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

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

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.

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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114. tornadofan
4:50 PM GMT on May 21, 2008
OSUWXGUY - thanks for your comments regarding tornadoes. It makes sense.
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113. pottery
12:46 PM AST on May 21, 2008
You know, now may be a good time, to go out and buy 50 X 45 gal drums of engine oil as an investment. The price of refined oils has not jumped accordingly with crude.
Could possibly make 50 % in 6 months or so ?
Any economists on board ?
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112. moonlightcowboy
11:46 AM CDT on May 21, 2008

Gas up. Food prices are going up! So, will everything else! Meanwhile, the dollar's value is plummeting. Recession, hhhhmmm! I think we're gonna start hearing the "d" word before too long!

Global warming? LOL! Much more serious things happening to the US economy...and long before the year 2100. AGW, LMAO - folks just don't get it! Can we get some money spent on a model for the economy? Seems to me, that'd be much more prudent!
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111. cchsweatherman
12:42 PM EDT on May 21, 2008

This is a very interesting scenario painted for June 1 by the GFS. It looks like it may be settling out the conflict between the powerful ridge in the Atlantic and a strong trough across the northern United States and has the trough winning, pulling the tropical system into South Florida. Looks like the model has now unconfused itself 456. But, I must remain skeptical with this still remaining outside medium-range.
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110. IKE
11:47 AM CDT on May 21, 2008
GFS 12Z on board with a SW Caribbean system...running through 252 hours....

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109. LafitteTman
4:45 PM GMT on May 21, 2008
Let's hope that feature in the BOC will bring some much needed rain to the Florida peninsula.
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12:43 PM EDT on May 21, 2008
i am telling ya it will be a 150 by july ike
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107. pottery
12:40 PM AST on May 21, 2008
Lakeshadow. I am with you there.
The only people that would find anything to complain about, would be the people who were in fact NOT stoned.
So ? Make it mandatory....
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4:42 PM GMT on May 21, 2008
Starting at the beginning of the comments - Tornadofan (4) and Mississippiwx23 (8)

There have been several studies recently about climate change and severe thunderstorms/tornadoes. As an avid storm chaser, I have tried to keep up on this research.

The gist of the results are that wind shear will decrease from reduced meridional temperature gradients, but enhanced moisture content in the atmosphere will lead to higher instability - CAPE values - which will more than counteract the decreased shear.

Many areas of the US - like Ohio where I live - have PLENTY of shear with spring-time storms but have a difficult time of getting enough surface based instability for tornadoes to form. This observation makes me feel more comfortable with the results of these studies.

Here is good paper to read:

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105. TexasGulf
4:34 PM GMT on May 21, 2008
That was a good point. We don't honestly KNOW that the Walker Circulation has decreased by 3.5% (or any other estimate) over the past century. We can speculate and model, but there is no verifiable past data.

Also, just because Walker Circulation decreases in one region doesn't mean that it won't increase in another area. The net result may be increased shear in some regions, decreased shear in others and a SHIFT in hurricane patterns.

There may not be fewer or weaker storms, just hurricanes in different areas or following 'untraditional' paths. We call this the "Masters Conundrum" after the person who first started this line of blogging.
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104. LakeShadow
4:27 PM GMT on May 21, 2008
I hate this every solution seems to end badly somehow. How can this be? Maybe the problem is people!

Hemp. Its renewable and cost efficient. It can be made into paper, cloth, plastic, rubber and combustible engine fuel to name a few uses. It can solve our energy problems and save our forests. The problem is the PEOPLE are afraid to allow the growth and development of the crops because their afraid everyone will get stoned ... forget about preserving our natural resources and helping the economy as well as the environment, dont wanna contact buzz!
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103. IKE
11:33 AM CDT on May 21, 2008
Here's something else uncalled for..the price of a barrel of crude oil...up $2.78 to $131.76.
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102. Patrap
11:31 AM CDT on May 21, 2008
Cure for those who cant-stop-thinking.

Zoloft and

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101. Buhdog
3:40 PM GMT on May 21, 2008
If it were any later than the middle of june.... I would not worry about rooting for a system as rainy season would soon be upon us climatologically. One thing though....in SW Florida where we get some of the most active weather in the country during the summer (sea breeze interactions) ..the last 2or 3 years the collisions have been scattered at best. IMO the reason is that the high has not been anchored in its normal spot. Troughiness and a weak high have really limited our rainy seasons lately! So I guess I am actively campaigning for a semi hit.
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100. smmcdavid
11:27 AM CDT on May 21, 2008
Wow, that was uncalled for.
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99. pottery
12:18 PM AST on May 21, 2008
Interesting blog, Dr. M. Food for thought there.

We should all note, however, that this is a theory based on current known parameters. Its the interpretation of a series of model runs, extrapolated for 90 years or so.

When are we going to get an accurate model interpretation for next month ? If you get my point.
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95. LakeShadow
4:21 PM GMT on May 21, 2008
The question is, which would you rather have: 4 CAT2s, or 2 CAT5s?

my guess is 4 CAT 2's???

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4:20 PM GMT on May 21, 2008
Good Afternoon Everyone! Long time lurker, only occasional poster.

I have a bunch to say on JM's blog plus many of the comments...

I'm a Master's student at Ohio State studying climate change and tropical cylones. I was at the AMS hurricane conference in Orlando and got the chance to speak with Knutson, Vecchi and Soden. My research is very similar to their work and I hope I can add a useful 2 cents to this conversation.
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93. cchsweatherman
12:22 PM EDT on May 21, 2008

By the way, this is not a blob, its a feature.
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91. Patrap
11:22 AM CDT on May 21, 2008
Always prepare for Catastrophe if planning to stay in a Warned area.
Opal..Charley and many other Hurricanes can and will go Beyond a Forecast Cat strength.

Never assume anything when a small cored system is traversing North-bound near the Loop eddy.

Charley..left lessons to be learned and remembered,Every one that is a Major usually does.

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90. smmcdavid
11:21 AM CDT on May 21, 2008
Does anyone know where I can find comparison pics of hurricane damage (ie: cat 1 vs cat 4, etc)?
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89. Floodman
4:20 PM GMT on May 21, 2008
Or for that matter, one storm in excess of CAT5 (I'm loath to say the other...call on the devil and he shall appear, as it were).
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88. captainhunter
4:20 PM GMT on May 21, 2008
81. cchsweatherman 4:09 PM GMT on May 21, 2008
Just taking another look at the Bay of Campeche feature and it looks strikingly similar to Tropical Storm Barry in its developing stages from last season. Almost the same exact structure and appearance.

And Barry was able to form under some pretty significant shear...
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87. Patrap
11:20 AM CDT on May 21, 2008
There is no Cat-6 ..and the SSS is not the best measure of a Hurricane,Size matters if ya pardon the Pun.
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86. StormStalker85
4:17 PM GMT on May 21, 2008
71. Patrap

Punta Gorda was indeed in the warned area; however, we were expecting a Cat1 to pass by us and head for Tampa. At no time did any of the meteorologists mention that it had any potential to go beyond that. By the time that it had increased to a Cat4 intensity, it was too late to prepare for something that catastrophic. After Charley hit, the local meteorologists quit using the "skinny black line" in the middle of the cone. My family was the only house on the block to even put up the shutters. No wonder this area was wiped out.
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85. Floodman
4:10 PM GMT on May 21, 2008
80. TexasGulf

All true...a reduced frequency in storms, but an increase in intensity. Under those conditions it stands to reason that for a storm to form the seed circulation would have to be strong; stronger systems make thier own weather in way (a major over simplification); the stronger the storm the less chance of dissipation...yes?

The question is, which would you rather have: 4 CAT2s, or 2 CAT5s?
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84. AWeatherLover
4:15 PM GMT on May 21, 2008
Can anyone post a sat pic of the BOC and GOM blobs? Are these the same feature? I'm a little confused by the discussion... Thanks!
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82. Floodman
4:06 PM GMT on May 21, 2008
Morning, everyone...yep, the models are all very primitive right now...the only way to sharpen them is with more data, and that takes time. Like Maxwell Smart used to say: Expect the unexpected...

smmcdavid, I agree...I rode out Jeanne in '04 and while I was rather excited to do it, I would NEVER sit in front of a CAT4...a CAT3 was plenty for me. I don't want to think about a CAT5, let alone a stronger storm
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81. cchsweatherman
12:05 PM EDT on May 21, 2008
Just taking another look at the Bay of Campeche feature and it looks strikingly similar to Tropical Storm Barry in its developing stages from last season. Almost the same exact structure and appearance.
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80. TexasGulf
3:10 PM GMT on May 21, 2008
If higher SST has caused the Walker Circulation to slow down by 3.5% over the past century, then ocean evaporation rates will have also reduced by at least that amount.

Ocean evaporation rates are a function of:
- Relative humidity
- Wind speed
- Diff sat vs. actual water vapor pressure (which is a function of temperature)

In the tropics, evaporation rates can be up to 80 inches/year due to trade winds. At the equator, evaporation rates tend to be about 50 inches/year due to reduced average surface wind speed.

Wind speed plays a double role, because it also affects the surface roughness. Larger waves and increased roughness causes more ocean surface area in contact with the air, increasing evaporation. The reverse is also true. Decreased wind speed reduces evaporation rates.

A slowing of the Walker Circulation by 3.5% would slow sea surface evaporation rate, causing higher ocean surface temperatures and a deeper ocean thermocline. Meanwhile, the reduced moisture in the atmosphere would lead to less cloud formation, resulting in more solar heat being absorbed by the oceans.

Forgetting the effects of higher wind shear (per Dr. Masters), wouldn't the reduced evaporation rates reduce the chances of tropical formation?

Once one forms, the higher SST may cause a hurricane to grow quickly, but might reduced evaporation cause fewer to form?
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79. smmcdavid
11:04 AM CDT on May 21, 2008
Just thinking about a storm strong enough to be a cat 6 makes me sick to my stomach.
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78. DocBen
4:02 PM GMT on May 21, 2008
Agreed Flood. I'm sure FL, GA, and AL would welcome several Cat 1's about now. Doubt they would like a Cat 6 though.
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77. Michfan
11:00 AM CDT on May 21, 2008
Yay a new blog finally. The other one was making my head spin. We could use some rain here in lower Alabama as well. My lawn is starting to turn brown.
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76. StormJunkie
4:00 PM GMT on May 21, 2008
Morning flood

To add to that, the Dr also mentions that these theories need more testing and that there are at least some that feel the opposite will occur.
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75. Floodman
3:53 PM GMT on May 21, 2008
69. StormHype 3:45 PM GMT on May 21, 2008
So global warming will have a negative impact on hurricane formation. Wait until Al Gore hears this one. It will ruin his day!

Read the whole thing: decrease in frequency, but an increase in SSTs...those that get through will likely be more intense...people talk about climate change as though the changes will be a static line, increasing incrementally from point A to point B. Nothing could be further from the truth: change will come, in many ways, in fits and starts. This process will be an average, not a straight line progression

Okay, I'm off the soapbox...for now
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74. JRRP
3:54 PM GMT on Mayo 21, 2008

nothing !!!
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73. TheCaneWhisperer
3:47 PM GMT on May 21, 2008
I see the silence is lifted on the BOC feature Patrap, lol. It's been an interesting little feature for the past couple days.
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72. Patrap
10:47 AM CDT on May 21, 2008
Like I always say,..one can root,..scream,Wish and west,WNW-cast all day and night.

They dont have ears or read da blogs.

But some Like to visit Coastal Cities on

National Hurricane Preparedness Week 2008 Link

History teaches that a lack of hurricane awareness and preparation are common threads among all major hurricane disasters. By knowing your vulnerability and what actions you should take, you can reduce the effects of a hurricane disaster. Hurricane Preparedness Week during 2008 will be held May 25th through May 31st.

The goal of this Hurricane Preparedness Web site is to inform the public about the hurricane hazards and provide knowledge which can be used to take ACTION. This information can be used to save lives at work, home, while on the road, or on the water.
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71. Patrap
10:45 AM CDT on May 21, 2008
Yeah..remember Charley and be Prepared for a right turn.
Always go with the Observed trends.

Punta Gorda was in a Hurricane Warned area and as Charley showed,.be Prepared cuz the Storm dont watch TV and didnt get the Word Tampa was His PING Target.

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70. cchsweatherman
11:45 AM EDT on May 21, 2008
65. captainhunter 11:39 AM EDT on May 21, 2008
Thanks for the analysis CCHSW.

Anytime man. That is what I'm here for; to help both myself and others learn about weather.

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69. StormHype
3:41 PM GMT on May 21, 2008
So global warming will have a negative impact on hurricane formation. Wait until Al Gore hears this one. It will ruin his day!
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68. StormJunkie
3:44 PM GMT on May 21, 2008
How much PH increaser would we need for the gulf

lmao! Good one Buhdog!
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67. cchsweatherman
11:42 AM EDT on May 21, 2008
66. KEEPEROFTHEGATE 11:41 AM EDT on May 21, 2008
dont rule any thing out cchs things can and will change
and always expect the unexpected

After last season, you should know that I'm always on the alert for the unexpected. The response I gave was just based on the current situation. I know things change as we have seen time and time again over the past several hurricane seasons. Just remember Hurricane Charley.
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11:36 AM EDT on May 21, 2008
dont rule any thing out cchs things can and will change
and always expect the unexpected
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65. captainhunter
3:37 PM GMT on May 21, 2008
Thanks for the analysis CCHSW.
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64. cchsweatherman
11:35 AM EDT on May 21, 2008
But, this Bay of Campeche feature could have a great impact on Florida as it appears the entire feature is heading towards Florida. If this does happen, it will interact with the sea breeze and the approaching frontal boundary tomorrow and Friday to create some much-needed widespread rainfall across the state. FYI Buhdog, in the situation we are currently in, I find no fault in rooting for a small system to hit Florida right now. We need it.
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