb-127N-532W.100pc.jpg
Invest 94L - Infrared satellite

Model forecasts still bring this area of interest over the eastern Caribbean, which is often not favorable for tropical development. None of the models go crazy with this system, but some do still show signs of development.

Click for full size model imagery:


Invest 94L - Track Forecast



Invest 94L - Intensity Forecast




Elsewhere in the Atlantic, there is an area of thunderstorms over the northeast Gulf of Mexico that has a slight chance of reaching tropical depression status before arriving over Florida. No threat is expected from this system and it could bring beneficial rains to the northern Florida area.

Gulf_IR.jpg
Area of storms approaching Florida

Writing for Jeff Masters for the week, Bryan Woods

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Bertha moves away from Bermuda, eyes still on Invest 94L

By: JeffMasters, 11:47 GMT le 15 juillet 2008

Tropical Storm Bertha is once again up near hurricane strength, but is fortunately moving away from Bermuda. While her maximum sustained winds have increased to 70 mph, it is more important to note that her central pressure is also up to 995 mb, indicating a weaker storm.


Bertha - Watches and Warnings

Bertha's satellite signature shows a very large circulation with one main band wrapping around most of the storm. Other than that, Bertha looks like a normal tropical system decaying over the North Atlantic.

Motion continuing to the north and then to the east is expected over the next few days with some more wobbles to be expected on top of the general progression to the east. Short term strengthening is possible, but Bertha will be transitioning to an extratropical system in the long term.

Click for full size model imagery:

Bertha - Track Forecast

Bertha - Wind Speed Forecast

The area of interest heading toward the Lesser Antilles has still yet to organize. Overall conditions remain favorable for Invest 94L but deeper convection is required. Right now the main burst of convection visible on the satellite imagery is located ahead of the easterly wave axis. If the convection can shift a bit to the east then we could see some development. However, in its current position this burst of convection will not lead to development.

The wave will continue to progress to the west over the medium term. The HWRF has backed off developing the system, but the GFDL is still agressive in its solution. All the models are in agreement on a track to the west-northwest across the northern Caribbean.

Click for full size model imagery:

Invest 94L - Track Forecast

Invest 94L - Intensity Forecast

Writing from cheery old London this week, Bryan Woods.

Updated: 11:49 GMT le 15 juillet 2008

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Bertha approaches Bermuda, Invest 94L likely to develop

By: JeffMasters, 11:00 GMT le 14 juillet 2008

One can tell that it is quickly becoming late July when the tropics start to heat up to the point when there are two systems to monitor in the Atlantic. Right now we are monitoring Tropical Storm Bertha close in on Bermuda at the same time as we watch Invest 94L approach the Lesser Antilles. Of the two systems, Invest 94L has the potential to create a much higher impact event.

Tropical Storm Bertha is holding on to strong tropical storm intensity as she wanders around the North Atlantic and slowly approaches Bermuda with maximum sustained winds of 65 mph and a central pressure of 990 mb. Bertha is finally making steady progress to the north-northwest at 8 mph which should brush her right up against Bermuda with 3-5 inches of rain, tropical storm force winds, and seas up to 18 feet. On Saturday I was flying from Hartford to Philadelphia and could see swells rolling off of Bertha reaching the East Coast of the U.S.


Bertha - Watches and Warnings

Storm's of Bertha's size and maturity rarely exhibit rapid intensity changes, especially as far north as Bermuda, unless they interact with some mid-latitude synoptic feature. Since Bertha is pretty well isolated in the middle of the Atlantic, significant changed in intensity are not expected over the coming couple of days. Bertha has managed to maintain decent convection for a mid-latitude tropical system and features one prominent band to the north of the circulation.

20080714.0845.goes12.x.ir1km.02LBERTHA.55kts-990mb-309N-634W.100pc.jpg
Bertha - Infrared satellite

Currently the main band of Bertha is still offshore of Bermuda, but is approaching the island. Some convection ahead of the storm is already bringing heavy rain and tropical storm force winds. To monitor Bertha's impact on Bermuda, check with the Bermuda Weather Service'sradar page and their weather web cam, as well as current surface observations.

westatlantic-radar-2008-07-14-0643.png
Bermuda radar

Motion first to the north and then to the east is expected over the next few days with some more wobbles to be expected on top of the general progression to the east. For the time being, I am disregarding the intensity forecast models as the general have little skill with tropical or hybrid systems in mid-latitudes. Instead 'persistence' seems like the best forecast for Bertha.

Click for full size model imagery:


Bertha - Track Forecast

Bertha - Wind Speed Forecast

Elsewhere in the Atlantic, halfway between Africa and the Lesser Antilles there is a strong tropical wave that looks likely to become a tropical depression in the next day or so. Convection has been persistent and is showing healthy curved outflow. Unfortunately we have yet to receive a good QuikScat pass of the system which is precluding me from making any bold statements about the circulation. While tropical waves do have associated cyclonic curvature in the flow, it seems unlikely at this point that a closed cyclonic circulation has yet developed. We'll wait for a more centralized core of convection and possibly verification from a QuikScat pass before we make that claim.

20080714.0815.goes12.x.ir1km.94LINVEST.25kts-1008mb-110N-405W.100pc.jpg
Invest 94L - Infrared satellite

Since yesterday is seems even more likely that Invest 94L will become Tropical Depression Three. It is far too early for public talk of a landfall at this time. You'll never see me issuing that kind of forecast until after a closed circulation is well-developed. Both the GFDL and the HWRF slowly develop the system as it tracks toward the west. The mostly likely situation is for us to see a strong tropical storm over the northeast Caribbean in about 5 days.

Click for full size model imagery:
94L_GFDL_5day.png
Invest 94L - GFDL 5-Day Forecast
94L_HWRF_5day.png
Invest 94L - HWRF 5-Day Forecast

However, the situation bears further watching. This system could eventually end up anywhere in the Caribbean, Gulf of Mexico, or western Atlantic, and at any strength. The model tracks are showing a huge spread which is expected. Weak systems, like invests, do not have a well developed circulation so differences in model initializations very often result in a wide range of track solutions, even among the members that agree on intensity.

Click for full size model imagery:


Invest 94L - Track Forecast

Invest 94L - Intensity Forecast


Writing from cheery old London this week, Bryan Woods.

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New disturbance 94L a threat to the Lesser Antilles

By: JeffMasters, 15:51 GMT le 13 juillet 2008

A tropical wave midway between Africa and the Lesser Antilles Islands has become organized enough this morning to be classified as a threat area (Invest 94L) by NHC. This morning's QuikSCAT pass missed the disturbance, but last night's pass revealed 94L's large, elongated circulation near 9N 35W. Visible satellite loops show this circulation getting better defined this morning, although there is not yet much organized heavy thunderstorm activity. The disturbance is embedded in a large area of tropical moisture, and 94L should be unaffected by dry air or Saharan dust over the next few days. Water temperatures are favorable for development--27.5°C. Wind shear is favorable for development--10 knots.


Figure 1. NHC's graphical tropical weather outlook for Sunday Jul 13 2008 at 8 am EDT. Area "1" is Invest 94L. Image credit: NHC.

The forecast for 94L
NHC is giving 94L a medium 20-50% chance of developing into a tropical depression by Tuesday morning. Wind shear is expected to remain low, less than 10 knots, for the next four days, and the water temperatures will be plenty warm enough to support development. Three of the four global models we trust predict 94L will develop into a tropical depression. The ECMWF develops it and puts 94L into the Lesser Antilles islands on Saturday. The UKMET model does not develop it, but also puts it into the Lesser Antilles islands on Saturday. The GFS develops it but foresees that Bertha will be close enough and strong enough to pull 94L north of the Lesser Antilles about, 6-8 days from now. The NOGAPS model is much slower, and does not foresee a threat to the Lesser Antilles this week. In summary, there is the potential for a tropical depression to form later this week from 94L, and residents of the Lesser Antilles should anticipate the possibility of tropical storm conditions affecting the islands by Saturday.

Bertha
Tropical Storm Bertha stayed too long in one spot, and has churned up so much cold water from the depths that it has weakened to a tropical storm. Visible satellite loops still show a large and well-developed circulation, and the storm does have the potential to re-intensify if it can move away from the cold water it upwelled. However, the steering currents are very weak and are expected to remain so for several more days, making it unlikely Bertha can find any warm water. The outer spiral bands of Bertha are very close to Bermuda (see links below), and the storm is now visible on Bermuda radar. Bertha spent six days as a hurricane, making it the fourth longest-lived July hurricane on record (Emily of 2005 holds the record, at seven days). Bertha has been a named storm for 10 days, and will easily break the record for longest-lived July named storm (12 days).

Links to follow:
Bermuda radar
Current weather at Bermuda
Bermuda WebCam

High surf of 12-18 feet is expected to affect Bermuda through Monday, according to the Bermuda Weather Service. There is about a 64% chance the island will experience sustained winds of tropical storm force (39 mph), according to the latest tropical storm wind probability forecast by NHC. Given the very weak steering currents predicted to affect Bertha through Tuesday, the storm could easily approach closer to the island than the current official forecast.

South Carolina disturbance
A small low pressure system has developed off the coast of South Carolina at the tail end of an old cold front. This morning's QuikSCAT pass shows the circulation quite clearly, as do visible satellite loops. The low is too small to develop quickly, and is under about 20 knots of wind shear, which should also keep any development slow. The low is expected to move northeast, parallel to the coast.

My last blog entry until Saturday
This will be my last blog entry for five days (until Saturday). I'm headed to Lake Michigan for some camping and vacation. If you happen to be in Mackinaw City Tuesday night, be sure to catch the Straits Area Concert Band in Concert at 8pm at the band shell by Lake Huron. My dad and I will be in the trombone section, puffing our way through the band's usual assortment of Sousa marches, Gershwin medleys, and patriotic fare.

In my absence, our guest tropical blogger, Bryan Woods, will be doing my blog this week. Bryan has done a great job over the past three years blogging on the tropics over at thestormtrack.com. Here's Bryan's bio:

Bryan received his BS in Meteorology from the University of Massachusetts in Lowell, MA in 2005, and his M.Phil. in Geology & Geophysics from Yale University in New Haven, CT in 2007. Bryan is currently a Ph.D. candidate at Yale where he is also the graduate and professional student body president.

Bryan has spent two summers working on a National Science Foundation (NSF) sponsored field micrometeorological research project in Atlanta, GA, studying evapotranspiration rates in urban forest canopies. Currently, Bryan's research is focused on combining wavelet techniques and aircraft data from the NSF/NCAR Gulfstream V to diagnose energy and momentum fluxes from atmospheric gravity waves. Bryan has spent the past three hurricane seasons writing blogs on the tropics for thestormtrack.com.

Jeff Masters

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Nothing new with Bertha

By: JeffMasters, 16:14 GMT le 12 juillet 2008

I'll echo the statement of Hurricane Specialist Lixion Avilla in the 11 am Hurricane Bertha discussion: "After a week or so...I am running out of things to say about Bertha." Hurricane Bertha has slowed to crawl, and remains in no hurry to recurve out to sea. The storm continues its rather sloppy appearance on satellite loops, thanks to the fact that the inner eyewall dissolved, leaving a new, much larger eye. Bertha's structure is sufficiently sloppy that significant intensification is not likely, despite the presence of wind shear less than 10 knots and water temperatures of 27.5°C. Shear is forecast to remain below 10 knots until Monday morning, when it should rise to 20 knots. At that time, Bertha should be over 26°C water, and will probably weaken to tropical storm strength. The Air Force Hurricane Hunters will make their second flight into Bertha this afternoon to check on the storm's strength. The outer spiral bands of Bertha are starting to affect Bermuda (see links below), and the storm is now visible on Bermuda radar.

Links to follow:
Bermuda radar
Current weather at Bermuda
Bermuda WebCam

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Figure 1. Microwave image of Bertha from 5:45 GMT 7/12/2008 showing the vary large eye. Image credit: Navy Research Lab.

High surf of 9-15 feet is already affecting Bermuda, according to the Bermuda Weather Service. There is about a 61% chance the island will experience sustained winds of tropical storm force (40 mph), according to the latest tropical storm wind probability forecast by NHC. Given the very weak steering currents predicted to affect Bertha through Monday, the storm could easily approach closer to the island than the current official forecast. None of the computer models currently foresee that hurricane force winds will impact the island, and I don't expect this will happen.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. The models are more scattered in their prediction of a tropical depression forming between Africa and the Lesser Antilles Islands 4-7 days from now. One model, the ECMWF, has dropped its forecast of a depression forming, and the two remaining models disagree on just when such a depression might form. There are not any impressive tropical waves near the coast of Africa presently, so I'm currently dubious of the model predictions of a Cape Verdes-type tropical storm forming next week.

I'll post an update Sunday morning.
Jeff Masters

Updated: 22:50 GMT le 12 juillet 2008

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Bertha heads towards Bermuda

By: JeffMasters, 13:13 GMT le 11 juillet 2008

Hurricane Bertha continues trundling along towards Bermuda, but is not in a hurry to get there. The storm has a rather sloppy appearance on satellite loops, thanks to the fact that the eyewall dissolved last night, with a new, much larger eye taking its place. Bertha will probably start to intensify today now that this Eyewall Replacement Cycle (ERC) is done. Wind shear is low (less than 10 knots) and water temperatures are warm (28°C). Shear if forecast to remain below 10 knots until Sunday, but the water temperatures will gradually cool to 26°C by the time Bertha reaches Bermuda's latitude on Sunday. Intensification back to Category 2 status seems probable, and Bertha still has an outside chance at reaching Category 3 status again. The Air Force Hurricane Hunters will make their first flight into Bertha this afternoon to check on the storm's strength.

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Figure 1. Forecast of wave heights for 8am EDT (12 GMT) on Saturday, July 12, from the World Wave Model. Wave heights in excess of 4 meters (13-14 feet) are likely near the center of Bertha.

High surf is already beginning to affect Bermuda, and the World Wave Model (Figure 1) predicts that waves will continue to build at Bermuda through Sunday as the storm approaches. The Bermuda Weather Service is predicting waves of 9-13 feet in the island's outer waters by Sunday. There is about a 47% chance the island will experience sustained winds of tropical storm force (40 mph), according to the latest tropical storm wind probability forecast by NHC. None of the computer models currently foresee that hurricane force winds will impact the island, but given the very weak and unpredictable steering currents forecast to affect Bertha by Sunday, the storm could easily approach closer to the island than the current official forecast. Wind shear is expected to rise above 20 knots on Sunday and Monday, and this could knock Bertha down to tropical storm strength by Monday. Bertha could affect Newfoundland as a tropical storm in a week's time, as predicted by the GFDL model.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. Three out of four of our reliable computer models are predicting that a tropical depression may form between Africa and the Lesser Antilles Islands 5-7 day from now. These models did a pretty good job forecasting the formation of Bertha this far in advance, so the potential of another Cape Verdes-type storm forming next week needs to be taken seriously.

I'll post an update Saturday morning.
Jeff Masters

Updated: 13:14 GMT le 11 juillet 2008

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Bertha back at Category 2 strength

By: JeffMasters, 13:01 GMT le 10 juillet 2008

Hurricane Bertha is back at Category 2 status, and appears destined to become one of the longest-lasting July hurricanes on record. Hurricane Emily of 2005 spent seven full days at hurricane strength in July, which I believe is the record. Bertha is halfway there. The storm is in no hurry to recurve out to sea, and will spend 3-4 more days over waters warm enough to maintain it at hurricane intensity. At that time, wind shear is expected to increase to 30 knots, which should knock Bertha down to tropical storm status. The models continue to show that Bertha may slow down and move erratically this weekend, and possibly threaten Bermuda. By the middle of next week, Bertha should be headed towards the Maritime Provinces of Canada, and may affect them as a tropical storm.


Figure 1. This natural-color image of Hurricane Bertha was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite on July 9, 2008, at 14:45 UTC (10:45 a.m. Eastern Daylight Time). Shortly after, the National Hurricane Center estimated that Bertha was a Category 1 storm, with maximum sustained wind speeds of 75 miles per hour. Bertha was compact when MODIS observed it, a small ball of clouds with a long line of thunderstorms trailing away to the southeast. The eye of the storm had clouded over. Image credit: NASA.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. The ECMWF model is predicting that a tropical depression may form between Africa and the Lesser Antilles Islands in seven days. The UKMET and GFS have shown a similar development during various runs over the past two days, but their current runs are showing just a strong tropical disturbance in this region next week. Stay tuned.

I'll post an update Friday morning.
Jeff Masters

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Bertha strengthening again, after rapidly falling apart

By: JeffMasters, 13:15 GMT le 09 juillet 2008

Hurricane Bertha fell apart yesterday to a minimal Category 1 hurricane about as quickly as her meteoric rise to major hurricane status had come. At Bertha's peak--a Category 3 hurricane with 120 mph winds and 948 mb pressure--Bertha was the third strongest hurricane on record so early in the season. Increased wind shear of about 20 knots plus some dry Saharan air that got injected into the core caused Bertha's sudden demise.

The intensity forecast
Shear is on the wane again over Bertha, down to about 15 knots. The storm is beginning to get a more symmetric cloud pattern and better organization, as seen on visible satellite loops. An eye is reappearing, and with Bertha over warm 28°C water, further intensification is likely today. None of the models are predicting a return to Category 3 status again, but Bertha could become a Category 2 hurricane again before wind shear increases once more later this week.


Figure 1. Bertha at peak intensity: 21:15 GMT Monday July 7, 2008. At this time, satellite estimates of Bertha's strength were 115 kt (135 mph), making the storm a weak Category 4 hurricane. Bertha was the third strongest hurricane on record so early in the season.

The track forecast
Bertha is slowing down and turning northward as it "feels" the approach of a trough of low pressure to the north. All of the computer model turn Bertha northwards east of Bermuda, and it currently appears that the island will feel only peripheral effects of Bertha. However, the trough of low pressure turning Bertha to the north will not be strong enough to fully drag the storm into the far North Atlantic, so Bertha will wander close to Bermuda early next week while it waits for another trough of low pressure to finish the job. It is very unlikely Bertha will threaten the U.S., but it could eventually affect the Maritime Provinces of Canada.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. The GFS model is predicting that a tropical depression may form off the coast of Africa on Monday.

Dramatic temperature difference in California
At 3pm PDT yesterday, the temperature at the Point Sur Light Station was 52°F. A shallow marine layer lay along the coast, keeping temperatures cool. Just 20 miles inland, the temperature was 111°F at Arroyo Seco RAWS! You can check out this remarkable contrast by viewing the Wundermap for the region. We just added a fire layer to the product yesterday, so you can see the smoke density in the region as well. It's another bad day in Big Sur!

I'll post an update Thursday morning.
Jeff Masters

Updated: 20:00 GMT le 09 juillet 2008

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Hurricane Bertha: 6th strongest early season hurricane on record

By: JeffMasters, 13:45 GMT le 08 juillet 2008

Hurricane Bertha put on a remarkable burst of rapid intensification Monday afternoon, reaching major hurricane status--Category 3--with 120 mph winds and an estimated central pressure of 948 mb. Among early season hurricanes that have formed before August 1, Bertha is the sixth strongest early-season Atlantic hurricane on record. Only 12 early season major hurricanes have formed since record keeping began in 1851, though several were no doubt undocumented before the advent of the aircraft reconnaissance in 1944. Bertha holds the record for the farthest east a major hurricane has formed so early in the season (52°W longitude), easily beating the mark set in 1996 (67°W) by a previous incarnation of Hurricane Bertha. This year's Bertha now holds the record for farthest east formation as a tropical storm, hurricane, and major hurricane, so early in the season.


Figure 1. Bertha at peak intensity: 21:15 GMT Monday July 7, 2008. At this time, satellite estimates of Bertha's strength were 115 kt (135 mph), making the storm a weak Category 4 hurricane.

Bertha may even have intensified to Category 4 status between 1900 and 2300 GMT yesterday. Satellite estimates of the storm's intensity (Figure 2) from both NHC and the University of Wisconsin during that period were 115 knots (135 mph), which would have made Bertha a low-end Category 4 hurricane. However, it is unclear what Bertha's final official intensity maximum will be, since it reached maximum strength in between the official 6-hourly times used by NHC to document a storm's strength.


Figure 2. Satellite estimates of Bertha's strength using the Automated Dvorak Technique (ADT) showed the storm peaking at 115 knots (135 mph) for a four hour period on July 7, 2008. The "Best Track" line is the official NHC advisory strength, which peaked at 105 knots (120 mph); the AMSU dots are estimates from a polar orbiting instrument, the Advanced Microwave Sounding Unit; SATCON is a SATellite CONsensus estimate of Bertha's intensity combining several satellite strength estimates. Image credit: University of Wisconsin CIMSS.

Bertha's intensification was somewhat surprising, because environmental conditions for intensification were good, but not great. Wind shear was a modest 10-15 knots, Sea Surface Temperatures were 27°C--only 1° above the minimum needed to support a hurricane--and the oceanic heat content was only 20 KJ/cm**2, which is well below the value of 80 typically associated with rapid intensification.

Bertha is probably not a major hurricane any more. Wind shear has increased to 15-20 knots, and the eye has become less distinct and the cloud pattern degraded, as seen on visible satellite loops. The Hurricane Hunter mission scheduled for today has been canceled, and no future missions are planned.

The intensity forecast
SSTs will continue to warm to 28°C Wednesday morning underneath Bertha, but wind shear is expected to remain high and possibly increase some later this week. Bertha should gradually decline in strength to Category 1 status by the end of the week.

The track forecast
Bertha is slowing down and turning northward as it "feels" the approach of a trough of low pressure to the north. All of the computer models turn Bertha northwards east of Bermuda, and it currently appears that the island will feel only peripheral effects of Bertha. However, the trough of low pressure turning Bertha to the north may not be strong enough to fully drag the storm into the far North Atlantic, so Bertha may wander close to Bermuda early next week while it waits for another trough of low pressure to finish the job. It is very unlikely Bertha will threaten the U.S., but it could eventually affect Newfoundland, Canada.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. The UKMET and GFS models are showing some possible development in the region between Africa and the Lesser Antilles Islands about a week from now.

I'll post an update Wednesday morning.
Jeff Masters

Updated: 15:47 GMT le 08 juillet 2008

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Bertha becomes the season's first hurricane

By: JeffMasters, 12:39 GMT le 07 juillet 2008

The 2008 hurricane season's first hurricane is here--Hurricane Bertha. Bertha is the earliest forming July hurricane since Hurricane Cindy (July 6, 2005) and Hurricane Dennis (July 7, 2005). Bertha took advantage of warmer Sea Surface Temperatures (SSTs) of 27°C and modest wind shear of 10-15 knots to put on a burst of intensification to hurricane strength overnight. The storm has continued to intensify since the 5 am EDT NHC advisory, with some satellite estimates giving Bertha 90 mph winds--just below the threshold of Category 2 status. Visible satellite loops show a well-formed eye with excellent upper-level outflow to the north. The Hurricane Hunters are scheduled to make their first visit to Bertha on Tuesday afternoon to get a better idea of the storm's true strength.


Figure 1. Bertha at 7:45am EDT Monday July 7, 2008.

The intensity forecast
SSTs will continue to warm to 28°C Tuesday morning underneath Bertha, but wind shear is expected to increase some, to 15-20 knots. Bertha should intensify into a Category 2 hurricane today. Higher wind shear should halt intensification on Tuesday. Wind shear is expected to stay in the 15-25 knot range for the remainder of the week, and it is unlikely that Bertha can intensify beyond Category 3 status. The GFDL predicts Bertha will peak at Category 2 status, and the HWRF takes it to Category 3 strength (941 mb) five days from now.

The track forecast
Bertha will start to slow down over the next few days as it "feels" the approach of a trough of low pressure scheduled to move off the U.S. East Coast on Thursday. This trough will force Bertha on a more northwesterly track towards Bermuda, and most of the computer models foresee that Bertha will pass close to Bermuda 5-7 days from now. The exception is the GFS model, which predicts that Thursday's trough of low pressure will not be strong enough to recurve Bertha so far to the north. The GFS keeps Bertha farther south, bringing the storm on a track to pass close to the Carolinas early next week. So far, the GFS has been the most reliable model (Figure 2) tracking Bertha. However, the current run of the GFS depicts a considerably weaker storm than Bertha has become, and its track for Bertha will likely be too far south. A stronger Bertha will "feel" the upper-level westerly winds of the approaching trough more strongly than the GFS is indicating. In short, the best forecast is to assume Bertha will recurve to the north and pass close to Bermuda by the end of the week. Whether or not this trough will be strong enough to fully recurve Bertha northeastward into the hurricane graveyard of the North Atlantic is uncertain at this time.


Figure 2. Animated .gif showing the official NHC forecast every 12 hours (gray line) along with the actual track Bertha took (black line) and the forecasts from several of the computer models. The GFS model has had the best overall performance of the models. The NOGAPS and HWRF model tracks are not shown here, but did not do as well as the GFS.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic, and none of the models are forecasting tropical storm formation in the next seven days.

I'll post an update Tuesday morning.
Jeff Masters

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Bertha over warmer waters, should strengthen

By: JeffMasters, 16:15 GMT le 06 juillet 2008

Tropical Storm Bertha continues west-northwestward over the open Atlantic, and has now moved over some warmer Sea Surface Temperatures (SSTs) of 26°C. Wind shear has remained about 10-15 knots, and the storm appears to be responding to these slightly more favorable conditions by generating a larger area of heavy thunderstorm activity near the center. The tops of these more vigorous thunderstorms reach higher into the atmosphere, and thus show up as colder in infrared satellite loops. This morning's QuikSCAT pass showed winds of about 50 mph north of the center. Visible satellite loops show some increased organization of Bertha's low level spiral bands, and it appears the storm is beginning to intensify. Additional data on Bertha's current state will be available Monday afternoon, when the Hurricane Hunters are scheduled to pay their first visit.


Figure 1. Oceanic heat content along Bertha's forecast track. At 8am EDT today, Bertha had just finished crossing a region of near zero heat content (black colors), but is poised to cross a region of increasing heat content over the next 120 hours. The oceanic heat content is a measure of the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. A high heat content in excess of 80 kilojoules per square centimeter is very beneficial to rapid intensification. Bertha is not passing over anything that warm in the next five days, so only slow intensification is anticipated. Image credit: NOAA/CIRA/RAMMB.

The intensity forecast
SSTs will warm quickly to 27°C by Monday morning and 28°C Tuesday morning underneath Bertha, and shear is expected to remain moderately low, in the 10-15 knot range. This should allow Bertha to intensify into a Category 1 hurricane by Tuesday. On Wednesday, Bertha is expected to hit a branch of the subtropical jet stream that will raise shear levels to 15-20 knots. The models have been steadily weakening the trough of low pressure associated with this shear in recent runs, and it now appears that the shear will not be strong enough to weaken Bertha, although it may be able to keep Bertha from intensifying further.

The track forecast
The models are now in better agreement that Bertha will track well north of the northernmost Lesser Antilles Islands, and tropical storm conditions are not likely in the islands. Bertha should continue to the west-northwest through Thursday, when a moderately strong trough of low pressure is forecast to exit the East Coast of the U.S. This trough is expected to turn Bertha to the northwest, and there is the potential for Bertha to affect Bermuda 6-8 days from now. Climatology, and the few models that we have that run out that far--the GFS and the ECMWF--suggest that this trough will be strong enough to fully recurve Bertha so that it misses the U.S. However, long range forecasts of this nature are highly erratic in their reliability, and if this trough is not strong enough to recurve Bertha, the storm may pose a risk to the U.S. East Coast 7-10 days from now.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic, and none of the models are forecasting tropical storm formation in the next seven days.

I'll post an update Monday morning.
Jeff Masters

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Bertha may pose a long-range threat to the U.S.

By: JeffMasters, 12:43 GMT le 05 juillet 2008

Tropical Storm Bertha has maintained it's strength overnight, but is having trouble with Sea Surface Temperatures (SSTs) of 25°C--one degree below the threshold of 26°C considered beneficial for tropical storms. This morning's QuikSCAT pass missed Bertha, but last night's pass confirmed winds of at least 40mph. Satellite estimates of Bertha's strength have consistently put the storm's strength at 50 mph over the past day. The storm is under about 10-15 knots of wind shear.


Figure 1. Latest satellite image of Bertha.

The forecast
The models are now more confident that Bertha will not take a turn to the north east of Bermuda, with two models--the UKMET and NOGAPS--calling for the storm to pass very near the northern Lesser Antilles Islands by Tuesday night and Wednesday morning. These islands should anticipate the possibility of tropical storm conditions as early as Tuesday night, even though the current official track keeps Bertha well north of the islands. Bertha may be large enough to bring tropical storm force winds 100-200 miles from the center by Tuesday. A threat to the East Coast of the U.S. is now possible, with such an event most likely to occur about 7-10 days from now. It is also possible that high wind shear will tear apart Bertha (as predicted by the ECMWF model), or that the storm will recurve to the north just east of North Carolina, missing the U.S. The long range track of Bertha is highly uncertain, and it is too early to speculate how likely each of these scenarios is.

By Sunday morning, Bertha will be over SSTs of 26°C, which will warm to 27°C by Monday and 28°C Tuesday. this should allow Bertha to intensify to near Category 1 hurricane strength. However, by Tuesday, wind shear is expected to increase to 20 knots and remain high for several days, as Bertha encounters a branch of the subtropical jet stream. The higher shear should weaken the storm.

Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic, and none of the models are forecasting tropical storm formation in the next seven days.

I'll post an update by Sunday afternoon.
Jeff Masters

Updated: 16:50 GMT le 05 juillet 2008

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Bertha slowly intensifies, continues west-northwest

By: JeffMasters, 12:22 GMT le 04 juillet 2008

Tropical Storm Bertha has slowly intensified overnight, with new heavy thunderstorm activity building up around the center. Sea Surface Temperatures (SSTs) are now slightly below the threshold of 26°C considered beneficial for tropical storms, and this is limiting Bertha's intensification. This morning's 4:01am EDT QuikSCAT pass revealed top winds of about 30 mph, but the satellite was not able to reliably detect Bertha's strongest winds, since QuikSCAT does poorly in heavy rain. Bertha's current intensity is based mostly on satellite imagery of the cloud patterns. The storm is under about 10 knots of wind shear. Bertha should continue to slowly intensify today.


Figure 1. Track chart of all Atlantic tropical storms that have formed east of 40°W longitude since 1851.

The forecast
Not much has changed in the forecast, with all of the computer models foreseeing a west-northwest track into the mid-Atlantic over the next five days, with a possible recurvature to the north by the end of the period. Whether this recurvature takes place depends on how strong Bertha gets. A larger, stronger storm will be more likely to "feel" the approach of the trough of low pressure expected to recurve Bertha, which a shallower, weaker storm might be able to avoid recurvature and continue west-northwest. Wind shear is expected to remain below 10 knots the next three days, then increase to 30-40 knots by day five, according to the GFS model, when Bertha hits a branch of the Subtropical jet stream. The GFDL model does not go along with this high shear forecast, and makes Bertha a Category 2 hurricane that begins recurving to the east of Bermuda. In contrast, the HWRF model keeps Bertha a weak tropical storm for the next six days. Suffice to say, there is a lot uncertainty in the long range intensity forecast for Bertha!

The hurricane season of 2008 sets a new record
Bertha's at 25° West longitude is the farthest east a tropical storm has ever formed in the Atlantic so early in the season. Reliable records of Eastern Atlantic storms go back to 1967, the beginning of the geostationary satellite era. It's remarkable that no other early July storm even comes close to matching how far east Bertha formed (Figure 1).

Is the formation of Bertha a harbinger of an active hurricane season?
Probably. According the the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average. So it could be said that a JJ storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."

Elsewhere in the tropics
A tropical wave (93L) that passed through the Lesser Antilles Islands Wednesday night is now in the central Caribbean and is very disorganized, thanks to high wind shear. This wave is not expected to develop, and no computer models are predicting development anywhere else in the Atlantic over the next seven days.

I'll post an update Saturday morning. Happy 4th of July weekend!
Jeff Masters

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Tropical Storm Bertha forms--and sets a record

By: JeffMasters, 15:18 GMT le 03 juillet 2008

Tropical Storm Bertha is here. Overnight, a significant amount of heavy thunderstorm activity built up around the center, aided by Sea Surface Temperatures (SSTs) about 26°C (1°C above average for this date). This temperature is right at the threshold of where tropical storms can form. Also aiding the storm is 5-10 knots of wind shear. Bertha has good organization and a favorable environment for intensification, and should continue to slowly intensify today. There is not much African dust or dry air near the storm, and the main impediment to future intensification will be a region of colder SSTs the storm will track over on Friday and Saturday. These cooler SSTs have created some stable air to the northwest of Bertha. Evidence of this stable air in satellite imagery (Figure 1) can be seen in the form of a large area of stratocumulus clouds to the northwest of Bertha.


Figure 1. Visible satellite image of Bertha shortly before becoming a tropical storm. Note the large field of stratocumulus clouds to the storm's northwest, indicating stable air lying over cooler SSTs.

The forecast
Most of the computer models foresee a west-northwest track into the mid-Atlantic over the next five days, with a possible recurvature to the north by the end of the period. Whether this recurvature takes place depends on how strong Bertha gets. A larger, stronger storm will be more likely to "feel" the approach of the approaching trough of low pressure and recurve. A shallower, weaker storm might be able to avoid recurvature and continue west-northwest towards Bermuda.

The hurricane season of 2008 sets a new record
Today's formation of Bertha at 25° West longitude is the farthest east a tropical storm has ever formed in the Atlantic so early in the season. It is also the farthest east a tropical storm has formed in the month of July. Reliable records of Eastern Atlantic storms go back to 1967, the beginning of the geostationary satellite era.

Is the formation of Bertha a harbinger of an active hurricane season?
Probably. According the the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average. So it could be said that a JJ storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."

Elsewhere in the tropics
A tropical wave (93L) that passed through the Lesser Antilles Islands last night has grow less organized this morning, due to an increase in wind shear to 15-20 knots. The disturbance is a rather small one, and thus vulnerable to wind shear. QuikSCAT satellite imagery from this morning shows no sign of a surface circulation. The SHIPS intensity model predicts wind shear will increase above 30 knots by Saturday morning over 93L, and it is unlikely this disturbance will develop into a tropical depression.

Both the GFS and NOGAPS models are forecasting the development of a tropical disturbance in the southern Gulf of Mexico's Bay of Campeche on Monday. The tropics are getting active!

I'll post an update Friday morning. Happy 4th of July weekend!
Jeff Masters

Updated: 15:33 GMT le 03 juillet 2008

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African disturbance 92L chugs west; California fires generating significant pollution

By: JeffMasters, 13:40 GMT le 02 juillet 2008

A strong tropical wave (Invest 92L) is a few hundred miles off the coast of Africa, south of the Cape Verde Islands. This low has the potential to develop into a tropical depression later this week as it moves westwards at 15-20 mph. The low is under about 5-10 knots of wind shear, which is favorable for development. Sea Surface Temperatures are about 28°C, which is about 2-3°C above average for this date, and well above the 26°C threshold for tropical storm formation. There is not much African dust or dry air near the storm, but the low appears much as it did yesterday, with little heavy thunderstorm activity. The low has plenty of spin, as seen on this morning's QuikSCAT pass (Figure 1). One 35 mph (30 knot) wind vector was noted to the southwest of the storm's center in the QuikSCAT data, so 92L is kicking up some strong winds. There aren't really any negatives for development, except for climatology--there has never been a tropical depression that has formed east of 34° longitude in the first half of July (see the first image I posted in my July Atlantic hurricane outlook). NHC is giving a medium (20-50% chance) that 92L will develop into a tropical depression by Friday afternoon. All the models are developing this system into a tropical depression by Saturday. Wind shear is forecast by the SHIPS model to remain below 10 knots for the next four days, and the waters will stay warm (above 26°C) for the next 2-3 days of 92L's life. None of the computer models foresee that 92L will become a threat to land, and it currently appears that the storm will recurve in the middle of the Atlantic Ocean, east of Bermuda.


Figure 1. QuikSCAT image of 92L from 3:12am EDT Wed July 2, 2008. Image credit: Paul Chang, NOAA/NESDIS/ORA.

California fires cause significant air pollution
It was a destructive April, May, and June for fires in California, with 889,000 acres burned (71% of this acreage since June 20!) This is over five times the acreage burned during the same months in 2007. Satellite-detected fires from April through June this year totaled over 13,000, compared to only 1,200 during the same time period in 2007. These fires have caused many cities in the state to exceed the EPA's daily standard of 35 µg/m3 for particulate air pollution (Figure 2).


Figure 2. Location of all satellite-detected fires from last year (left panel) and this year (right panel) for the months of April through June, along with the number of days when surface particle pollution exceeded EPA's daily standard in several major cities in California. Image credit: Environmental Protection Agency (EPA), National Aeronautics and Space Administration (NASA).

Air quality is improving this week in California due to the onset of west-southwesterly winds aloft that have enhanced mixing and dispersion in the atmosphere. The moist flow of air off the ocean has brought fog and raised humidity levels, aiding firefighting efforts. However, with fire season's peak still months away, expect many more fires and significant pollution episodes later this summer.

Jeff Masters

Fire

Updated: 21:32 GMT le 16 août 2011

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African disturbance 92L could develop into tropical depression

By: JeffMasters, 18:55 GMT le 01 juillet 2008

A strong tropical wave (Invest 92L) moved off the coast of Africa last night, south of the Cape Verde Islands. This low has the potential to develop into a tropical depression late this week as it moves westwards at 15-20 mph. The low is under about 10 knots of wind shear, which is favorable for development. Sea Surface Temperatures are about 28°C, which is about 2-3°C above average for this date, and well above the 26°C threshold for tropical storm formation. There is not much African dust or dry air near the storm, but so far the low has not been able to generate much heavy thunderstorm activity. The low has plenty of spin, and has developed some broad, curved bands that are a sign of organization. I can't really find any negatives for development, except for climatology--there has never been a tropical depression that has formed east of 34° W longitude in the first half of July (see the first image I posted in yesterday's July Atlantic hurricane outlook). NHC is giving a low chance (less than 20%) that 92L will develop into a tropical depression by Thursday afternoon. Given the system's current disorganization, that's a reasonable forecast. However, all the models are developing this system into a tropical depression by late this week, and I think that the odds rise to a 50% chance of a tropical depression by Saturday. Wind shear is expected to remain low, and the waters will stay warm (above 26°C) for the next 2-3 days of 92L's life.


Figure 1. Latest satellite image of Invest 92L.

2008 African dust forecast
According to an experimental dust forecast by Amato Evan of the University of Wisconsin's Cooperative Institute for Meteorological Satellite Studies, 2008 should have fairly normal levels of African dust activity, similar to what was seen in 2007. This is a change from 2004, 2005, and 2006, which had fairly low levels of dust activity. This lack of dust resulted in 0.2°C less cooling of the eastern Atlantic Sea Surface Temperatures in 2004, 2005, and 2006 compared to 2007. High levels of dust block sunlight, leading to cooling of the sea surface.

Jeff Masters

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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