Dr. Jeff Masters' WunderBlog

August hurricane outlook

Posted by: Dr. Jeff Masters, 15:02 GMT le 31 juillet 2009

The Atlantic remains quiet today, with no threat areas to discuss and no models calling for tropical storm formation over the seven days. Not much has changed in the Atlantic since my mid-July Atlantic hurricane outlook posted two weeks ago. However, we are now at the cusp of when hurricane activity begins a steep rise (Figure 1). Early August is typically when wind shear begins a major decline, sea surface temperatures continue to rise, African dust and dry air outbreaks diminish, and the African Monsoon and Intertropical Convergence Zone (ITCZ) become quite active, spawning frequent and powerful tropical waves. These tropical waves serve as the instigators of about 85% of all major hurricanes.

Since the current active hurricane period began in 1995, ten out of fourteen years (71%) have had a named storm form during the first half of August, with an average of 1.4 named storms per year. The last nine years in a row have had a named storm form during the first half of August, but the previous four year stretch (1996 - 1999), did not have any storms form.

Figure 1. The seasonal distribution of Atlantic hurricane activity shows a steep rise at the beginning of August. Image credit: NHC.

Sea Surface Temperatures
Eighty-five percent of all major hurricanes form in the Main Development Region (MDR) of the Atlantic, from the coast of Africa to the coast of Central America, between 10° and 20° latitude. This region also spawns 60% of all weaker hurricanes and tropical storms. Sea Surface Temperature (SST) anomalies in the MDR have slowly but steadily risen during July, and now stand at a respectable 0.5°C (0.9°F) above average (Figure 2). SSTs are well below the record levels observed in 2005 and 2006, when they were up to 2°C above average over large portions of the Main Development Region. Still, there is plenty of heat energy available for strong hurricanes to form this year. The strength of the Azores-Bermuda high has been below average over the past month, driving below average trade winds. Weaker trade winds don't mix up as much cold water from the depths, and cause less evaporative cooling. The latest 2-week run of the GFS model predicts continued slightly below average-strength trade winds through mid-August, so SST anomalies should continue to warm during this period.

Figure 2. Sea Surface Temperature (SST) departure from average for July 30, 2009. SSTs were about 0.5°C (0.9°F) above average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of an El Niño episode. Image credit: NOAA/NESDIS

El Niño
El Niño conditions have remained steady over the tropical Eastern Pacific over the past month. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niño 3.4 region", remain at 0.8°C above average, which is 0.3°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.2°C would push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month, and predicts that El Niño conditions will intensify over the next few months, and last through the coming winter. The latest set of mid-July runs of the El Niño computer models are almost universally calling for El Niño conditions to remain well-established for the peak months of hurricane season, August - October. It is likely that Atlantic hurricane activity will be suppressed in 2009 due to the strong upper-level winds and resulting wind shear an El Niño event usually brings to the tropical Atlantic. The NOAA CFS model is calling for continued above-average wind shear over most of the tropical Atlantic for the August-September-October peak part of hurricane season.

Figure 3. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niño 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 31, 2009, SSTs in the Niño 3.4 region had risen to 0.83°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: Australian Bureau of Meteorology.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in July over the Atlantic hurricane breeding grounds, but in August the jet stream retreats to the north, and wind shear typically falls.

Wind shear over the past month (Figure 4) has mostly been above average over the tropical Atlantic, particularly over the Caribbean. The presence of El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. However, wind shear has been slowly falling over the southern portion of the Caribbean and southern MDR over the past week, and is forecast by the GFS model to fall to near-average levels by mid-August. This should present a more favorable environment for hurricanes to form in by mid-month.

Figure 4. Departure of wind shear from average in m/s for the 1-month period ending July 27, 2009. Higher than average wind shear (blue colors) was observed over the Caribbean. The El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. Image credit: NOAA Climate Prediction Center.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past month. Expect dust from Africa to diminish in the coming month, allowing a greater chance for African tropical waves to develop.

Steering currents
The steering current pattern has remained virtually the same all summer. A persistent trough of low pressure has remained entrenched over the Eastern U.S., bringing cool and relatively moist weather to the northeastern portion of the country. This trough is strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. It is often difficult to break a months-long steering current pattern like the current one, and it's reasonable to forecast that the current steering pattern will continue to dominate into September.

Summary
Recent history suggests a 71% chance of a named storm occurring in the first half of August. However, this is not a typical year. The ITCZ has been remarkably inactive, and there have been an unusually low number of tropical waves coming off the coast of Africa. Although SST anomalies should continue to rise and wind shear should slowly fall over the next few weeks, the computer models suggest no significant changes to the current inactive weather pattern. I'll go with a 30% chance of a named storm forming in the first half of August.

I'll have a new post on Monday.

Jeff Masters

Categories: Hurricane

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1601. Cavin Rawlins 12:27 GMT le 02 août 2009
Quoting cg2916: Not to sound like someone who doesn't know much about weather, but how does that affect the steering of the system? Does the system become attracted to the trough? It follows the path of least resistance, so basically it becomes attracted. Like thowing a bucket of water on the side walk and it flows down the drain (trough). Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1602. Cavin Rawlins 12:29 GMT le 02 août 2009
A little closer to home, this upper low has continued to perist across the NW Atlantic slowly become from something entirely non tropical. Though i don't expect anything from it, even if it develops it will absorbed shortly. But an interesting feature. been in the Atlantic for a weak or so Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1603. IKE 12:34 GMT le 02 août 2009
Last Update: 2-AUG-2009 08:21am EDT Though the majority of the Atlantic Basin is free of activity, there are a few tropical waves we are watching heading into early next week. The eastern-most wave, which just emerged off the African coast continues to look impressive on satellite imagery this morning. This wave is maintaining deep convection as well as a circulation as it moves westward. The waters have warmed over this area the past several weeks and the wind shear is currently light. Though conditions are not ripe for any quick development, some slow organization could occur the next 24-36 hours if thunderstorms can maintain themselves near the center. Other waves are located near 86 west and south of 20 north, 60 west and south of 25 north and 40 west near 25 north. With the exception of the western-most wave, the lack of convection and unfavorable winds will inhibit the chance for development. The wave near 86 west is interacting with Belize and Honduras, which will offset the favorable warm waters for development. The Intertropical Convergence Zone shows some thunderstorms from 55 west to 30 west south of 15 north, although the lack of any low pressures will inhibit any development into early next week. By AccuWeather.com Meteorologist Justin Povick Member Since: 9 juin 2005 Posts: 23 Comments: 37044

1604. Cavin Rawlins 12:36 GMT le 02 août 2009
CrownWx weather next update was suppose to be Monday but they posted this, this morning: Here are my thoughts: Currently this particular tropical disturbance is in a moist environment and wind shear values are running at 5 to 10 knots. At this point, I think slow development is possible this week as environmental conditions are conducive for development. For now it is something to watch over the coming days and I give it a LOW to at very most MODERATE chance of developing into a tropical cyclone between today and Wednesday. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1605. IKE 12:37 GMT le 02 août 2009
If accuweather is talking about the western most blob in this picture, it looks less impressive to me(less convection)...more w/the ITCZ..... Member Since: 9 juin 2005 Posts: 23 Comments: 37044

1606. cg2916 12:46 GMT le 02 août 2009
Quoting IKE: If accuweather is talking about the western most blob in this picture, it looks less impressive to me(less convection)...more w/the ITCZ..... Really, I think the western most one looks the best. Member Since: 21 décembre 2007 Posts: 13 Comments: 2908

1607. IKE 12:50 GMT le 02 août 2009
Quoting cg2916: Really, I think the western most one looks the best. It doesn't look RIP or terrible...just doesn't look as circular to me. Maybe it's some dust on the northern side. Maybe I should put my glasses on... Member Since: 9 juin 2005 Posts: 23 Comments: 37044

1608. Cavin Rawlins 12:53 GMT le 02 août 2009
Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1609. Cavin Rawlins 12:55 GMT le 02 août 2009
Ur right cg, it does look good but the models isnt latching onto it. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1610. ALCoastGambler 13:00 GMT le 02 août 2009
Quoting Weather456: Ur right cg, it does look good but the models isnt latching onto it. Weather, do they typically get much model support before they exit. Or do the models start picking up once they exit Africa?

1611. Cavin Rawlins 13:04 GMT le 02 août 2009
Quoting ALCoastGambler: Weather, do they typically get much model support before they exit. Or do the models start picking up once they exit Africa? normally you can get an idea if a feature will develop as the models tend yo forecast development before they emerge. In cases like this it's best to wait for the feature to emerge cuz its not all the time they will verify. Bertha, Dean, Helene, and Ivan had model support before they emerged. In addition, eastern atlantic waves are delicate systems so when you have model conensus out there, you can often guarantee development. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1612. ALCoastGambler 13:05 GMT le 02 août 2009
Quoting Weather456: normally you can get an idea if a feature will develop as the models tend yo forecast development before they emerge. In cases like this it's best to wait for the feature to emerge cuz its not all the time they will verify. Bertha, Dean, Helene, and Ivan had model support before they emerged. Thanks

1613. stoormfury 13:08 GMT le 02 août 2009
wave is embedded in a very moist environment Link Member Since: 22 août 2006 Posts: 0 Comments: 2302

1614. sporteguy03 13:09 GMT le 02 août 2009
Weather 456, In case you wondering still about Levi: Link Member Since: 7 juillet 2005 Posts: 0 Comments: 4824

1615. Cavin Rawlins 13:12 GMT le 02 août 2009
Quoting sporteguy03: Weather 456, In case you wondering still about Levi: Link I see, thanks. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1616. ALCoastGambler 13:14 GMT le 02 août 2009
Quoting sporteguy03: Weather 456, In case you wondering still about Levi: Link I knew I had been missing someone this season. Thanks

1617. aquak9 13:17 GMT le 02 août 2009
ok, since there's not a gazillion people in here at once, i'm referencing this question to post 1608. There's two major areas of convection, one over land, towards the northeast of a convection-free area, then another to the southwest (over water), of the same convection-free area. Is this all part of the same "blob", with the convection-free area being the central point? Or are these two completely separate creatures? Member Since: 13 août 2005 Posts: 163 Comments: 25005

1618. Orcasystems 13:25 GMT le 02 août 2009
Blog Update Reflector site for those at work, which includes Weather456, daily update. AOI AOI Member Since: 1 octobre 2007 Posts: 77 Comments: 26077

1619. wunderkidcayman 13:27 GMT le 02 août 2009
good morning everyone whats up I see we could have 99L in the makings Member Since: 13 juin 2009 Posts: 2 Comments: 5385

1620. stoormfury 13:29 GMT le 02 août 2009
good morning the tropics are again quiet this morning, but there is this feature in the tropical east atlantic, which could be something to watch down the road. this topical wave is in a better environment than all the others that exited the african coast the last few weeks. it is my thought that this system will develop very slowly and will be able to fight off the dry and stable air in the CATL , because of the moisture shield in which it is engulfed. the system i believe willtrack west for the next two days and will begin to take a WNW track for a further two days ,before coming back on more westerly track west of 45W. the strengh of the system i cannot forecast but i expect it to be the first named storm of the season Member Since: 22 août 2006 Posts: 0 Comments: 2302

1621. Cavin Rawlins 13:35 GMT le 02 août 2009
Quoting aquak9: ok, since there's not a gazillion people in here at once, i'm referencing this question to post 1608. There's two major areas of convection, one over land, towards the northeast of a convection-free area, then another to the southwest (over water), of the same convection-free area. Is this all part of the same "blob", with the convection-free area being the central point? Or are these two completely separate creatures? i was asking my self the same question but it does not seem to be one system at the moment based on vortcity charts and satellite imagery. Tho, the GFS shows the 2 features become one in 36 hrs, with the feature offshore dominating the source of convection. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1622. ALCoastGambler 13:39 GMT le 02 août 2009
Quoting Weather456: i was asking my self the same question but it does not seem to be one system at the moment based on vortcity charts and satellite imagery. Tho, the GFS shows the 2 features become one in 36 hrs, with the feature offshore dominating the source of convection. Wouldn't that mean that the one offshore has more low level dominance. Or am I grabbing at straws

1623. WxLogic 13:41 GMT le 02 août 2009
Quoting aquak9: ok, since there's not a gazillion people in here at once, i'm referencing this question to post 1608. There's two major areas of convection, one over land, towards the northeast of a convection-free area, then another to the southwest (over water), of the same convection-free area. Is this all part of the same "blob", with the convection-free area being the central point? Or are these two completely separate creatures? Well... I believe there're multiple pieces of energy which the GFS has nicely shown as strong areas of convection feedback. I will have to say that one of these will become the central focus for another possible area of development as it combines with the other convective burst/energy pockets... should be interesting to see if anything does manage to spin up out of those. Member Since: 14 août 2008 Posts: 4 Comments: 4666

1624. nrtiwlnvragn 13:41 GMT le 02 août 2009
Quoting aquak9: ok, since there's not a gazillion people in here at once, i'm referencing this question to post 1608. There's two major areas of convection, one over land, towards the northeast of a convection-free area, then another to the southwest (over water), of the same convection-free area. Is this all part of the same "blob", with the convection-free area being the central point? Or are these two completely separate creatures? My opinion based on Hovmöller Diagram (satellite images ~12 hours apart) is one system, you can see the progression of the system across Africa (look top to bottom). Member Since: 23 septembre 2005 Posts: 11 Comments: 8918

1625. AussieStorm 13:42 GMT le 02 août 2009
Quoting Weather456: i was asking my self the same question but it does not seem to be one system at the moment based on vortcity charts and satellite imagery. Tho, the GFS shows the 2 features become one in 36 hrs, with the feature offshore dominating the source of convection. Interesting Member Since: 30 septembre 2007 Posts: 5 Comments: 13272

1626. aquak9 13:43 GMT le 02 août 2009
thanks everyone. I stared and stared, could not figure out if it was one or two areas. If it's one, it's huge, but vort charts do not support it. If it's two, then one will obviously have to become dominant, I think. Aw heck, who am I kidding, COC's can wander around like ants on an anthill. Member Since: 13 août 2005 Posts: 163 Comments: 25005

1627. Cavin Rawlins 13:50 GMT le 02 août 2009
Everything is embedded with a broad area of low pressure with an ill define but closed low se the cape verdes Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1628. Cavin Rawlins 13:51 GMT le 02 août 2009
Quoting ALCoastGambler: Wouldn't that mean that the one offshore has more low level dominance. Or am I grabbing at straws lol yea, I just posted a graphic not before I read your comment. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1629. stormsurge39 13:53 GMT le 02 août 2009
Goodmoning, I noticed a burst of convection, on the NW side of the circulation on land. Is this a sign of anything or is it just a burst?

1630. ALCoastGambler 13:53 GMT le 02 août 2009
Quoting Weather456: lol yea, I just posted a graphic not before I read your comment. Thanks

1631. Cavin Rawlins 13:55 GMT le 02 août 2009
Quoting nrtiwlnvragn: My opinion based on Hovmöller Diagram (satellite images ~12 hours apart) is one system, you can see the progression of the system across Africa (look top to bottom). This shows it too, but I'm not seeing 1 system on quikscat Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1632. aquak9 13:55 GMT le 02 août 2009
closed low already? Lotta black barbs on that graph, 456...but I know you have looked at many more charts than that, so I'll take your word for it. I sure ain't gonna argue with a featured blogger, hehehehe... So IF that area's gotta closed low, then there oughtta be a moist enough environment to at least SUSTAIN convection NE and SW of that area, probably healthier convection stays to the SW. Could be better consolidation in another 48-72 hours. Member Since: 13 août 2005 Posts: 163 Comments: 25005

1633. Cavin Rawlins 14:00 GMT le 02 août 2009
Quoting aquak9: closed low already? Lotta black barbs on that graph, 456...but I know you have looked at many more charts than that, so I'll take your word for it. I sure ain't gonna argue with a featured blogger, hehehehe... So IF that area's gotta closed low, then there oughtta be a moist enough environment to at least SUSTAIN convection NE and SW of that area, probably healthier convection stays to the SW. Could be better consolidation in another 48-72 hours. lol, yea there is one but remember I said it's ill define. I'm gonna post something shortly but in the mean time you can see it on the GFS 00hr or the observed conditions inputted into models Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1634. stormsurge39 14:03 GMT le 02 août 2009
It also looks like on the last frame of the eumetsat in infared, that the circulation on land is starting too show a little more moisture around it.

1635. aquak9 14:08 GMT le 02 août 2009
re-1633 Dang, look at that flat line across the Atlantic. That high is just ALL stretched from one end to the other. (sigh) but what it is today, ain't the same as what it is in four days...that's why we track. Thanks for all the input, ya'll- so much easier to learn when there's not a buncha screamin' kids around. Member Since: 13 août 2005 Posts: 163 Comments: 25005

1636. ALCoastGambler 14:11 GMT le 02 août 2009
Where is the trough everyone was talking about in that graph?

1637. nrtiwlnvragn 14:10 GMT le 02 août 2009
Quoting Weather456: This shows it too, but I'm not seeing 1 system on quikscat I see what you are saying, but I am skeptical of the QuikScat due to directional ambiguities in the rain contaminated area. However, the 06Z surface analysis supports the low: But the 12Z does not: Member Since: 23 septembre 2005 Posts: 11 Comments: 8918

1638. Cavin Rawlins 14:11 GMT le 02 août 2009
Here's it is....I've said the feature southeast the cape verde wasnt a wave but by the looks of the inverted V pattern seen here, the feature is likely a wave, and when you had the ill-define surface circulation and 500 vort max it seems the attention may shift here. In the far right is the the other circulation over Africa which the GFS shows emerging and merging with this feature. We'll see what happens, but seems to a bit clearer than yesterday Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1639. aquak9 14:15 GMT le 02 août 2009
probably healthier convection stays to the SW. YES! I think I got it right!! and I saw the makings of the inverted V also, on RGB. But wasn't sure if I wasn't just trying too hard to see it. Member Since: 13 août 2005 Posts: 163 Comments: 25005

1640. lawntonlookers 14:16 GMT le 02 août 2009
Quoting Weather456: A little closer to home, this upper low has continued to perist across the NW Atlantic slowly become from something entirely non tropical. Though i don't expect anything from it, even if it develops it will absorbed shortly. But an interesting feature. been in the Atlantic for a weak or so 456 I just saw this one and thought it looked like something developing. But you said that it has been around awhile and will be absorbed by the front to the north I assume. Member Since: 22 Mars 2006 Posts: 9 Comments: 1570

1641. stormsurge39 14:18 GMT le 02 août 2009
456, please tell me if im looking at convection starting to wrap a little bit around the low on land ? I saw this on the last animated frame on the Eumetsat on the AVN page?

1642. Cavin Rawlins 14:18 GMT le 02 août 2009
August Outlook Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1643. Cavin Rawlins 14:20 GMT le 02 août 2009
Quoting stormsurge39: 456, please tell me if im looking at convection starting to wrap a little bit around the low on land ? I saw this on the last animated frame on the Eumetsat on the AVN page? There was an area of convection that develop to the NW but that has since died. It does not appear the feature wants to develop convection since its been like that for 24 hrs. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1645. stormsurge39 14:23 GMT le 02 août 2009
oh, I must be looking at an old frame. Thanks

1646. AussieStorm 14:25 GMT le 02 août 2009
Quoting Weather456: Here's it is....I've said the feature southeast the cape verde wasnt a wave but by the looks of the inverted V pattern seen here, the feature is likely a wave, and when you had the ill-define surface circulation and 500 vort max it seems the attention may shift here. In the far right is the the other circulation over Africa which the GFS shows emerging and merging with this feature. We'll see what happens, but seems to a bit clearer than yesterday The blob over the ATL has got a spin to it. Click here to view the spin Link Member Since: 30 septembre 2007 Posts: 5 Comments: 13272

1647. mikatnight 14:28 GMT le 02 août 2009
Link Member Since: 18 octobre 2005 Posts: 0 Comments: 1946

1648. stormsurge39 14:30 GMT le 02 août 2009
456 do the bursts over land help developement?

1649. Cavin Rawlins 14:33 GMT le 02 août 2009
Quoting stormsurge39: 456 do the bursts over land help developement? building and dying convction moistens the atmosphere which could help the feature as it emerges but currently i'm not seeing any signs of it building convection. Member Since: 24 juillet 2005 Posts: 407 Comments: 19076

1650. HIEXPRESS 14:33 GMT le 02 août 2009
Everybody mentions how vertical wind shear affects TC development, but we don't hear much about horizontal wind shear. Most here are familiar with how SST anomalies change the wind patterns over the Pacific ocean, then teleconnect to the Atlantic basin, but I was wondering about local effects of SST anomalies in the Atlantic. Since they are "right there", wouldn't they have an influence at least as strong on the weak systems trying to develop overhead, either supporting or inhibiting those systems? Is there an ENSO like effect locally over the Atlantic basin? To restate, If a developing weak cyclonic circulation moved into an area where the easterly winds to the south are faster (or to the North are slower than usual), would development be hindered? Can SST anomalies cause this? (keep it simple, nothing like this). I had asked the same question on an earlier blog, but no one commented. I figured one of these * It was a stupid question (there are no stupid questions, only stupid people) No one here knows the answer or has an opinion It was a good question & experts are saving the answer for next year's hurricane conference It was cut off by a "new blog" I am on so many "Ignore" lists, that no one saw it. (LOL?) Member Since: 13 octobre 2005 Posts: 4 Comments: 2153

1651. Walshy 14:36 GMT le 02 août 2009
Still watching Lana hold its convection. This frame put into motion shows some convection on the southern side trying to rotate around what appears to be the center of circulation. Is there anyone here to confirm this? Thanks, in advance. Link Member Since: 17 mai 2009 Posts: 2 Comments: 618

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