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

By: Dr. Jeff Masters , 1:55 PM GMT on May 21, 2008

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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

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264. 786
7:25 PM GMT on May 21, 2008
ok help now you are going overboard...one man ain't gonna do shizznits
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263. Floodman
7:24 PM GMT on May 21, 2008
257. Patrap 7:24 PM GMT on May 21, 2008
Gee..Im gonna have to redo my 401K then. Call the Lawyer.

CAn I just quit mowing the LAwn too?




Livestock, Patrap! Have your sheep and cattle mow the lawn for you!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
262. pearlandaggie
7:25 PM GMT on May 21, 2008
Patrap, I would advise against that unless you live in an HOA-free area! :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
261. KEEPEROFTHEGATE (Mod)
3:23 PM EDT on May 21, 2008
lol
Member Since: July 15, 2006 Posts: 163 Comments: 52257
260. smmcdavid
2:22 PM CDT on May 21, 2008
I'm all for people doing what they can to help with energy costs and the environment... I do my part. However, the minute the government decides I have to start driving a deathtrap and gives me a curfew we have problems.

Individuals need to be responsible for themselves. I think the government needs to stay out of certain aspects of my private life.
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259. Floodman
7:23 PM GMT on May 21, 2008
248. help4u 7:20 PM GMT on May 21, 2008
This country has to go down.We have destoryed the world.ChanGe is coming in just a few months.



The ignore button finds a use...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
257. Patrap
2:23 PM CDT on May 21, 2008
Gee..Im gonna have to redo my 401K then. Call the Lawyer.

CAn I just quit mowing the LAwn too?
Member Since: July 3, 2005 Posts: 414 Comments: 125572
256. pearlandaggie
7:22 PM GMT on May 21, 2008
251. LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
255. IKE
2:22 PM CDT on May 21, 2008
251. KEEPEROFTHEGATE 2:22 PM CDT on May 21, 2008
i got some help4u
its called the mental hospital


T-MINUS 11 hours, 53 minutes until he's admitted!
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254. pearlandaggie
7:21 PM GMT on May 21, 2008
i wonder what the HOA would say 'bout solar panels and a wind turbine! LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
253. 786
7:17 PM GMT on May 21, 2008
Anything going on in the Atlantic? still panning out to be a slow start? What do you think - end of June/beginning of July for any significant development?

BTW smm I agree with help to a certain degree, clearly people will not do it themselves and choose to make sacrifices, in hot summer months in London for ex. the govn't rations the water and there are laws concerning usuage. It forces people to be conscious about how much they use and learn to appreciate its value. In NYC on the other hand water is completely free and people can use how ever much they want without being concerned about a water bill or reaching a limit..you do the math smm and you'll see how the two stategies differ in resource consumption
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252. IKE
2:21 PM CDT on May 21, 2008
Destroyed the world help4u? Come on now.
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251. KEEPEROFTHEGATE (Mod)
3:18 PM EDT on May 21, 2008
i got some help4u
its called the mental hospital
Member Since: July 15, 2006 Posts: 163 Comments: 52257
250. smmcdavid
2:21 PM CDT on May 21, 2008
You are a strange one help4u, but I guess everyone is entitled to their opinion.
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249. IKE
2:20 PM CDT on May 21, 2008
What a difference 4 months makes, huh?

Yup.
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247. pearlandaggie
7:12 PM GMT on May 21, 2008
236. you know, i'm starting to think the only way to blunt the oncoming hardtimes is to install solar and wind for my house. at least that would give us some energy while helping to hold down prices.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
246. Floodman
7:18 PM GMT on May 21, 2008
231. IKE 6:59 PM GMT on May 21, 2008
$5 a gallon?


Oil prices may fall further, energy agency says
The Associated PressPublished: January 16, 2008



PARIS: Crude oil prices could fall further from this month's record high of $100 per barrel, the International Energy Agency said Wednesday, citing U.S. economic weakness.

Still, rising demand in China, tensions in Nigeria and the Middle East, and falling oil stocks remain "supportive factors" for prices, the Paris-based agency said in its monthly oil market report.

A "rapid retreat" in crude oil prices from the $100 mark earlier this month shows "how volatile markets remain and, in early 2008, there would appear to be some downside potential," the report said.


What a difference 4 months makes, huh?

Member Since: August 2, 2006 Posts: 10 Comments: 9922
245. TexasGulf
7:01 PM GMT on May 21, 2008
The world's largest known remaining petroleum reserves are in the Oil Shale and Oil Sands of North America. Some of the largest dnown natural gas reserves on earth are located just off the coast of Florida.

If you're interested in Nuclear Fission energy power, where do you think the world's largest easily obtainable Uranium deposits are found. YUP, North America. If you want Nuclear Fusion (futuristic, but possible)... does anyone truly think that NASA hasn't already determined and made plans to exploit the richest, most easily mined sectors for Helium-3 on the moon?

Why hasn't the U.S. began to seriously tap into some of these KNOWN sources of energy? We certainly have the technology and Oil Shale has been economically feasible since oil went over $50/barrel. Well, it's not environmentally friendly... and lots of other excuses. (see Third rule - Provide Excuses) Don't tap into your own supply, but set up your industry to be ready any time you want.

In a few decades, the U.S. will be in a strong position of energy independence and being the world's largest food producer. Iraq will be depleted, Saudi Arabia will be a wealthy sand pile, UAE will be a tourist destination (Disney-esque).

If Global Warming is true, then the sea lanes north of Siberia will become open to shipping traffic year round. That brings the rich oil fields, natural gas and resources of the Siberian region to the point of being nicely accessible.

The government has a strategy and it doesn't involve giving away our advantage.
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244. IKE
2:13 PM CDT on May 21, 2008
238. moonlightcowboy 2:13 PM CDT on May 21, 2008
IKE, be careful there man, you're going to run out of furniture!


LOL....this is pitiful....every time I go to town I check to see what the gas prices have done. They've already gone up since yesterday...I'm 99.99% sure they'll go up another 5-10 cents a gallon just in time for the holiday weekend.



12Z ECMWF has backed off on an east-PAC storm and now has it crossing over land heading into the Atlantic side.....maybe the GFS isn't alone...

Link
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243. smmcdavid
2:14 PM CDT on May 21, 2008
help4u, are you on drugs? I'm pretty sure that freedom... you know, what this country is all about, doesn't include the government telling me when and what I can eat or drive.
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242. TEXASYANKEE43
7:12 PM GMT on May 21, 2008
237



Yesterday was 95 here in Liberty and add to it the mushy air, we broke a 2yr record.....
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241. jholmestyle
7:14 PM GMT on May 21, 2008

219. presslord 6:44 PM GMT on May 21, 2008 Hide this comment.
flood...Thanks.....interestingly, many municipal gov'ts in USA are beginning to get on the band wagon....mixed use, pedestrian friendly communities are beginning to get traction...goes by several names and takes various forms...but the idea is that development and redevelopment marginalize auto use...I'm doing a project near Charleston which is driven by the concept...live, work, play, purchase...all within walking distance...

Where in Charleston Press? I live in Johns Island, and thats how it sounds like the current development model of the island is going to be.
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239. Floodman
7:12 PM GMT on May 21, 2008
234.

LOL...good luck with that, TexasGulf...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
238. moonlightcowboy
2:12 PM CDT on May 21, 2008
IKE, be careful there man, you're going to run out of furniture!

You may need to save it for stove wood!
Member Since: July 9, 2006 Posts: 184 Comments: 29594
237. Floodman
7:10 PM GMT on May 21, 2008
235.

I hear you, Pearland...I'm not looking forward to it...the temp at the house on Monday was 99. You can say "dry heat" all you want, but hot is hot...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
236. Floodman
7:00 PM GMT on May 21, 2008
232. pearlandaggie

They're coming, no matter what we do, unfortunately. By the way, TexasGulf, nice use of game strategy...

I know I sound like a tree hugger here, but for the last 30 years the sight of some monster over-sized passenger vehicle just got under my skin...imagine how yanked I am living here, in the land of the stegosaurus-sized SUV?
Member Since: August 2, 2006 Posts: 10 Comments: 9922
235. pearlandaggie
7:00 PM GMT on May 21, 2008
233. we lived in DFW for almost 5 years. 97F at 10PM is INSANE! the first summer there, the temperature hit 117F!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
234. smmcdavid
1:58 PM CDT on May 21, 2008
Hey TexasGulf... I wasn't aware you wanted to take over the world! Good luck with that... lol
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233. Floodman
6:57 PM GMT on May 21, 2008
227. pearlandaggie

Yeah, buddy! What's even better, I moved here from New Orleans, and to New Orleans from St Louis...from the suana to the oven...I think my next move will be Wisconsin, maybe...LOL
Member Since: August 2, 2006 Posts: 10 Comments: 9922
232. pearlandaggie
6:58 PM GMT on May 21, 2008
i hope you're right, texasgulf...if not, hard times are in store for ALL of us!
Member Since: September 14, 2007 Posts: 3 Comments: 3963
231. IKE
1:58 PM CDT on May 21, 2008
$5 a gallon?
Member Since: Posts: Comments:
230. IKE
1:57 PM CDT on May 21, 2008
OIL
4.32
+3.35%
$133.30

Hits hand on computer desk...breaking it.
Member Since: Posts: Comments:
229. TexasGulf
6:45 PM GMT on May 21, 2008
Think of the global economy in terms of basic gaming strategy.

The first rule of limited resources is to use the other person's resources first. Take advantage of their low initial prices (cheap fuel) to build up your own infrastructure.

The second rule is (once you have built up your own infrastructure) to buy and continue to use their resources as long as they are willing to sell. Drive the price of resources up to make it more expensive for later players to develop their own infrastructure.

The third rule is, as resources begin to become very limited and demand is high, develop your own resources but refuse to sell any of your resources until other players either stop building or begin to become desperate. Provide excuses, but don't sell out or listen to pleading.

The last rule is to dominate every player who stopped building their own infrastructure, while bleeding those who became desperate to maintain their way of life.

In the end game, you want to dominate control of the essential food, transportation and energy. You win.

We're on Step-3 of the four step program. :>O
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228. Floodman
6:56 PM GMT on May 21, 2008
Thnx, TexasGulf...I have a few more years than my avatar would suggest; not only that, but it's not as much the years as the mileage!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
227. pearlandaggie
6:55 PM GMT on May 21, 2008
that ol' oven better known as DFW! LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
226. pearlandaggie
6:43 PM GMT on May 21, 2008
see, now you took it too far. "no possibility of a return?" part of the subsidies that make "renewable" energy even remotely affordable are in part derived from taxes on oil companies. short-sided squeezing of the energy supply now limits the amount of money available for development of alternative energy sources. it really boils down to an energy density problem...we have very few technologies out there that will provide the amount of energy needed to replace or even significantly offset the energy generated by fossil fuels.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
225. Patrap
1:54 PM CDT on May 21, 2008


GOES-12 CH-3 WV With Dry Air Shaded Loop Link
Member Since: July 3, 2005 Posts: 414 Comments: 125572
224. Floodman
6:53 PM GMT on May 21, 2008
220. Weather456

I believe it's 6, 456...does anyone know what the average is for this yime of year (African waves, that is)
Member Since: August 2, 2006 Posts: 10 Comments: 9922
223. Floodman
6:49 PM GMT on May 21, 2008
219. presslord 6:44 PM GMT on May 21, 2008
flood...Thanks.....interestingly, many municipal gov'ts in USA are beginning to get on the band wagon....mixed use, pedestrian friendly communities are beginning to get traction...goes by several names and takes various forms...but the idea is that development and redevelopment marginalize auto use...I'm doing a project near Charleston which is driven by the concept...live, work, play, purchase...all within walking distance...



I have an old friend who lives in San Francisco; beautiful city, and one, for the most part, that you can walk to nearly any place you want to go...it's a great concept! I, on the other hand, live the Dallas/Fort Worth area, and driving is a necessity...

**steps back on soapbox**

Sprawl is bad!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
222. 69Viking
1:42 PM CDT on May 21, 2008
Sweet, some weather in the FL Panhandle tonight! We need the rain too, just not as bad as south Florida. We had our drought last year. Great conversations today, mostly weather with a little GW mixed in to satisfy the political itch!
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221. groundswell
6:42 PM GMT on May 21, 2008
Wind capture for energy-starting to take off-but then you are contributing to climate change in some way too, as the normal surface wind pattern will be reduced-the "butterfly effect" on a much larger scale.
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220. Weather456
2:30 PM AST on May 21, 2008
An impressive tropical wave is located just off the coast of West Africa. The wave lies under favorable upper winds and over warm sea surface temperatures. However, future development is not expected to occur because of dry air and unfavorable upper level winds ahead of this feature. This is the 5th or 6th wave of the season. I lost count.

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219. presslord
2:38 PM EDT on May 21, 2008
flood...Thanks.....interestingly, many municipal gov'ts in USA are beginning to get on the band wagon....mixed use, pedestrian friendly communities are beginning to get traction...goes by several names and takes various forms...but the idea is that development and redevelopment marginalize auto use...I'm doing a project near Charleston which is driven by the concept...live, work, play, purchase...all within walking distance...
Member Since: August 13, 2007 Posts: 0 Comments: 10458
218. pearlandaggie
6:40 PM GMT on May 21, 2008
213. there's no question there's climate change....there always has been and there always will be. the rub comes when humans are blamed for it and the "proposed solution" is to tax energy and drive up costs of food, fuel, and essentially everything else when so many are already hurting from high prices and the weak dollar.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
217. TexasGulf
6:24 PM GMT on May 21, 2008
Floodman, you are wise beyond your years.

New construction must follow individual county standards, but it also must comply with U.S. Uniform Building Code, ASCE & ASME & AISC & ACI standards.

After every major (disasterous) hurricane season, ASCE has revised the wind load standards or requirements for coastal areas. Latest revision was ASCE-05 (Dec. '05).

The federal government is responsible for establishing the basic construction standards... the "Thou Shalt Not's" and "Thou Shall's". Counties are responsible for interpreting and enforcing the UBC regulations for their region, but also for providing their own standards. Counties can only add to the UBC regulations, not subtract.

I agree that Miami - Dade County, Florida has the best, most strict building codes in the nation. That doesn't stop people from building on sand bar islands. I still see examples of shoddy construction from previous hurricane damage studies of relatively new buildings.

Retrofitting existing buildings is more effort than we can afford or enforce. Allowing counties to create and enforce their own standards hasn't really worked very well in many cases. Some do better than others.

To prevent massive losses in the future, the government can certainly create and enforce tougher construction standards for coastal counties. I don't think the FED does enough to enforce and promote stronger coastal construction standards.
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216. Floodman
6:40 PM GMT on May 21, 2008
209. pearlandaggie 6:34 PM GMT on May 21, 2008
207. i totally agree. we should put our energy and attention to the future while doing what we have to in the short term to ensure stable energy supplies. vilifying those that produce energy does nothing productive for the future.



Outstanding! We agree on that point...we need to stop spending money on things that have no possibility of a return and concentrate on reality
Member Since: August 2, 2006 Posts: 10 Comments: 9922
215. smmcdavid
1:40 PM CDT on May 21, 2008
I see...
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214. IKE
1:39 PM CDT on May 21, 2008
Statement as of 2:30 PM EDT on May 21, 2008

The NWS Storm Prediction Center has issued a
Severe Thunderstorm Watch for portions of

far southeastern Alabama
northern Florida
southeastern Georgia
coastal waters

Effective this Wednesday afternoon and evening from 230 PM until
1000 PM EDT.

Hail to 2 inches in diameter... thunderstorm wind gusts to 70
mph... and dangerous lightning are possible in these areas.

The Severe Thunderstorm Watch area is approximately along and 55
statute miles north and south of a line from 40 miles southwest
of Crestview Florida to 25 miles east northeast of Brunswick
Georgia. For a complete depiction of the watch see the
associated watch outline update (wous64 kwns wou2).

Remember... a Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings. Severe thunderstorms can and occasionally
do produce tornadoes.

Discussion... convection has recently developed near quasi-stationary
front as temperatures warmed into the upper 80s. MLCAPES around 2000
j/kg and deep layer shear near 40 kt are favorable for severe
storms. Hail should be the main threat... given the amount of
instability and steep mid level lapse rates. However...
unidirectional wly winds increasing with height may result in short
line segments with wind damage.

Aviation... a few severe thunderstorms with hail surface and aloft
to 2 inches. Extreme turbulence and surface wind gusts to 60
knots. A few cumulonimbi with maximum tops to 500. Mean storm
motion vector 29025.
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About JeffMasters

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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