Dr. Jeff Masters' WunderBlog

Alma's remains could become a Gulf of Mexico tropical depression

By: JeffMasters, 15:30 GMT le 31 mai 2008

The remnants of Tropical Storm Alma reorganized over the Western Caribbean this morning, just off the coast of Belize. The storm (now called Invest 90L), whipped up winds over 40 mph over the ocean just east of Belize, according to this morning's 7:11am EDT QuikSCAT pass. Observations from Buoy 42056, just to the north in the Yucatan Channel, showed sustained winds of 30 mph, gusting to 35, with 10 foot seas. The center of 90L has now moved inland over Belize, and the storm has missed its chance to become the first tropical depression of the 2008 Atlantic hurricane season. It will have one more chance to do so on Sunday afternoon, when several models, including the GFDL and NOGAPS, are predicting that 90L will continue west into the Gulf of Mexico's Bay of Campeche and reorganize. The other models keep 90L inland over Mexico and do not foresee development into a tropical depression. If 90L does emerge into the Gulf of Mexico, it will not stay there long--a strong ridge of high pressure is forecast by all the models to force 90L on a west-southwesterly track into Mexico, giving the storm perhaps 12 hours to reorganize. Wind shear will be low, 5-10 knots, and I give 90L a 40% chance of becoming a tropical depression by Sunday night or Monday morning. The storm should bring heavy rains of 3-6 inches to Honduras, El Salvador, Belize, Guatemala, and Mexico's Yucatan Peninsula over the next two days. If 90L does manage to reorganize into a tropical depression or weak tropical storm, southeast Mexico could end up with 5-10 inches of rain. Here's NHC's take on the system:

Special tropical disturbance statement
1130 am EDT Sat May 31 2008

The broad area of low pressure previously located over the western Caribbean Sea has moved inland over Belize and the Yucatan Peninsula this morning. This system is accompanied by a large area of squalls and gusty winds primarily over the waters north and east of the circulation center. Significant development is not expected today as the system moves slowly westward over the Yucatan Peninsula. However...there is some potential for a tropical cyclone to form if the area of low pressure moves over the Bay of Campeche on Sunday.

Even if no development occurs...localized heavy rains and floods are possible during the next couple of days over portions of Honduras...El Salvador..Guatemala...Belize...and southeastern Mexico. Future tropical disturbance statements will be issued on this system as necessary. For information specific to your area...please consult statements from your local weather office.

Forecaster Avila/Rhome



Figure 1. Current satellite image of Alma's remnants, now called 90L.

Rare severe weather outbreak today along East Coast
More tornadoes raked the Midwest yesterday, adding to the extensive damage already wreaked by one of the worst months of tornado damage in U.S. history. Yesterday's most significant destruction occurred at 5:15am CDT in Attica, Iowa, when an EF-2 tornado smashed through town, injuring 10 people. Other tornadoes hit Wyoming, Minnesota, Illinois, Indiana, and Ohio.

Today, the action shifts to the East Coast, where the Storm Prediction Center is calling for a "Moderate" risk of severe weather from New York City to Washington D.C. The primary threat will be damaging thunderstorm wind gusts and large hail, but some brief tornado touchdowns are also possible.

Jeff Masters

Updated: 15:34 GMT le 31 mai 2008

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Alma dies without causing a major disaster; tornadoes rip Midwest

By: JeffMasters, 15:03 GMT le 30 mai 2008

Tropical Storm Alma has dissipated over the high mountains of Honduras, and thankfully did not dump enough rain to cause a major flooding disaster in Central America. At 3pm EDT yesterday, Alma became the first tropical storm since records began in 1949 to make landfall on the Pacific coast of Central America. All previous Eastern Pacific storms have hit Mexico, which is considered part of North America. Alma came ashore in Nicaragua, near the Honduras border, as a tropical storm with 65 mph winds. So far, one person has been killed in Nicaragua, due to a fallen power cable. However, Alma has not dumped enough rain to cause widespread flooding--rainfall amounts in Nicaragua the past two day have been 3-8 inches. Costa Rica has had rains of similar magnitude, which have caused isolated mudslides that have blocked roads. Additional rain from Alma's remnants should total less than two inches in Costa Rica and four inches in Nicaragua. However, Honduras, El Salvador, Belize, Guatemala, and Mexico's Yucatan Peninsula are likely to get 4-8 inches of rain over the next two days from this system, which could cause significant flooding and mud slides.


Figure 1. Current satellite image of Alma's remnants.

There is a large area of disturbed weather that has developed in the Western Caribbean between Jamaica and the Yucatan Peninsula in the past few hours, in association with a trough of low pressure extending from the center of Alma. This morning's 7:37am EDT QuikSCAT pass showed winds of up to 50 knots (58 mph) in this region, but no hint of a circulation or wind shift. It is possible that this disturbed area could start to develop on its own later today, as a westward-moving tropical wave currently near Jamaica interacts with it. This area should expand and spread into Honduras, Belize, and Mexico's Yucatan by Saturday. However, I'm not expecting a major flooding disaster with heavy loss of life anywhere in Central America from Alma's remnants. The GFS model is predicting that moisture from Alma will eventually work its way north and bring heavy rains to Florida by June 7. No models are predicting a tropical storm in the Atlantic during the coming week.


Figure 2. Doppler radar winds from the Kearney, Nebraska tornadoes of May 29, 2008. Note that a twin set of vorticies appears in this image, denoting that two adjacent tornadoes may have hit.

Tornadoes rip the Midwest again Thursday; tornadoes expected today in Illinois, Indiana, and Missouri
Numerous strong tornadoes raked the Midwest last night, adding to the extensive damage already wreaked by one of the worst months of tornado damage in U.S. history. Last night's most significant destruction occurred in Jewell, Kansas, and Kearney, Nebraska. In Jewell, numerous businesses were destroyed and the town water tower toppled. In Kearney, multiple twisters hit, and a 90-car train was knocked off of its tracks. Wunderblogger Mike Theiss caught up to the Kearney tornado, and describes his experience in his blog today. All told, there were 55 tornado reports Thursday, but no deaths or injuries.

The Storm Prediction Center is calling for a "Moderate" risk of severe weather across Illinois, Indiana, and Missouri this afternoon---one level below their "High" risk level that was posted yesterday. Expect another significant tornado outbreak today. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. Mike is in Tornado Alley this week, performing his annual chase efforts.

Jeff Masters

Tornado

Updated: 20:45 GMT le 24 octobre 2011

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Alma nears hurricane strength, takes aim at Nicaragua; major tornado outbreak today

By: JeffMasters, 17:03 GMT le 29 mai 2008

Tropical Storm Alma, in the Eastern Pacific off the coast of Nicaragua, is steadily intensifying, and appears likely to develop into a hurricane later today. The latest QUikSCAT pass from 8:03am EDT showed winds of 50 knots (58 mph) near the center, and a recent microwave image (Figure 1) showed the formation of an eye. Alma is generating very heavy rains in excess of six inches per day near its center. So far, satellite estimates of rainfall (Figure 2) indicate that 3-6 inches of rain has fallen over portions of Costa Rica and Nicaragua, and rainfall will continue to increase in these nations through Saturday. All of Central America, except for Panama, is at risk of flash flooding that will create dangerous mud slides over the next three days, and Alma has the potential to be a major disaster for Central America. Nicaragua, in particular, is at high risk of experiencing flash flooding and mud slides capable of causing heavy loss of life, due to its high mountainous terrain that will receive up to 20 inches of rain. However, the mountainous regions along the Pacific coast of Costa Rica, Honduras, El Salvador, and Guatemala are also at high risk of destructive flooding and mud slides.


Figure 1. Microwave image (colored, left side) and visible satellite image (gray colors, right side) of Tropical Storm Alma taken at 10:14am EDT Thursday May 29, 2008. An eye is visible in the color microwave image, surrounded by intense echoes (red colors) of an eyewall on the west side of the eye. Microwave instruments carried on polar-orbiting satellites can only "see" a swatch of Earth's surface a few hundred kilometers in diameter, and the edge of this swath happened to fall very near the eye of Alma at this time. Image credit: Navy Research Lab, Monterey.

Since Alma now dominates the circulation pattern of the region, none of the computer models are predicting that a tropical depression will form in the Western Caribbean in the coming week. It is possible that Alma could cross Central America and pop out in the Gulf of Mexico's Bay of Campeche. However, the crossing of Central America will severely disrupt the storm, and the odds of Alma becoming a depression in the Atlantic basin are very low. I am not expecting moisture from the storm to reach the U.S. Satellite loops show that Alma has developed a large circulation that extends into the Western Caribbean, and rains from Alma will affect Jamaica, the Cayman Islands, Mexico's Yucatan Peninsula, and western Cuba through Saturday. These areas can expect heavy downpours with rainfall totals of 3-6 inches through Saturday. Rainfall may be heavier, perhaps 5-10 inches, in Belize and along the north coast of Honduras.


Figure 2. Observed precipitation for the 24 hours ending at 12Z (8am EDT) Thursday May 29, 2008. Rainfall amounts in excess of 150mm (six inches, green colors) occurred near the coast of Costa Rica. Image credit: U.S. Navy Monterey.

Major severe weather outbreak today in the Plains
The Storm Prediction Center is calling for a "High" risk of severe weather across Iowa and Nebraska this afternoon--the highest level of severe weather alert. Expect another significant tornado outbreak today in the Plains. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. Mike is in Tornado Alley this week, performing his annual chase efforts.

I'll have an update Friday morning, or tonight if there's major tornado action to talk about.

Jeff Masters

Updated: 22:07 GMT le 29 mai 2008

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Major flooding disaster in Central America possible; severe weather outbreak today

By: JeffMasters, 13:08 GMT le 29 mai 2008

Tropical Depression 1-E, in the Eastern Pacific off the coast of Costa Rica, is steadily organizing and appears likely to develop into Tropical Storm Alma later today or tomorrow. Satellite loops show that the low has developed a very large and expanding circulation. This circulation is likely to expand across Central America into the Western Caribbean, allowing the storm to tap moisture from the Atlantic and Pacific. Storms that are able to tap the moisture sources of both oceans can be extremely dangerous rainmakers, even if they are weak tropical depressions. Already, TD 1-E is generating very heavy rains in excess of six inches per day near its center. So far, satellite estimates of rainfall (Figure 1) indicate that 3-6 inches of rain has fallen over portions of Costa Rica and Nicaragua, and rainfall will continue to increase in these nations through Saturday. All of Central America is at risk of flash flooding that will create dangerous mud slides over the next three days, and TD 1-E has the potential to be a major disaster for Central America. Nicaragua, in particular, is at high risk of experiencing flash flooding and mud slides capable of causing heavy loss of life, due to its high mountainous terrain that will receive rains up to 20 inches.

Since TD 1-E now dominates the circulation pattern of the region, none of the computer models are predicting that a tropical depression will form in the Western Caribbean in the coming week. It is possible that TD 1-E could cross Central America and pop out in the Western Caribbean near the Yucatan Peninsula, or in the Gulf of Mexico's Bay of Campeche. However, the crossing of Central America will severely disrupt the storm, and the odds of the storm becoming a depression in the Atlantic basin are low. I am not expecting moisture from the storm to reach the U.S., although I expect Jamaica and western Cuba will get heavy downpours from the system over the next three days.


Figure 2. Observed precipitation for the 24 hours ending at 06Z (2am EDT) Thursday May 29, 2008. Rainfall amounts in excess of 150mm (six inches, green colors) occurred near the coast of Costa Rica. Image credit: U.S. Navy Monterey.

Major severe weather outbreak today in the Plains
The Storm Prediction Center is calling for a "High" risk of severe weather across Iowa and Nebraska this afternoon--the highest level of severe weather alert. Expect another significant tornado outbreak today in the Plains. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. Mike is in Tornado Alley this week, performing his annual chase efforts.

I'll have an update Friday morning, or tonight if there's major tornado action to talk about.

Jeff Masters

Updated: 13:50 GMT le 29 mai 2008

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Parkersburg tornado an EF-5; major flooding in Central America likely from 90E

By: JeffMasters, 15:46 GMT le 28 mai 2008

The tornado that devastated Parkersburg, Iowa on Sunday has now been rated an EF-5 by the National Weather Service. An EF-5 is the strongest possible classification a tornado can receive, and is only given to those tornadoes with estimated winds over 200 mph. The winds in the Parkersburg tornado were estimated at 205 mph. At those wind speeds, total destruction of homes occurs. Even those sheltering in basements are not safe--several of the six deaths from the Parkersburg tornado were from people sheltering in basements.

The Parkersburg tornado cut a path 43 miles long and between 3/4 miles and 1.2 miles wide across Iowa, killing six people, completely destroying 350 buildings in Parkersburg, and injuring 70 people. It was only the second EF-5 tornado this decade in the U.S. The other EF-5 occurred in May 2007, when much of Greensburg, Kansas got leveled. The Parkersburg tornado was the first F5 or EF5 tornado in Iowa since the Jordan, Iowa tornado of June 13, 1976, and was the second deadliest in Iowa since official record-keeping began in 1950. Iowa's deadliest tornado hit Charles City on May 15, 1968, killing 13 while producing F5 damage.


Figure 1. EF-5 damage from the May 25, 2008 Parkersburg tornado. At EF-5 winds speeds (over 200mph), homes are completely destroyed or removed from their foundations. Image credit: Iowa Helicopter. The NWS Des Moines office has posted ground damage photos from their damage survey.

Major flooding likely in Central America from 90E
An area of low pressure (90E) in the Eastern Pacific off the coast of Costa Rica, near 10N 88W, is steadily organizing and appears likely to develop into a tropical depression later today or tomorrow. The National Hurricane Center is currently assigning a "High" probability (>50% chance) that this will be a tropical depression, in its new experimental Tropical Weather Outlook. Satellite loops show that the low has developed a very large and expanding circulation. This circulation is likely to expand across Central America into the Western Caribbean, allowing the storm to tap moisture from the Atlantic and Pacific. Storms that are able to tap the moisture sources of both oceans can be extremely dangerous rainmakers, even if they are weak tropical depressions. Already, 90E is generating very heavy rains in excess of six inches per day near its center. The storm is expected to move northeastward over Costa Rica or Nicaragua by Thursday or Friday, and should being dangerous flooding rains of 5-10 inches to those nations and Panama. Most of the computer model guidance suggests that the storm will then track to the north, spreading very heavy rains across Guatemala, El Salvador, Honduras, Belize, and southern Mexico by Saturday. These heavy rains will cause life-threatening flash flooding, particularly in mountainous regions.

Since 90E is beginning to dominate the circulation pattern of the region, it appears unlikely that a tropical depression will form in the Western Caribbean in the coming week, as some computer models have been predicting. It is possible that 90E could cross Central America and pop out in the Western Caribbean near the Yucatan Peninsula, or in the Gulf of Mexico's Bay of Campeche. However, the crossing of Central America will severely disrupt the storm, and the odds of 90E becoming a depression in the Atlantic basin are low.


Figure 2. Observed precipitation for the 24 hours ending at 12Z (8am EDT) Wednesday May 28, 2008. Rainfall amounts in excess of 2000mm (eight inches, yellow colors) occurred near the center of disturbance 90E off the Pacific coast of Costa Rica. Image credit: U.S. Navy Monterey.

I'll have an update Thursday morning.

Jeff Masters

Tornado

Updated: 20:46 GMT le 24 octobre 2011

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The tornado season of 2008: climate change to blame? And, tropical update

By: JeffMasters, 17:07 GMT le 27 mai 2008

Residents of Parkersburg, Iowa continue to assess damage and clean up from the tornado that killed six people on Sunday. The tornado was rated EF-5, the highest possible rating for a tornado. An EF-3 tornado also hit Hugo, Minnesota on Sunday, killing one person. Only five new tornado reports occurred yesterday, and severe weather is expected to remain relatively low for the next two days. A new storm system is expected to bring an enhanced chance of severe weather to the upper Midwest beginning Thursday. The deaths Sunday push this year's tornado death toll to 110. This makes 2008 the 12th deadliest tornado season since 1950, and the deadliest since 1998, when 130 deaths were recorded. Assuming that the Parkersburg, Iowa tornado was an EF-4 or EF-5, there have been nine violent EF-4 or EF-5 tornadoes this year. This is the most since 1999, when 13 such twisters were recorded. The total (preliminary) number of tornadoes so far this year is 1191. I doubt that we will break the all time record of 1817 tornadoes in a year, set in 2004, but 2008 may vault into second place if we can top 1998's 1424 tornadoes. Could this year's tornadoes be a sign of climate change?


Figure 1. Tornadoes deaths in the U.S. by year since 1950. Year 2008 deaths are as of May 26.

Well, let's be clear that human-caused climate change is occurring, and will significantly affect nearly all aspects of weather and climate in the decades to come. However, many of these changes will be so small or gradual that they will not become detectable until many decades hence, since there is a large natural variability in weather. As I noted in my February blog, Are tornadoes getting stronger and more frequent?, there is new research that predicts that we may see an increase in the severe thunderstorms that spawn tornadoes by the end of the century. However, the computer modeling efforts that predict this rise in severe weather are just beginning, and much more research remains to be done before we can believe these preliminary results.

Will we be able to detect changes in tornado frequency if they occur?
We won't be able to detect changes in tornado frequency due to climate change, unless there is a very large change. We need a technology that can detect all tornadoes, all the time in order to be able to evaluate changes in tornado frequency. Doppler radar can only "see" perhaps 50% of all tornadoes, and many of those it detects never touch down. Thus, we rely on human observers to spot tornadoes, or look for buildings that got in the way of a tornado, using the damage pattern to identify a tornado. If there are no humans around to see a tornado, and if a tornado does not encounter any structures, it will go unrecorded. As the population increases and more buildings are erected, tornado reports will increase. This factor alone can account for the observed increase in total tornadoes since 1950 (Figure 2).

Is there evidence that strong and violent tornadoes are increasing?
Strong tornadoes (EF2 and EF3 on the Enhanced Fujita Scale) and violent tornadoes (EF4 and EF5, or F4 and F5 on the pre-2007 Fujita Scale), which make up less than 25% of all tornadoes, cause a large fraction of the tornado deaths. These storms are less likely to go uncounted, since they tend to cause significant damage along a long track. Thus, the climatology of strong and violent tornadoes may offer a clue as to how climate change may be affecting severe weather. Unfortunately, we cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. Tornadoes are categorized using the Enhanced Fujita (EF) scale, which is based on damage. So, if a strong or violent tornado happens to sweep through empty fields and never destroy any structures, it will not get a rating. Thus, if the number of violent tornadoes has actually remained constant over the years, we should expect to see some increase in these storms over the decades, since more buildings have been erected in the paths of tornadoes.

However, if we look at the statistics of strong and violent U.S. tornadoes since 1950 (Figure 2), there does not appear to be any increase in the number of these storms. In fact, there appears to be a decrease, although the quality of the data base is probably not good enough to say this with confidence. It appears likely that climate change has not caused an increase in the strongest tornadoes in recent decades. I believe we can blame 2008's nasty tornado season on an unusually far south loop that the jet stream has taken this year over the U.S., thanks to natural variability in the weather.


Figure 2. Total, strong and violent tornadoes in the U.S. by year since 1950. The year 2008 (not pictured) has had 128 strong or violent tornadoes as of May 26, according to Wikipedia.

Possible development in the Western Caribbean or Eastern Pacific late this week
A weak low pressure area (Invest 90E) has developed in the Eastern Pacific off the coast of Guatemala, near 10N 90W. This low has the potential to develop into a tropical depression by the end of the week, according to the UKMET model. Other models, such as the GFS, Canadian, and ECMWF, foresee that this area of disturbed weather will not have time to develop before moving northwards over Central America by the end of the week, bringing heavy rains to the region. Once over land, this low might move over the waters of the Western Caribbean and allow a tropical depression to form, as predicted by the GFS model. The NOGAPS model, in contrast, predicts that a tropical depression will form in the Western Caribbean south of Cuba, with no development in the Eastern Pacific. Given the persistence of these computer models over the past week in developing something in the region, I'd put the odds of a tropical depression forming within 7 days at about 40% in the Eastern Pacific, and at 20% in the Western Caribbean. There is a lot of wind shear predicted to prevail near or over the Western Caribbean late this week and early next week, reducing the odds that any such development could hold together long enough to affect the U.S. Regardless, residents of Guatemala, El Salvador, Honduras, Belize, and southern Mexico can expect heavy rains and possible flash flooding late this week from this system.


Figure 3. Area of disturbed weather over the Eastern Pacific that is forecast by some models to develop into a tropical depression. The NHC Graphical Tropical Weather Outlook is a good tool to track this disturbance.

I'll have an update by Wednesday afternoon.

Jeff Masters

Tornado

Updated: 20:46 GMT le 24 octobre 2011

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Killer tornadoes rip Iowa, and Minnesota; tropical depression possible late this week

By: JeffMasters, 14:08 GMT le 26 mai 2008

The 2008 Memorial Day Weekend tornado outbreak will continue to hammer the U.S. today, even as residents from Iowa and Minnesota clean up from the devastating tornadoes that killed eight people Sunday afternoon. A mile-wide tornado plowed through Parkersburg, Iowa between 5pm and 6pm CDT yesterday, killing five people in that city, and two in nearby New Hartford. It was the deadliest tornado in Iowa in more than 40 years. The tornado passed just north of the airport in Waterloo, Iowa, which recorded sustained winds of 64mph, gusting to 94 mph at 5:37pm CDT. Damage appeared to be at least EF-4 in photos I saw, and possibly EF-5 (over 200 mph). In Minnesota, another powerful twister killed at least one person and injured 9 in the Minneapolis suburb of Hugo.


Figure 1. Satellite image of the supercell thunderstorms that spawned the Parkerburg, Iowa and Hugo, Minnesota tornadoes on May 25, 2008.

The slow-moving low pressure system responsible for all the mayhem began in Colorado on Thursday, when the Storm Prediction Center (SPC) recorded 48 reports of tornadoes, including the EF-3 mile-wide twister that killed one person in Windsor, Colorado. On Friday, an additional 63 tornado reports occurred, mostly in Kansas. The tornado that hit Quinter, Kansas on Friday was the eighth violent EF-4 tornado of the year. Also on Friday, two people were killed in Cairo, Kansas when a tornado smashed a car trying to flee the storm. If you want to see why one should not try to escape a tornado in a car, take a look at what the tornado did to the car. Saturday was relatively quiet, with only 13 tornado reports, but Sunday's tally of 43 brought the 4-day total from the 2008 Memorial Day weekend outbreak to a remarkable 157 tornado reports. Some of these tornado reports are undoubtedly of the same tornado, so the actual number of tornadoes for the 4-day outbreak may be less than 150. BBC has some awesome aerial footage of the weekend tornadoes.



Figure 2. Radar reflectivity image (top) of the May 25, 2008 Parkersburg, Iowa tornado. The position of Parkersburg is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex.



Figure 3. Radar reflectivity image (top) of the May 25, 2008 Hugo, Minnesota tornado. The position of Hugo is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex. We've also saved a 12-frame radar animation of the Hugo cell, thanks to wunderground member Todd S.

Tallying up the numbers
The death toll from Sunday pushes this year's tornado deaths to 111, the most since 1998, when 130 were recorded. Assuming that the Parkersburg, Iowa tornado was an EF-4 or EF-5, there have been nine violent EF-4 or EF-5 tornadoes this year. This is the most since 1999, when 13 such twisters were recorded. The total number of tornadoes this year is approaching 1100, and we may challenge the all time record for tornadoes in a year of 1817, set in 2004. Could this be a sign of climate change? No, I don't think so, and I'll explain why in a blog later this week.

Severe weather forecast
Severe weather will pound the U.S. again this Memorial Day, with the main action expected to stretch more than halfway across the country--from Texas to New York. NOAA's Storm Prediction Center has placed much of Kansas, Oklahoma, and the Texas Panhandle under its "Moderate Risk" category for severe weather, one step below its highest level of concern, "High Risk". Yesterday was also a "Moderate Risk" day in Iowa and Minnesota. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. After today, it appears the severe weather outbreak will finally diminish, with only a slight risk of severe weather expected Tuesday, and no severe weather expected Wednesday.

Possible development in the Western Caribbean or Eastern Pacific late this week
For the past 3-6 days, our most reliable global computer weather forecast models have been predicting the development of a low pressure system near or over Central America by Friday of this week. Given the persistence in the models in developing this low, we need to be alert to the possibility of a tropical depression forming in either the Western Caribbean or Eastern Pacific, on either side of Costa Rica and Nicaragua. It is uncertain which ocean basin such a storm might form in, and whether or not there will be a tropical wave around to help kick off development. It may be that the low pressure region will stay anchored over land south of the Yucatan Peninsula, preventing any development. This is the solution preferred by the ECMWF model in its last few runs. However, the GFS, NOGAPS, and Canadian models all predict a tropical depression might form in the Western Caribbean near Mexico's Yucatan Peninsula. In contrast, the UKMET shows development in the Eastern Pacific, on the Pacific side of Central America. Climatologically, May tropical storms are much more common in the Eastern Pacific than the Western Caribbean, so we should not discount the UKMET solution, even though it is an outlier. All five models predict that the Central American low pressure area will move northward towards the Gulf of Mexico, and wind shear may fall enough to allow a tropical depression to form should the low's center emerge over water. I'll be posting daily updates on the situation this week.

Jeff Masters

Tornado

Updated: 20:47 GMT le 24 octobre 2011

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Killer tornadoes smash Iowa and Minnesota

By: JeffMasters, 03:09 GMT le 26 mai 2008

An exceptional tornado outbreak continues to hammer the U.S. tonight, in what has been an extraordinary year for tornadoes. A mile-wide tornado plowed through Parkersburg, Iowa this afternoon, killing at least five people and injuring 15, according to KCRG. The tornado passed just north of the airport in Waterloo, Iowa, which recorded sustained winds of 64mph, gusting to 94 mph at 5:37pm CDT. Damage appeared to be at least EF-4 in photos I saw, and possibly EF-5 (over 200 mph). In Minnesota, another powerful twister killed at least one person and injured 9 in the Minneapolis suburb of Hugo this afternoon.


Figure 1. Satellite image of the supercell thunderstorms that spawned the Parkerburg, Iowa and Hugo, Minnesota tornadoes on May 25, 2008.

A slow-moving low pressure system that began in Colorado on Thursday spawned 48 reports of tornadoes Thursday, including the EF-3 mile-wide twister that killed one person in Windsor, Colorado. On Friday, an additional 63 tornado reports occurred, mostly in Kansas. The tornado that hit Quinter, Kansas on Friday was the eighth violent EF-4 tornado of the year. The last year that had more violent tornadoes was 1999, when 13 such twisters were recorded. Two people were killed in Cairo, Kansas when a tornado smashed a car trying to flee the storm. If you want to see why one should not try to escape a tornado in a car, take a look at what the tornado did to the car. Saturday was relatively quiet, with only 13 tornado reports, but Sunday's tally (so far) of 37 has brought the 4-day total to a remarkable 150+ tornado reports. Some of these tornado reports are undoubtedly of the same tornado, so the actual number of tornadoes for the 4-day outbreak may be less than 150.



Figure 2. Radar reflectivity image (top) of the May 25, 2008 Parkersburg, Iowa tornado. The position of Parkersburg is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex.



Figure 3. Radar reflectivity image (top) of the May 25, 2008 Hugo, Minnesota tornado. The position of Hugo is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex.

Severe weather forecast
Severe weather will pound the U.S. again Memorial Day, with the main action expected to stretch more than halfway across the country--from Colorado to New York. NOAA's Storm Prediction Center has placed central Kansas under its "Moderate Risk" category for severe weather, one step below its highest level of concern, "High Risk". Yesterday was also a "Moderate Risk" day in Iowa and Minnesota. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss.

I'll have an update Monday morning, including a look at the tropics, where the computers models persist in predicting a tropical storm may form in the Western Caribbean or Eastern Pacific late this week.

Jeff Masters

Tornado

Updated: 20:48 GMT le 24 octobre 2011

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Mile-wide tornado smashes Windsor, Colorado; plus, hurricane season commentary

By: JeffMasters, 13:23 GMT le 23 mai 2008

A mile-wide tornado swept through Colorado between 11am and noon yesterday, ripping the roofs off buildings, tossing cars into the air, and killing at least one person in Weld County, northeast of Denver. Hail up to 2.75" in diameter accompanied the storm, which took an unusual north-northwesterly track , parallel to the Rocky Mountains north of Denver. Hardest hit was the town of Windsor, between Fort Collins and Greely, where damage appeared to be at least EF3. A local TV station took some impressive live video of the tornado, and a wunderground web cam video in Windsor, Colorado (Figure 1) caught the funnel as it passed east of the camera.


Figure 1.Webcam view looking east at 11:45am MDT in Windsor, Colorado as the tornado passed by. Note the golf ball-sized hail covering the ground. Image credit: windsorweather.com.

The Weather Underground's tornado expert, Rob Carver, had this explanation of yesterday's tornadoes:

Two low pressure systems over the Western U.S. were the cause of the severe weather outbreak of May 22. A strong upper-level low over the Great Basin formed a surface low in the lee of the Rockies, and brought a strong southerly mid-level jet over the Plains of Colorado and Kansas. As the surface low formed, it brought warm moist air northwards, forming a warm front. This warm air moved westward, rising with the terrain, causing thunderstorms to form. Once these storms formed, the juxtaposition of easterly flow at the surface with southerly flow aloft (i.e., wind shear) produced significant spin in the horizontal direction, which was tilted by the storms to vertical spin, which triggered the formation of tornadic thunderstorms. This movement of the warm front up the slope of the Rockies to help trigger tornadic thunderstorms is a rare occurrence.



Figure 2. Radar reflectivity image (top) of the Windsor, Colorado tornado at 11:45am MDT May 22, 2008. Note the classic hook-shaped echo associated with the tornado. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex. For those interested, we've saved an animation of the reflectivity and Doppler velocity.

Severe weather forecast
Severe weather is expected today over Kansas, Nebraska, and surrounding states. NOAA's Storm Prediction Center has placed portions of this area under its "Moderate Risk" category for severe weather, one step below its highest level of concern, "High Risk". Yesterday was also a "Moderate Risk" day, which SPC later upgraded to "High Risk" once the tornadoes started pounding Colorado. More severe weather is expected Saturday and Sunday over the Midwest as the upper-level low pressure system responsible moves slowly eastward. The Weather Underground Severe Weather page and Tornado page and WebCam page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss, who was in Kansas yesterday, and has posted many spectacular photos of yesterday's storms. According to Mike's blog:

Today Cloud 9 Tours saw 4 tornadoes and ONE developing almost overhead. We experienced winds over 100mph from the circulation of the meso that passed overhead as a cone tornado developed. Stay Tuned....


Possible development in the Eastern Pacific late next week
The past four days, the ECMWF model has been predicting the formation of a tropical storm in the the Eastern Pacific, just off the coast of Guatemala, around May 29. The GFS model has also been predicting something might develop, but in the Western Caribbean near Mexico's Yucatan Peninsula. The GFS has been rather inconsistent with its handling of this potential storm, and I am inclined to discount its forecast--especially since last night's long range runs of the NOGAPS, Canadian, and UKMET models all show development in the Eastern Pacific, not the Caribbean. All five models predict a northward shift in the jet stream and substantial relaxation in wind shear over the Eastern Pacific and Western Caribbean next week. It is common to see May tropical storms in the Eastern Pacific, and it would not be a surprise to see something develop there. I'd put the odds of something popping up in the Western Caribbean or Gulf of Mexico at less than 10%, though.

NOAA's seasonal hurricane forecast
NOAA issued its annual seasonal hurricane forecast yesterday, which I discussed in detail in yesterday's blog entry. Yesterday's forecast came out with a little more uncertainty attached to it than previous forecasts, which is a good thing. The media attention and fanfare that accompanies these forecasts is rather excessive, given the low skill they have. In fact, we don't even know if the NOAA forecasts have ANY mathematical skill, because they've never released a verification study of their forecasts. I doubt that the skill is very high--as Eric Berger of the Houston Chronicle pointed out in a blog yesterday, NOAA has blown its forecast of Accumulated Cyclone Energy (ACE) each of the last six years. NOAA held its usual press conference to announce the forecast; this year, it was held at the home of the NOAA Hurricane Hunter aircraft, MacDill AFB in Tampa. The NOAA dignitaries present said all the right things, preaching the need for preparedness regardless of the forecast for the upcoming hurricane season. Still, I wonder if NOAA might be hurting themselves by making such a public spectacle over the release of a forecast that no one knows the accuracy of, and has performed poorly by some measures in recent years. I do like the fact they are issuing public hurricane forecasts, as I expect their accuracy and value will improve in coming years, but they're definitely not worth the attention they're getting at present.

Jeff Masters

Tornado

Updated: 20:48 GMT le 24 octobre 2011

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A 65% chance of an above-average 2008 hurricane season : NOAA

By: JeffMasters, 18:25 GMT le 22 mai 2008

There is a 65% chance that year's Atlantic hurricane season will have above average activity, and only a 10% chance it will be below average, according to today's seasonal hurricane forecast issued by the National Oceanic and Atmospheric Administration (NOAA). In an effort to emphasize the uncertainties in their forecast, NOAA for the first time included a probability with their "numbers" forecast. The numbers NOAA predicts are 12-16 named storms, 6-9 hurricanes, 2-5 intense hurricanes, and an Accumulated Cyclone Energy (ACE) index 100%-210% of the median. They put the odds of experiencing those numbers at 60-70%. This probability is based on the numbers seen in past hurricane seasons that had May climate patterns similar to those of 2008. An average hurricane season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The forecasters also call for a 70% chance of a below normal season in the eastern Pacific off the coast of Mexico, and a below normal season in the Central Pacific near Hawaii.

The forecasters considered two main climate signals to make their forecast:

1) Continuation of conditions that have been conducive to above-normal Atlantic hurricane activity since 1995, including above-average sea-surface temperatures in the eastern tropical Atlantic Ocean (Figure 1). Between 1995-2007, the Atlantic has averaged 14.5 named storms, 8 hurricanes, and 4 major hurricanes, with an average ACE index of 167% of the median. NOAA classifies nine of the thirteen seasons since 1995 as above normal, with seven being hyperactive (ACE > 175% of median). Only four seasons since 1995 have not been above normal. These include the three El Niño years (1997, 2002, and 2006) and the 2007 season.

2) La Niña or neutral conditions during the peak months (August-October) of hurricane season. The current weak La Niña episode has substantially weakened the upper-level west-to-east winds over the tropical Atlantic. These winds create much of the wind shear that inhibits hurricane development. La Niña is expected to persist (or weaken slightly to neutral) by hurricane season, and these weak westerly winds are predicted to persist over the Atlantic, bringing low levels of wind shear.



Figure 1. SST image from NOAA's May 22, 2008 seasonal hurricane forecast.

Where will this year's storms hit?
NOAA does not present any forecasts of where the steering currents might take this year's hurricanes. In general, steering currents are not predictable more than 5-7 days in advance, although sometimes one can stretch this to two weeks. As far as U.S. landfalls go, I believe that the climatology of landfalls since 1995 offers the best forecast. Between 1995-2007, the U.S. experienced an average of 4.7 named storms making landfall, of which 1.9 were hurricanes and 0.8 were intense hurricanes.

How believable is the NOAA forecast?
Mathematical analysis of forecasts made in late May and early June by two groups--Tropical Storm Risk Inc., and the Colorado State University group led by Phil Klotzbach and Dr. Bill Gray--have shown that these forecasts had reasonable skill over the past decade or so. Unfortunately, NOAA has never performed a mathematical analysis of their May forecasts, so we don't know whether their forecasts have any skill. Their forecast did poorly last year, and the head of NOAA's forecast team, Dr. Gerry Bell, provides an interesting analysis of why last year's forecast failed.

Commentary
NOAA's forecast is a reasonable one--but then, since 1995, it's always been smart to forecast an above-average hurricane season, barring an obvious El Niño event. I like that NOAA is looking to insert more uncertainty into their forecast by providing a percent chance of their expected numbers to verify. However, they really need to provide some verification numbers of how much skill these forecasts have, if they want to make them more useful. In addition, they should consider dropping the "numbers" forecast of named storms, since the number of named storms often does not give a true picture of how active a hurricane season is. For example, recall the 15 named storms from 2007, which was about 50% above average! Yet the ACE index, a more true measure of the destructive power of the storms, was 31% below average. ACE index is a better measure because it uses the square of each storm's peak winds, summed up over the lifetime of the storm. Wind damage from a hurricane is proportional to the square (and possible the cube) of the peak winds, and total damage increases by a factor of 5-10 for each Saffir-Simpson Category between Cat 1 and Cat 4. According to Pielke and Landsea (1998), a doubling of wind speed--from a 74 mph Category 1 hurricane to a 148 mph Category 4 hurricane--has historically resulted in 250 times more damage for U.S. landfalling hurricanes. This figure includes storm surge, flooding, and wind damage.

The Colorado State University group led by Phil Klotzbach and Dr. Bill Gray issues their seasonal hurricane forecast on June 3 this year, and I'll be sure to provide additional commentary then.

Jeff Masters

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The future of wind shear: will it decrease the number of hurricanes?

By: JeffMasters, 13:55 GMT le 21 mai 2008

Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? Several modeling studies are now predicting this, and it is a reasonable hypothesis. The most recent study, "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", was published Sunday in Nature Geosciences. The authors, led by Tom Knutson of NOAA's GFDL laboratory, showed that global warming may reduce the number of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. However, their model also found that the strongest hurricanes would get stronger.

An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

The main sources wind shear over the tropical Atlantic:
1) The jet stream is the primary year-round source of high wind shear over the Atlantic. The jet can have two branches--the main northerly polar jet, and a weaker subtropical jet that blows over the Gulf of Mexico or Caribbean. In winter, the jet stream is far to the south, bringing very high levels of wind shear to the tropical Atlantic. The Caribbean Sea is warm enough year-round to support hurricane formation, but high levels of wind shear from the southerly position of the jet stream prevents wintertime hurricanes from forming. In the summer, the jet stream retreats to the north, but can still loop far enough south to create hurricane-hazardous wind shear.

2) The large-scale tropical atmospheric circulation pattern known as the Walker Circulation (Figure 1) can bring high wind shear to the Atlantic. A weak Walker Circulation brings high wind shear, while a strong Walker Circulation--rising air over the tropics near Australia, combined with sinking air of the coast of South America near Peru--brings weak upper-level winds over the Atlantic, resulting in low levels of wind shear.

3) The presence or absence of an El Niño event has a critical impact on wind shear levels. El Niño events weaken the Walker Circulation, bringing strong upper-level winds out of the west to the Atlantic, creating high wind shear.

4) In summer and fall, Tropical Upper Tropospheric Troughs (TUTTs) and upper-level cold-core low pressure systems ("cold lows") that are cut off from the jet stream often wander through the tropics, bringing high wind shear with them.

5) A strong east-to-west flowing jet of air is frequently found at the southern boundary of the Saharan Air Layer (SAL), a hot, dry region of air found off the coast of Africa during hurricane season. This easterly jet often is strong enough to cause significant wind shear over the hurricane development region of the tropical Atlantic.


Figure 1. Schematic drawing of the Pacific Ocean's Walker Circulation. Warm ocean waters over the Western Pacific near Australia heat the air above, causing it to rise. When the rising air reaches the top of the troposphere, it can't rise any further, and is forced to flow eastwards towards the Atlantic. This air then sinks back to the surface near the Pacific coast of South America, then flows back towards Australia as easterly trade winds. Image credit: Wikipedia.

The future of wind shear
In their 2007 paper, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Gabe Vecchi of NOAA's GFDL laboratory and Brian Soden of the University of Miami looked at 18 of the models used to formulate the "official word" on the science of climate change, the 2007 Intergovernmental Panel on Climate Change (IPCC) climate report. Vecchi and Soden found that in the scenario where CO2 doubles to 720 ppm by year 2100 (the so-called "A1B" scenario), these models predict a 1.5-3.5°C increase in global surface air temperature. However, in the Caribbean and some surrounding regions, at least 13 of the 18 models predict that the amount of wind shear rises by 1-2 mph per degree C of warming (Figure 2). The shear increases largely as a result of a weakening of the Walker Circulation. This weakening brings strong upper-level westerly winds to the Eastern Pacific and Caribbean.

The implications
If true, Vecchi and Soden's results imply that we may see fewer hurricanes in the Atlantic and Eastern Pacific by the end of the century, since wind shear is such an important ingredient in their formation. How reliable are these model predictions? If global warming is expected to cause a slowdown in the Walker Circulation and increased wind shear over the tropical Atlantic, shouldn't we be able to see these effects already? There is some evidence that we are seeing these effects. According an article by the same authors published in 2006 in Nature, the observed 0.5-0.6°C global warming in the past century has caused the Walker Circulation to slow down by 3.5%--in line with what theory predicts. Moreover, Wang and Lee (2008) documented a 3 mph increase in wind shear over the tropical Atlantic between 1949-2006 (despite some rather low shear years recently, such as during the record-breaking Hurricane Season of 2005). These results, plus the fact that 13 of the 18 IPCC models predict a tropical Atlantic wind shear increase in the coming century, make the hypothesis that we may see increased wind shear over the Atlantic in coming decades a reasonable one. However, climate scientists Ray Pierrehumbert and Rasmus Benestad argue in a 2006 post on realclimate.org that we need another ten years of observations of the Walker Circulation to confirm that we really are seeing a slowdown. In addition, we need to see if the model predictions of increased wind shear hold up when improved simulations with better data and higher resolutions are performed. These models are fairly primitive in their abilities to simulate these sort of regional climate shifts, and some models predict a strengthening of the Walker Circulation in coming decades--the opposite of what Vecchi and Soden found.


Figure 2. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

Caveats
All other things remaining constant, an increase in wind shear will cause fewer hurricanes to form. However, all other things will not remain constant. As the climate warms, Sea Surface Temperatures (SSTs) will warm, which may partially or completely offset the effects of increased wind shear. Vecchi and Soden's research also show a substantial increase in wind shear over most of the Southern Hemisphere's hurricane breeding grounds during their hurricane season, but a significant decrease in wind shear over the Western Pacific and North Indian Oceans. Typhoons and cyclones in these ocean basins may well get more numerous and stronger in the future as a result of the lower wind shear. Much more research remains to be done, and it is far too early to be confident of how wind shear might change in a warming world.

References
Vecchi, G.A., B.J. Soden, A.T. Wittenberg, I.M. Held, A. Leetmaa, and M.J. Harrison, 2006, "Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing", Nature, 441(7089), 73-76.

Vecchi, G.A., and B.J. Soden, 2007, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Geophysical Research Letters, 34, L08702, doi:10.1029/2006GL028905, 2007.

Wang, C., and S. Lee, 2008, "Global warming and United States landfalling hurricanes", Geophysical Research Letters 35, L02708, doi:10.1029/2007GL032396, 2008.

realclimate.org has a nice discussion of the Veccu and Soden paper.

Jeff Masters

Climate Change

Updated: 16:49 GMT le 23 février 2010

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Chile's volcano not likely to affect the climate

By: JeffMasters, 14:01 GMT le 19 mai 2008

It's been a busy month for natural disasters, and I haven't found time to talk about Chile's Chaiten volcano, 760 miles (1,220 km) south of the capital Santiago. The volcano started erupting on May 2 for the first time in thousands of years, spewing ash, gas and molten rock into the air, forcing the evacuation of thousands of people. Did this mighty eruption have a cooling effect on the climate?


Figure 1. This May 5, 2008 image from NASA's Terra satellite caught Chaiten erupting. Image credit: NASA.

Many historic volcanic eruptions have had a major cooling impact on Earth's climate. However, Chaiten is very unlikely to be one of them. To see why this is, let's examine recent volcanic eruptions that have had a significant cooling effect on the climate. In the past 200 years, Mt. Pinatubo in the Phillipines (June 1991), El Chichon (Mexico, 1982), Mt. Agung (Indonesia, 1963), Santa Maria (Guatemala, 1902) Krakatoa (Indonesia, 1883), and Tambora (1815) all created noticeable cooling. As one can see from a plot of the solar radiation reaching Mauna Loa in Hawaii (Figure 2), the Mt. Pinatubo and El Chichon eruptions caused a greater than 10% drop in sunlight reaching the surface. The eruption of Tambora in 1815 had an even greater impact, triggering the famed Year Without a Summer in 1816. Killing frosts and snowstorms in May and June 1816 in Eastern Canada and New England caused widespread crop failures, and lake and river ice were observed as far south as Pennsylvania in July and August. Volcanic eruptions cause this kind of climate cooling by throwing large amounts of sulfur dioxide gas into the stratosphere. This gas reacts with water to form sulphuric acid droplets (aerosol particles), which are highly reflective, and reduce the amount of incoming sunlight.

You'll notice from the list of eruptions above that all of these climate-cooling events were from volcanoes in the tropics. Above the tropics, the stratosphere's circulation features rising air, which pulls the sulfur-containing volcanic aerosols high into the stratosphere, where the upper-level winds circulate them all around the globe. These aerosol particles take a year or two to settle back down to earth, since there is no rain in the stratosphere to help remove them. However, if a major volcanic eruption occurs in the mid-latitudes or polar regions, the circulation of the stratosphere in those regions generally features downward subsiding air, and the volcanic aerosol particles are not able to penetrate high in the stratosphere and get carried all around the globe. Chaiten is located near 40° south latitude, far from the tropics, and thus is unlikely to be able to inject significant amounts of sulfur aerosols into the stratosphere. Furthermore, the character of Chaiten's eruptions so far has been to eject a lot of silica and not much sulfur into the air. The total amount of sulfur ejected has been only about 1/10000 of what Mt. Pinatubo put into the air, according to NASA.


Figure 2. Reduced solar radiation due to volcanic aerosols as measured at Mauna Loa Observatory, Hawaii. Image credit: NOAA/ESRL.

Realclimate.org has a nice article that goes into the volcano-climate connection in greater detail. One interesting quote from the article: There can be some exceptions to the tropics-only rule, and at least one high latitude volcano appears to have had significant climate effects; Laki (Iceland, 1783-1784). The crucial factor was that the eruption was almost continuous for over 8 months which lead to significantly elevated sulphate concentrations for that whole time over much of the Atlantic and European regions, even though stratospheric concentrations were likely not particularly exceptional.

My next blog will talk about new research regarding the hurricanes/global warming connection.

Jeff Masters

Climate Change

Updated: 19:56 GMT le 16 août 2011

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Dean, Felix and Noel get their names retired; 13th warmest April on record

By: JeffMasters, 13:53 GMT le 16 mai 2008

For the first time since the the incredible Hurricane Season of 2005, a new set of Atlantic hurricane names has been permanently retired. Members of the World Meteorological Organization's Regional Association IV Hurricane Committee decided to retire the names of Hurricane Dean, Hurricane Felix, and Hurricane Noel during their annual meeting in Orlando this week. These names will not be used again because of the significant death and destruction these storms caused in 2007. The names Dorian, Fernand, and Nestor will serve as replacements of the 2013 hurricane season, when the names from 2007 are scheduled to repeat. The list of retired hurricane names now features 21 storms from the decade of the 2000s, and 70 storms since 1954.


Figure 1. Satellite images of the fearsome threesome of 2007: Dean, Felix, and Noel.

The names for the coming 2008 Atlantic hurricane season are Arthur, Bertha, Christobal, Dolly, Edouard, Fay, Gustav, Hanna, Ike, Josephine, Kyle, Laura, Marco, Nana, Omar, Paloma, Rene, Sally, Teddy, Vicky, and Wilfred. Looking at the latest long range GFS model forecast, there's no sign that we'll be seeing Tropical Storm Arthur during the last half of May. Wind shear remains seasonably high over the tropical Atlantic, and there is plenty of dry air evident.

In the Eastern Pacific, where hurricane season began yesterday, the names for 2008 are Alma, Boris, Cristina, Douglas, Elida, Fausto, Genevieve, Hernan, Isell, Julio, Karina, Lowell, Marie, Norbert, Odile, Polo, Rachel, Simon, Trudy, Vance, Winnie, Xavier, Yolanda, and Zeke. There's nothing brewing in that ocean basin, either.

April 2008: 13th warmest April on record for the globe
April 2008 was the 13th warmest April for the the globe on record, according to statistics released by the National Climatic Data Center. The January-April year-to-date period ranked twelfth warmest. A weak La Niña event continues to cool ocean waters in the central and eastern equatorial Pacific. The La Niña event weakened considerably in April, but has stabilized just above the threshold for being classified as neutral, during the first half of May.

A cool April in the U.S.
For the contiguous U.S., April was the coolest April in 11 years for the lower 48 United States, and fell into the lowest twenty-five percent of all Aprils based on records going back to 1895, making it the 29th coolest April on record. Precipitation was near average for the month.

April arctic sea ice extent
April 2008 Northern Hemisphere sea ice extent was the eighth lowest on record for the month of April, 7% below its extent in 1979 when satellite measurements began, according to the National Snow and Ice Data Center. April was the fifth straight month that a new monthly minimum arctic sea ice record was not set, following a string of five months in a row where monthly records were set. The past four years had the least April sea ice extent since records began in 1979, with 2007 having the least April sea ice extent on record. However, while the ice extent is not at a record low this year, the volume of the arctic sea ice is probably at a record low for April. The ice is exceptionally thin across the Arctic this winter, and the edge of this thin first-year ice extends beyond the North Pole.

I'll have more on Cyclone Nargis next week. The Southwest Monsoon has continued to push northward, and is expected to move into the cyclone-devastated region on Saturday, bringing heavy rains.

Jeff Masters

Climate Summaries

Updated: 22:25 GMT le 21 octobre 2011

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Monsoon rains approaching Myanmar

By: JeffMasters, 20:22 GMT le 14 mai 2008

The weather in the region ravaged by Cyclone Nargis in Myanmar has taken a turn for the better today, after a low pressure system that brought heavy rains and 20-30 mph winds yesterday weakened and moved off to the north. This system appeared to be a threat to develop into a tropical depression yesterday, but interaction with land is hindering its development, and the low is no longer expected to become a tropical depression. You can view the latest satellite images of the low (dubbed 96B) at the Navy/NRL web site.

The monsoon is coming
However, the respite from bad weather will be short-lived, as the mighty summer monsoon is almost upon the disaster area. The Southwest Monsoon (called that because the winds typically blow from the southwest) is an annual rainy period lasting from late May to mid-September in the regions surrounding the North Indian Ocean. The monsoon forms in response to the unequal summertime heating of the air over the land and oceans. The land heats up quicker than the oceans, creating low pressure and rising air over the Indian subcontinent. Moist air from the oceans is drawn in over the land areas to replace this hot, rising air, and the moist oceanic air brings heavy rains to the region. Truly prodigious rains accompany the arrival of the monsoon. The capital of Yangon averages about one inch of rain per month in the period just before the monsoon starts, and twenty inches per month thereafter.


Figure 1. Current position of the Southwest Monsoon, (northernmost green line), compared to average. The northern edge of the monsoon is almost upon the region hit by Cyclone Nargis. Image credit: India Meteorological Department.

As of today, the edge of the monsoon was just 100 miles south of Yangon and the Irrawaddy delta region (Figure 1). The monsoon is expected to push northwards into the region by Saturday--about one week earlier than average. The monsoon will greatly complicate relief efforts in Myanmar, which can expect flooding rains and problems with mud-choked and washed out roads. The monsoon will continue to affect the area until September. One bright side: once the monsoon arrives, it greatly reduces tropical cyclone formation in the North Indian Ocean. Major tropical cyclones in the North Indian Ocean are most common in May and November, just before and just after monsoon season.

Was the population warned?
Many of you have expressed amazement that so many could die from a tropical cyclone in this day and age of satellites and modern communications. Why did it happen? I believe there are two main reasons: the historical lack of tropical cyclones that have hit the Irrawaddy delta, and the unwillingness of Myanmar's leaders to provide adequate warnings for fear of jeopardizing their May 10 referendum to consolidate their power.

I've been sent an image of the warning for Cyclone Nargis as it appeared on May 2 in one of Myanmar's main newspapers, "The New Light of Myanmar". The warnings for Nargis on the day it made landfall as a major cyclone did not make the front page, but instead were buried on page 15 of the obituaries and miscellaneous section. The story did not talk about the storm surge or the cyclone's maximum sustained winds, and only mentioned that Myanmar might experience 50 mph winds in squalls. At the time the newspaper was likely preparing this article, both the Joint Typhoon Warning Center and the India Meteorology Department were calling for Nargis to be a Category 1 or Category 2 storm at landfall in Myanmar.

Jeff Masters

Updated: 22:14 GMT le 14 mai 2008

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May 2008: month of the natural disaster

By: JeffMasters, 13:18 GMT le 13 mai 2008

We live on a dangerous planet. In the wake of Cyclone Nargis, one of the deadliest tropical cyclones of all time, China's worst earthquake in 32 years has struck just 12 days later. Rarely in recorded history have twin natural disasters claiming 10,000 or more lives struck so close to each other in time. The last such occurrence I could find was in China in 1931. That summer, the world's deadliest natural disaster of all time--the Yellow River Flood of 1931--killed between one and four million people. On August 10 of the same summer, a magnitude 8.0 earthquake killed 10,000 people near Fuyun, China. Another notable twin disaster occurred on September 1, 1923 when the Great Kanto earthquake hit Japan. Winds from a passing typhoon fanned fires that sprang up after the quake, and the resulting fire storm engulfed Tokyo, killing over 100,000 people. Still, with a major volcanic eruption in Chile and an unusually severe tornado season pounding the U.S. with killer tornadoes, May 2008 will long be remembered as one of the worst months for natural disasters in world history.

Cyclone Nargis update
A tragedy of truly epic proportions continues to unfold in Myanmar in the wake of Cyclone Nargis. The United Nations now unofficially estimates that the death toll from the storm is at least 100,000, with up to 220,000 people missing. It is now 12 days since the cyclone struck, and aid efforts are only about 10-20% of what is needed to bring life-saving water, food, and medicine to the 1.5 million people affected by the storm. The death toll will now start to rise sharply, as the failure to provide adequate relief within ten days greatly increases the risk of disease and death in survivors of a cyclone. The indifference of Myanmar's leaders to the plight of its suffering people could make the death toll from Nargis the second highest in history, next to the 500,000 people killed in Bangladesh's Great Bhola Cyclone of 1970. Cyclone Nargis' unofficial death toll of 100,000 currently ranks the storm as the 10th deadliest in world history.


Figure 1. Topography of Myanmar, with track of Cyclone Nargis superimposed. Image credit: NASA.

Was the population warned?
Many of you have expressed amazement that so many could die from a tropical cyclone in this day and age of satellites and modern communications. Why did it happen? I believe there are two main reasons: the historical lack of tropical cyclones that have hit Burma's Irrawaddy delta, and the unwillingness of Myanmar's leaders to provide adequate warnings for fear of jeopardizing their May 10 referendum to consolidate their power.

According to irrawaddy.com:

Appearances on Burma's state television by the country's director general of the Department of Meteorology and Hydrology, Tun Lwin, always attract a large following.

Viewers like his style and informative approach to weather reporting. But now those same viewers are asking: "Why did he fail to warn us of the approach of Cyclone Nargis?"

According to well-informed sources close to his department, Burma's leading meteorologist passed those warnings on to the government in Naypyidaw, together with information about the cyclone's strength, expected course, and timing.

Tun Lwin reportedly suggested the warning should be carried by state media, but sources said he was told by his bosses in the capital: "Don't create public panic ahead of the referendum."

Warnings of the approaching cyclone were finally published in the official press, but they were buried amid news of the approaching constitutional referendum.


I've been sent an image of the warning for Cyclone Nargis as it appeared on May 2 in one of Myanmar's main newspapers, "The New Light of Myanmar". The warnings for Nargis on the day it made landfall as a major cyclone were buried on page 15 of the obituaries and miscellaneous section of the newspaper.

Figuring into the junta's logic for ignoring the approaching cyclone was the history of tropical cyclone strikes on the country. Since reliable records began in 1970, there have been only six hurricane-strength tropical cyclones to hit Myanmar. See the paper, "Simulation of Storm Surges Along Myanmar Coast Using a Location Specific Numerical Model" (Jain et al., Natural Hazards 39, 1, September 2006) for more information. The storms are:

1975 Pathein cyclone, Cat 1 (75 knots), hit just north of Irrawaddy Delta. An estimated 1.2 meter storm surge occurred. This storm did inundate the Irrawaddy delta, and 187 people died.

1982 Gwa cyclone, Cat 4 (120 knots), hit Gwa, north of Irrawaddy delta. An estimated 4 meter storm surge occurred.

1992 Sandoway cyclone, Cat 1 (65 knots), hit Sandoway, farther north than Gwa. An estimated 1.2 meter storm surge occurred.

1994 Sittwe cyclone, Cat 4 (125 knots), hit Sandoway/Sittwe. An estimated 1.2 meter storm surge occurred.

Mala of 2006 was a Cat 4, (115 knots) and also hit north of the Irrawaddy delta. No storm surge estimate available.

Nargis of 2008.

In the pre-1970 years, I could find only one mention of a hurricane-force storm hitting the country, a Cat 1 cyclone in 1936 that killed 36 people. A significant cyclone hitting the Irrawaddy delta causing thousands of deaths would very likely have been recorded, had this happened any time in the past 300 years. Such events were recorded in both India and Bangladesh during that period. Nargis appears to have been the only major tropical cyclone to hit the Irrawaddy River delta in recorded history, and may be a once-in-500-year event.

Comments from Chris Burt
I've been in regular communication about this disaster with Chris Burt, author of the excellent book Extreme Weather. He has been visiting Myanmar every year for 30 years, and has much insight on the situation there:

Anecdotally, I can say in all the time I've spent in Burma I have never heard anyone talk about or worry about tropical storms, it simply is not in their consciousness. This is why people really didn't heed the warnings. People were warned at least 48 hours in advance--I got an email from a friend two days before the storm telling me about the warnings in Rangoon.


More severe weather today
The Storm Prediction Center has placed much of eastern Texas and surrounding states under their "Slight Risk" category for severe weather today. The Weather Underground Severe Weather page and Tornado page are good places to go to follow today's severe weather. A slight risk of severe weather is also expected Thursday over the deep south, from eastern Texas to Alabama.


Jeff Masters

Updated: 12:25 GMT le 14 mai 2008

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An unusually early and violent tornado season

By: JeffMasters, 13:51 GMT le 12 mai 2008

An EF-4 tornado with winds of 166-175 mph swept through Oklahoma and Missouri Saturday, killing 21 people. Hardest hit were the towns of Picher, OK, where six died, and Seneca, MO, where ten died. The violent tornado was up to a mile wide. It's been an unusually early and violent tornado season in the U.S. There have been 905 tornadoes so far this year, a total usually not seen until late July (Figure 1). Saturday's deaths bring the 2008 U.S. tornado death toll up to 96--the most tornado fatalities since 1998, when 130 people died. With at least another month left in peak tornado season, 2008 ranks as the 12th deadliest year in the 59-year record. The Picher tornado was the sixth violent EF-4 tornado of the year.


Figure 1. Cumulative tornado activity in the U.S. through May 11, compared to average. This year's 905 tornadoes match the total usually seen by late July. Image credit: NOAA Storm Prediction Center.

Severe weather forecast
Severe weather is not expected today over the U.S., but more severe weather and tornadoes are expected Tuesday through Thursday in association with another powerful spring storm. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. Mike is in Tornado Alley this week, performing his annual chase efforts.

Amazing video: why not to take shelter in your car during a tornado
If you haven't seen it, the video captured by a surveillance camera during the Leighton, Alabama tornado on May 8 is a stunning testimonial of why one should not try to escape a tornado using a car. The EF-2 tornado with winds of 111-135 mph picked up cars like toys and tossed them into the air. A large number of tornado deaths and injuries occur when people try to escape the twister in their car and get caught by the violent winds.

Jeff Masters

Tornado

Updated: 21:54 GMT le 24 octobre 2011

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Tornadoes kill 21 in Missouri and Oklahoma

By: JeffMasters, 17:44 GMT le 11 mai 2008

Killer tornadoes swept through Oklahoma and Missouri, yesterday, killing at least 21 people. Hardest hit were the towns of Picher, OK, where seven died, and Seneca, MO, where ten died. Damage photos I saw from these towns showed buildings swept clean from their foundations, indicative of at least EF-4 damage, and possibly EF-5. Yesterday's deaths brings the 2008 U.S. tornado death toll up to 96--the most tornado fatalities since 1998, when 130 people died. With at least another month left in peak tornado season, 2008 already ranks as the 12th deadliest year in the 59-year record.


Figure 1. Storm damage reports from the Saturday, May 10 tornado outbreak. Image credit: NOAA Storm Prediction Center.

Today's severe weather forecast
Severe weather is not done with the region yet--the Storm Prediction Center has placed much of the Southeast under their "Slight Risk" category for severe weather today. Two tornadoes have already been reported today, one in Georgia, and one in Kentucky. The Weather Underground Severe Weather page and Tornado page are good places to go to follow today's severe weather. A more significant chance of tornadoes and severe weather is expected Wednesday and Thursday over the deep south, from eastern Texas to Alabama.

Storm chasing with Mike Theiss
No word yet from Wunderblogger Mike Theiss on which storms he intercepted during yesterday's mayhem. Mike is in Tornado Alley this week, performing his annual chase efforts. Be sure to catch his spectacular photos and chase accounts.

Jeff Masters

Tornado

Updated: 21:54 GMT le 24 octobre 2011

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Cyclone Nargis death toll may top 100,000

By: JeffMasters, 13:58 GMT le 08 mai 2008

The tragedy of Cyclone Nargis' aftermath in Myanmar continues to intensify, with the top U.S. diplomat in the county now predicting a death toll of 100,000. A death toll of 80,000 has been estimated by a local official in just one district of the country--Labutta--so the death toll may go much higher than 100,000. Although the first major U.N. relief flight of emergency supplies has finally landed, the criminal indifference of the nation's leaders towards the plight of the cyclone's survivors will doom hundreds or thousands more to death or terrible suffering. One can only hope that the people of Myanmar will rise up and put an end to Myanmar's dictatorship as a result of this awful tragedy.

There is historical precedent for this sort of occurrence. The deadliest tropical cyclone of all time, the Great Bhola Cyclone of 1970, killed upwards of 550,000 people is what was then called East Pakistan (and now called Bangladesh). A statement released by eleven political leaders in East Pakistan ten days after the cyclone hit charged the government with "gross neglect, callous indifference and utter indifference". They also accused the president of playing down the news coverage. The dissatisfaction with the government response to the disaster boiled over into full-fledged civil war the next year, which ultimately led to the overthrow of the government and the establishment of the new nation of Bangladesh. As bad as the West Pakistani response to the Great Bhola Cyclone of 1970 was, the response of the Myanmar government to Nargis is far worse. The slowness of response to this tropical cyclone disaster is unprecedented in modern times.

Nargis is one of the 20 deadliest cyclones in history
Cyclone Nargis' official death toll of 22,500 ranks the storm as the 19th deadliest in world history. Fourteen of the world's twenty deadliest cyclones have been Bay of Bengal storms. Until Nargis, these storms have all affected Bangladesh and India. The bay's shallow bathymetry and low-lying, heavily populated river deltas make the Bay of Bengal the world's most notorious tropical cyclone graveyard. If Nargis's death toll does exceed 100,000, the storm will still only rank as the tenth deadliest tropical cyclone of all time.


Figure 1. This pair of images from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite use a combination of visible and infrared light to make flood waters obvious. Water is blue or nearly black, vegetation is bright green, bare ground is tan, and clouds are white or light blue. On April 15 (top), rivers and lakes are sharply defined against a backdrop of vegetation and fallow agricultural land. The Irrawaddy River flows south through the left-hand side of the image, splitting into numerous distributaries known as the Mouths of the Irrawaddy. The wetlands near the shore are a deep blue green. Cyclone Nargis came ashore across the Mouths of the Irrawaddy and followed the coastline northeast. The entire coastal plain is flooded in the May 5 image (bottom). The fallow agricultural areas appear to have been especially hard hit. For example, Rangoon (Yangon), the capital city with population over 5 million, is almost completely surrounded by floods. Several large cities (population 100,000-500,000) are in the affected area. Muddy runoff colors the Gulf of Martaban turquoise. Image credit: NASA Earth Observatory.

Nargis' storm surge
Nargis took the worst possible path, tracking right along the low-lying, heavily populated Irrawaddy River delta. Moreover, the storm hit at high tide, greatly increasing the impact of the storm surge. Tidal range in the Irrawaddy River delta is about five feet between low tide and high tide, and the death toll would have been much, much lower had the storm hit at low tide. Further amplifying the storm surge's height was the fact that Nargis was moving rather slowly--about 11 mph. Slow moving tropical cyclones can drive a much higher storm surge into narrow estuaries that connect to the ocean, since there is more time for the surge to penetrate inland. Nargis' track, forward speed, and high tide timing created a "perfect storm" able to cause an unprecedented storm surge in the Irrawaddy River delta. The only saving grace was the relatively small size of the cyclone.

I talked to Stephen Baig, the National Hurricane Center's storm surge expert, about Nargis. He confirmed that the surge from Nargis was likely about 12 feet, and that had Myanmar asked, NHC would have happily made them custom storm surge forecasts for the storm. He offered that NHC would do the same for any country in need of storm surge forecasts for an approaching storm.

Human factors helped make the storm surge worse. About 80% of the mangrove forest along Myanmar's coast has been destroyed, to make room for rice paddies and shrimp farms. Mangroves--tall, gnarly, salt-tolerant trees--act to blunt and slow down the progress of the storm surge and reduce the wave action of the ocean. Had more mangroves been left to survive, the impact of the storm surge would have been lessened. How much so, no one can say, for there are few observations of the storm surge to verify models of this. Keep in mind that the mangroves are far more effective in protecting against a sudden, powerful wave like the 2004 tsunami, than the slower, hours-long inundation of a storm surge.

Comments from Chris Burt
I've been in regular communication about this disaster with Chris Burt, author of the excellent book Extreme Weather. He has been visiting Myanmar every year for 30 years, and has much insight on the situation there:

Yet another official Chinese news agency has gone on record about the disaster, going so far as to quote the U.S Chief of Mission (the top ranking US diplomat in Myanmar since we have had no ambassador in the country for 30 years now):

http://news.xinhuanet.com/english/2008-05/08/content_8127548.htm

Again, I can tell you that this is VERY significant. China is Myanmar's only real friend in the world and when THEY begin to publish critical reports, I really am now starting to think this dreadful regime in Myanmar has met its match.

Here is a letter I received from a friend in the capital city, Yangon today:

Dear Chris,

Help received within the country is insufficient and inefficient. We need so much of outside professional, efficient help. NOT in three days. SHOULD be in three hours. Should have been in the last three days. Why are they waiting for three more days? We cannot risk any more life. The dead toll is shockingly too high and it is increasing minute by minute. Please keep me posted with international support. I will also keep you posted.


Great Plains storm chasing
Wunderblogger Mike Theiss is in Tornado Alley this week, performing his annual chase efforts. Be sure to catch his spectacular photos of the amazing storms that form over our Great Plains.

Jeff Masters

Updated: 13:24 GMT le 23 mai 2008

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Cyclone Nargis death toll in the tens of thousands

By: JeffMasters, 12:42 GMT le 06 mai 2008

A disaster of staggering magnitude continues to unfold in Myanmar, where the death toll from Tropical Cyclone Nargis exceeds 22,000. In one city alone--Bogalay, about 50 miles southwest of the capital of Yangon--10,000 people are thought to have died. Bogalay is a decrepit city of 100,000 that lies at the head of a estuary that leads to the sea. No doubt this narrow waterway served to funnel a storm surge over ten feet high into the city. News reports have not yet been received from the coast southeast of Yangon, which also received a significant storm surge, and the toll from Nargis is certain to go much higher.

Nargis hit the coast of Myanmar Friday night as powerful Category 3 cyclone with winds of 130 mph. The cyclone took the worst possible track, passing directly over the densely populated and low lying Irrawaddy River delta. A deadly storm surge--probably around 12 feet high--inundated the delta region, accounting for most of the deaths. The storm's fierce winds killed many more. The only fortunate thing about Nargis was its small size. Hurricane-force winds covered an area about 90 miles in diameter (Figure 1). In contrast, the wind field of Katrina at landfall spanned an area about 205 miles in diameter. Winds from both storms at landfall were about the same (strong Category 3), but Katrina's winds covered an area four times larger than Nargis.



Figure 1. The wind field (black contours, in knots) of Nargis shortly after landfall, when it was a Category 3 storm with top winds of 105 knots (120 mph). Hurricane force winds (red wind barbs) spanned an area about 90 miles in diameter. Image credit: CSU/CIRA/RAMMB.

Loss of the rice harvest
Nargis' arrival came at the worst time possible, during the winter bora rice crop harvest. This crop, planted in January, and very heavily focused in the Irrawaddy River delta, was significantly affected by Nargis. Rice prices have nearly tripled in the past year, and now Myanmar must wait until the summer rice crop is harvested in September and October before adequate supplies of rice will be at hand. The impact will spread beyond Myanmar, since they export rice to Banladesh and Sri Lanka, according to Reuters.

Comments from Chris Burt
I've been in regular communication about this disaster with Chris Burt, author of the excellent book Extreme Weather. He has been visiting Myanmar every year for 30 years, and has much insight on the situation there:

Note this: No word yet about casualties from the Mon or Karen States; those areas not in the Delta region but to the SE of Rangoon where a major storm surge and flooding from rains may have occurred.

The government considers these areas 'minority states' and these states have traditionally been looked down upon by ethnic Burmese, They are heavily populated. I will bet they will be the last areas to receive aid, and the last regions from which we hear news so far as storm damage is concerned.

I might add this is indicative of just how bad the situation in Burma is. People who are not aware of the isolation of Myanmar, one of the largest most populated countries in the world, will not be able to grasp the gravity of this disaster. It is a country under the thumb of complete ignorants: the leaders of this county have NEVER traveled outside of Burma before. They have no education whatsoever. They live in a dream world of astrology and have maintained their control by funneling all the nation's vast resources to crony patronage and the military. China is their only steadfast friend in the international arena. Even Thailand, Malaysia, Singapore, and India who do business with this regime do so at hands length.

If you can understand what I am saying here you will understand just how bad the situation is.

Jeff Masters

Updated: 14:58 GMT le 06 mai 2008

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Thousands dead in Myanmar tropical cyclone

By: JeffMasters, 14:33 GMT le 05 mai 2008

A disaster of horrific proportions has befallen Myanmar, where the death toll is now over 4,000, with thousands more missing, in the wake of Cyclone Nargis. Nargis--a popular woman's name in India--is the deadliest and most destructive tropical cyclone ever to hit Myanmar (Burma). The storm hit the coast of Myanmar Friday night as borderline Category 3/Category 4 cyclone, with winds of 130-135 mph. After passing over the low-lying and densely populated Irrawaddy River delta region, Nargis made a direct hit on the capital city of Rangoon (Yangon), as a Category 1 storm with top winds of 80 mph. Winds at the Yangon airport hit 69 mph, gusting to 138 mph, at 5:30am local time on Saturday. The anemometer failed at that point, and the winds likely rose higher.


Figure 1. Population density of Myanmar, with Nargis' track superimposed. Nargis passed over some of the most densely populated regions of the country. Image credit: Columbia University's CIESEN.

However, it was the storm surge, not the winds, that was the big killer in Nargis. The storm tracked over the low-lying Irrawaddy River delta region, which is highly vulnerable to storm surge deaths due to its low elevation, dense population, and limited hurricane awareness of the people. I could find no records of a major tropical cyclone ever making a direct hit on the Irrawaddy River delta. The ocean bottom off the coast of Myanmar is quite shallow (Figure 2). A large area of Continental Shelf waters with depth 200 meters or less extends far out to sea. This is a situation similar to the Gulf of Mexico, and is ideal for allowing large surge surge to pile up over the shallow waters. The counter-clockwise circulation of winds around Nargis likely built up a storm surge of at least 4 meters (13 feet), that then smashed ashore into the Irrawaddy Delta region, drowning thousands of people.


Figure 2. Bathymetry of the Bay of Bengal. The shallow waters of the Continental Shelf (mostly shallower than 200 meters) are shaded whitish-grey. The shallow waters south of Rangoon allowed the counter-clockwise circulation or winds around Nargis to pile up a large storm surge to the right of the storm's track. Image credit:geomapapp.org.

Storm surges of four meters have been recorded along the Myanmar coast in at least one other cyclone. The Gwa cyclone of May 4, 1982--a Category 4 storm with 140 mph winds--hit just north of where Nargis struck, and carried a four meter high storm surge to the coast. Fortunately, that storm hit far enough north that it was not able to pile up a huge storm surge along the southern shore of the Irrawaddy Delta. The official death toll from the Gwa cyclone was only five people, but was probably very much higher. The military junta that has controlled Myanmar since 1962 has been known to conceal the number of people killed in natural disasters. The highest official death toll from a tropical cyclone in Myanmar is 187, during the Category 1 storm that hit on May 7, 1975. According to a email I received form Chris Burt, author of the excellent book Extreme Weather:


Statistics concerning disasters in Burma since the mid-1960s must be viewed with some skepticism since the authorities always want to pretend they have control of all situations whether natural or societal, and outsiders are never allowed access to devastated sites--I am sure this will be the case again with Cyclone Nargis. We may never know the true magnitude of what happened. I was in Rangoon just 7 weeks ago (I've been visiting Burma for 30 years on a regular basis--every year since) and can tell you that if winds as strong as reported occurred, the damage must be enormous (the vast majority of structures in the city are poorly built and even the newer construction was not constructed with CAT 3 winds in mind.

During the tsunami event of Dec. 26, 2005 the 'official' death toll was something like 69, but in reality many hundreds were killed (estimates 300-800) in the Mergui Archipelago just north of the Thai border. In fact, not included in the official death toll (in Burma or Thailand) were hundreds and perhaps 1,000 illegal Burmese immigrants working in the hotel industry at Kao Lak in Thailand (just north of Phuket where the worst damage occurred in Thailand).



Figure 3. Simulated storm surge of the May 4, 1982 cyclone that hit Gwa, Myanmar. The Gwa cyclone was a Category 4 storm with 140 mph winds shortly before landfall. Storm surge values of four meters (13 feet) were measured and simulated just to the right of where the the cyclone hit the coast. Note that the counter-clockwise circulation around the cyclone also drove a high storm surge into the bay just east of Rangoon. The reported death toll was only five, but was probably much higher. Image credit: "Simulation of Storm Surges Along Myanmar Coast Using a Location Specific Numerical Model" (Jain et al., Natural Hazards 39, 1, September 2006).

Jeff Masters

Updated: 14:36 GMT le 05 mai 2008

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Hundreds dead from Burma's worst tropical cyclone on record

By: JeffMasters, 15:57 GMT le 04 mai 2008

Cyclone Nargis, the deadliest and most destructive tropical cyclone ever to hit Burma (Myanmar), is finally dissipating today over Thailand. Nargis--a popular woman's name in India--slammed into the coast of Myanmar Friday night as borderline Category 3/Category 4 cyclone, with winds of 130-135 mph. After passing over the low-lying and densely populated Irrawaddy Delta region, Nargis made a direct hit on the capital city of Rangoon (Yangon), as a Category 1 storm with top winds of 80 mph. Winds at the Yangon airport hit 69 mph, gusting to 138 mph, at 5:30am local time on Saturday. The anemometer failed at that point, and the winds likely rose higher.

The death toll from Nargis is already at least 351, and is certain to rise as reports from hard-hit areas still cut off from communications begin to arrive. In particular, the low-lying Irrawaddy Delta region where Nargis initially made landfall is densely populated, and a storm surge in excess of 10 feet likely occurred there. It is unusual for a Bay of Bengal cyclone to pass so far south, and Nargis is the strongest tropical cyclone on record to hit the capital city of Rangoon. The previous highest death toll from a tropical cyclone in Myanmar was 187, during the Category 1 storm that hit on May 7, 1975. Nargis is the most powerful cyclone to hit Myanmar since Category 3 Cyclone Mala hit on April 28, 2006. Mala hit a less populated area less prone to storm surge, and killed 22 people and damaged 6000 buildings.


Image credit:NASA.

I'll have an analysis of Nargis' storm surge on Monday.

Jeff Masters

Updated: 16:13 GMT le 04 mai 2008

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First tropical wave of the year; Postcards V: Hurricane warning probabilities

By: JeffMasters, 20:44 GMT le 02 mai 2008

The first respectable tropical wave of the year rolled off the coast of Africa into the tropical Atlantic today. While sea surface temperatures are too cold this time of year to support development of this tropical wave, today's event does serve as a reminder that hurricane season begins in just one month. Hurricane season in the Eastern Pacific begins May 15, and tropical waves such as this one sometimes serve as the nucleus for May tropical storms, once they cross Central America and enter the Eastern Pacific.


Figure 1. Visible satellite image from 12pm EDT May 2, 2008, showing the year's first African tropical wave. Image credit: Navy research lab, Monterey.

Hurricane conference news
The 28th Conference on Hurricanes and Tropical Meteorology in Orlando is over. A final postcard from the conference is below, and next week, I'll write some long letters about some of the presentations, focusing the hurricanes/global warming debate.

Hurricane warning probabilities
Hurricane specialist Michelle Mainelli of the National Hurricane Center evaluated the usefulness of NHC's new hurricane wind probabilities product. The product, which was developed via funding from the immensely valuable Joint Hurricane Testbed research project, provides users with information regarding the chances of experiencing winds of tropical storm force and hurricane force at specific locations within the five-day forecast period. They also indicate, in probabilistic terms, the range of possibilities regarding when these wind conditions could begin at specific locations (an important factor in the timing of evacuation orders). While these products are primarily intended for use by emergency managers (for decision-making) and the media (for communication of risk and uncertainty), the availability of the products via the NHC website makes it possible for anyone in the general public to use them for their own decision-making.



One of the significant challenges with the probabilities thus far has been relating them to coastal watches and warnings issued by the NHC. A hurricane watch is issued when hurricane conditions are possible within the watch area, generally within 36 hours. A hurricane warning is issued when hurricane conditions are expected within the warning area, generally within 24 hours (or less in some cases). Between 2000-2006, an area of coast under a hurricane watch actually received hurricane-force winds about 20% of the time, while areas under a hurricane warning got hurricane force winds 25% of the time. Mainelli's research shows that at the first issuance of a watch or warning, the new wind probability product typically gives a 10% chance of hurricane force winds impacting the end points of the watch/warning area. These probabilities increase to 32% at the center of the warning area. Her research will be used to help develop an automatic technique that will recommend to NHC forecasters exactly where to place their watches and warnings.

What's your forecast?
For those of you who have a hunch about how many tropical storms and hurricanes we'll see this year, you can log your forecast as part of our first ever "Wundercast" competition. It's a friendly competition between weather enthusiasts to determine the best of the best in weather forecasting. Don't worry if you do not have any experience in forecasting, learn from Weather Underground meteorologist--as well as experienced website community members--to become an expert forecaster. It's free, and you can sign up at http://www.wunderground.com/wundercast/. The first forecast day is Tuesday, May 13, and competitors will be forecasting for San Francisco, California for the first two weeks. Thereafter, you'll be asked to forecast for Omaha, Seattle, Philadelphia, and Miami.

Jeff Masters

Updated: 20:49 GMT le 02 mai 2008

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Postcards IV: Katrina's storm surge, and the Rita evacuation

By: JeffMasters, 21:03 GMT le 01 mai 2008

I'm in Orlando this week for the 28th Conference on Hurricanes and Tropical Meteorology, sponsored by the American Meteorological Society. The conference, held once every two years, brings together the world's experts on hurricane science. Two postcards from today's talks:

Hurricane Rita evacuation survey
The evacuation effort for Hurricane Rita as it approached the Texas/Louisiana coast was one of the largest evacuation efforts of all time. Rebecca Mouss of the National Center for Atmospheric Research reported on the results of a survey of 120 residents of Houston, Galveston, and Port Arthur, Texas that evacuated from Rita. Since Rita came so soon after Hurricane Katrina's record devastation, 55% of the people surveyed reported that Katrina influenced their decision to evacuate, and 10% said it was their primary reason for evacuating. For future storms, a majority of the people said they would evacuate if evacuation orders were given for a Category 3 hurricane, but not for a weaker storm. Five percent said they would not evacuate ever, even for a Category 5 hurricane.


Figure 1. Hurricane Rita bears down on the Texas/Louisiana coast.

Katrina's storm surge
Pat Fitzpatrick of Mississippi State University studied the effect of the levee system along the Mississippi River on Hurricane Katrina's storm surge. Katrina had the highest storm surge on record in the Atlantic--an astonishing 27.8 feet along the Mississippi coast. Fitzpatrick showed that the levees along the Mississippi River acted to dam up the storm surge along the east side of the Mississippi River, increasing the storm surge by 2-3 feet within 15 miles of the levees. Calculations from a storm surge model showed that inundation of Chalmette and the Ninth Ward of New Orleans was accelerated by 1-5 hours, thanks to the presence of the levees, with a higher surge of 3-7 feet. In contrast, the levees had little impact on the timing or height of the surge on the Mississippi coast. It is possible that, without the river levees, hard-hit Chalmette and the 9th Ward may have experienced significantly less flooding.

Fitzpatrick also modeled the effect of the loss of wetlands due to erosion on Katrina's storm surge. The general rule of thumb developed for work in the 1960's credited wetlands with reducing storm surge by one foot for every 2.7 miles the storm surge had to pass over a wetland. The SLOSH storm surge model found that wetlands in Louisiana were actually twice as effective in reducing storm surge--each three miles of wetland the surge passed over reduced Katrina's storm surge by two feet. Note that this effect varied with the depth of the surge--an eight foot high surge was knocked down about 13% by wetlands, while a one foot high surge was reduced 59%.

Jeff Masters

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About

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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