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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914. cchsweatherman
1:47 PM EDT on May 22, 2008
...TROPICAL WAVES...

TROPICAL WAVE IS ALONG 23W S OF 12N MOVING W 10 KT. THIS WAVE
REMAINS FAIRLY WELL DEFINED WITH VISIBLE IMAGES SHOWING
LOW-LEVEL CYCLONIC TURNING ALONG THE AXIS AND A CLEAR
PERTURBATION IN THE ITCZ. HOWEVER...MUCH OF THIS TURNING IS LOW
LATITUDE CENTERED NEAR 6N. SCATTERED SHOWERS AND TSTMS ARE
WITHIN 150 NM EITHER SIDE OF THE AXIS S OF 9N.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
913. Weather456
1:42 PM AST on May 22, 2008
2004

2008

Most conditions are expected to be the same except for reduce wind shear from La-Nina-Neutral in 2008.
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912. Patrap
12:46 PM CDT on May 22, 2008
GOM GOES-12 Ch-3 WV Loop Link

Wow, if I'm analyzing correctly

You must be thinking Ne..and East
Member Since: July 3, 2005 Posts: 424 Comments: 128261
911. weatherdead
5:43 PM GMT on May 22, 2008
It seems "global warming" is used to explain a lot of things. More hurricanes, less hurricanes, even global cooling. Is it just possible that no one really knows what is going on?
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910. FLWeatherFreak91
5:41 PM GMT on May 22, 2008
Wow, if I'm analyzing correctly I would say that the low in the gomex should actually move more e ward than ne ward and affect Tampa.... The storm track feature on the WU radar is showing a few hail cells which should move onshore n of Tampa in the next 4 hours.
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909. 882MB
5:37 PM GMT on May 22, 2008
Thanks weather456, Ill be back later to see the new ECMWF 12UTC model run.
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908. Weather456
1:35 PM AST on May 22, 2008
906. 882MB 1:33 PM AST on May 22, 2008
weather456 that is from november.


That was a mistake...Now the one below....look at the time frame of the possible disturbance, that is May 29-June 5

LINK
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906. 882MB
5:32 PM GMT on May 22, 2008
weather456 that is from november.
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905. weathermanwannabe
1:18 PM EDT on May 22, 2008
Current ENSO forecast from AMO:

CURRENT STATUS as at 21st May 2008
Next update expected by 4th June 2008 (two weeks after this update).

Summary: Pacific climate patterns generally neutral

Pacific climate patterns remain generally neutral, although some aspects of the 2007/08 La Niña persist. Lingering La Niña signals include the continued presence of cooler than average water on the equator near the date-line, accompanied by reduced cloud and, in the first few weeks of May, enhanced Trade Winds.

Overall though, the tropical Pacific has been warming gradually during the past few months, with ocean temperatures now near-normal in central and eastern areas. In addition, the Trade Winds have decreased to near-normal levels in the western half of the Pacific and, confirming the return to neutral conditions, the Southern Oscillation Index (SOI) is about zero for the past 30 days.

Computer model predictions show Pacific temperatures continuing to gradually increase over the next two seasons, but remaining near-average. The models indicate a low chance of either a stronger warming to El Niño levels or a re-intensification of La Niña conditions during 2008.

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904. Weather456
1:28 PM AST on May 22, 2008
.
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903. Weather456
1:27 PM AST on May 22, 2008
The tropics today

updates are in the comment section
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902. Patrap
12:25 PM CDT on May 22, 2008
There only Models.
When we start seeing some consensus among the ensembles.
Then we give more interest,least for me.

The seasonal Tropics (reg.Patent.Pending)...are arriving on schedule.



Member Since: July 3, 2005 Posts: 424 Comments: 128261
901. 882MB
5:27 PM GMT on May 22, 2008
Weather456, do you have any images of how the MJO will look in a week or 2
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900. cchsweatherman
1:17 PM EDT on May 22, 2008
Here is the Tropical Update I just posted on my website.

We are now only days away from the 2008 Atlantic Hurricane Season and it appears the entire Tropical Atlantic has started showing signs of that. Right now, wind shear (upper-level winds that can "shear" the tops right off thunderstorms) remains relatively low for this time in the year, sea surface temperatures (SSTs) continue to rise across the entire basin and overall, is warmer than average (with the exception of the Central Atlantic), and there has been more frequent and progressively more impressive tropical waves coming off Africa.

But, despite all these favorable factors right now, there is no tropical development occuring. There is a rather strong mid-level disturbance moving into Northern Florida, but there will be no tropical development as this disturbance will interact with a front and produce some heavy rains in northern Florida and southern Georgia over the next three days.

Now, the major headline in the tropics is something that isn't even out there. According to a few computer models, there will be a developing tropical system in the Southwest Caribbean by late this weekend into early next week. Based upon these models, the system would travel northward and arrive in South Florida during the first few days of June. There has been remarkable consistency with the long-range models on this and now, the shorter-range models also hint at development occuring in the area. But, there is no need to get excited or panic as the system that would cause this to form hasn't even developed yet, but the models need to be taken into consideration. Stay tuned to further updates as the supposed event comes closer.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
899. mississippiwx23
5:19 PM GMT on May 22, 2008
I still don't like the location in which this storm forms. The likelihood of a storm to form that close to Panama and then move north is so low that I can't jump on board. I still say its either a Pacific storm or no storm.
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898. Patrap
12:22 PM CDT on May 22, 2008
Ghosts in the Machine...Or the trend?

UNYSIS GFSx Day-10 June 1 Link
Member Since: July 3, 2005 Posts: 424 Comments: 128261
897. IKE
12:20 PM CDT on May 22, 2008
Here's the 12Z CMC....forms an east-PAC storm in 144 hours...Link
Member Since: June 9, 2005 Posts: 23 Comments: 37858
896. Weather456
1:08 PM AST on May 22, 2008
880. cchsweatherman 1:00 PM AST on May 22, 2008

Formation continues to be consistent and the track is much more straight-forward that 24 hrs ago. I also notice, the system is moving very slowly early in the cycle, spending much more time in the Western Caribbean Sea. There is a possible reason for that. Surface ridging continues to project far across the Western Atlantic and across the SE USA and Gulf. The system only begins moving faster when an upper trough dips across the area and pulls the system more north and east. In the near term, the confidence of something developing in one week is moderate. MJO is expected to enhance convection over the area soon and zonal shear is expected to remain favorable for atleast the development of a tropical depression.
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895. AWeatherLover
5:16 PM GMT on May 22, 2008
I think I need some cycloactive drugs... LOL
this blog is becoming more and more addictive as H-season approaches.
Member Since: November 2, 2007 Posts: 0 Comments: 431
894. Patrap
12:20 PM CDT on May 22, 2008
Indiana Jones Premiere Gets a 8.5 from me.

The Hot Dog got a 9.5.Twas a tad lukewarmish.
Member Since: July 3, 2005 Posts: 424 Comments: 128261
893. 882MB
5:15 PM GMT on May 22, 2008
Hey everybody, Just seen the new GFS 12Z and it continues to show a well defined low pressure system forming in the SW CARRIBEAN,only time will tell,im waiting on the new run of the ECMWF and CMC models.
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892. franck
5:11 PM GMT on May 22, 2008
Someone say rotation?
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891. IKE
12:12 PM CDT on May 22, 2008
880. cchsweatherman 12:00 PM CDT on May 22, 2008
IKE and 456,

Could you offer your thoughts on the latest GFS model? What I have now seen for the last five runs that I had not seen before is remarkable consistency in the track of the system and the intensity (although we all know all to well that all models tend to underestimate the intensity of systems down the road). The whole setup looks quite reasonable considering the current environmental conditions and climatology. Starting to lean more towards believing the model considering all the action around Panama and Costa Rica as well as favorable wind shear throughout the Caribbean.


I believe in the GFS. It's been consistent with this...I think it's gonna happen...if I'm wrong serve me some crow. Plus the ECMWF is trending toward the GFS...time will tell.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
890. IKE
12:10 PM CDT on May 22, 2008
887. weathermanwannabe 12:06 PM CDT on May 22, 2008
875. IKE 12:52 PM EDT on May 22, 2008

Hey Ike; how have you been....I for one am quite upset right now that the bulk of the rain is missing us up here in North Florida...Have you gotten any over in Defuniak?...Tallahasse has been overcast but bone dry this morning.......


Doing OK...no rain so far...I noticed yesterday both the NAM and GFS had the precip taking a turn right as it approached the Florida panhandle...sure enuff, that's what is happening.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
888. Weather456
1:02 PM AST on May 22, 2008
879. AndyN 12:58 PM AST on May 22, 2008
Could be a scary year weather456


That is the point some have been stressing on for the past months. SSTs in April and May are not an indicator of SSTs in September and August.
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887. weathermanwannabe
1:04 PM EDT on May 22, 2008
875. IKE 12:52 PM EDT on May 22, 2008

Hey Ike; how have you been....I for one am quite upset right now that the bulk of the rain is missing us up here in North Florida...Have you gotten any over in Defuniak?...Tallahasse has been overcast but bone dry this morning.......
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886. Patrap
12:06 PM CDT on May 22, 2008
I feel so doomed..Doomed I say

NEXRAD Radar
New Orleans Base Reflectivity 0.50 Degree Elevation Range 124 NMI Link
Member Since: July 3, 2005 Posts: 424 Comments: 128261
885. nash28
1:04 PM EDT on May 22, 2008
Let's see if by Saturday the GFS still hangs onto this.
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884. cchsweatherman
1:01 PM EDT on May 22, 2008
OMG!!! Rain is about to arrive in my neighborhood for the first time in about five weeks! It may not be much, but we'll take it.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
883. sporteguy03
5:02 PM GMT on May 22, 2008
Entire 12Z GFS model run....Link

Is it gonna happen? I say yes.......

Ike,
Or it will be one of the most forcasted syatems to not happen, guess we'll see.
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881. Weather456
1:00 PM AST on May 22, 2008
880. cchsweatherman 1:00 PM AST on May 22, 2008

Let me look at the latest run
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880. cchsweatherman
12:55 PM EDT on May 22, 2008
IKE and 456,

Could you offer your thoughts on the latest GFS model? What I have now seen for the last five runs that I had not seen before is remarkable consistency in the track of the system and the intensity (although we all know all to well that all models tend to underestimate the intensity of systems down the road). The whole setup looks quite reasonable considering the current environmental conditions and climatology. Starting to lean more towards believing the model considering all the action around Panama and Costa Rica as well as favorable wind shear throughout the Caribbean.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
879. AndyN
4:57 PM GMT on May 22, 2008
Could be a scary year weather456
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877. OrchidGrower
4:52 PM GMT on May 22, 2008
One thing I've never heard anyone discuss is the effect the relentless widening of the Atlantic Ocean might/should have on frequency of tropical systems. As I understand it, the Atlantic Rift is broadening the Atlantic by an inch a year, and while that doesn't sound like much, for an ocean that stretches pole to pole, a lot of water has to be added to fill that extra "real estate" - a column one inch wide but thousands of feet deep and thousands of miles long.
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876. Weather456
12:50 PM AST on May 22, 2008
This is from Noaa's outlook regarding SSTs

In contrast, SSTs over the central and western MDR were below average during the last several months in association with the combination of La Niña and a persistent pattern of anomalous northeasterly and easterly surface winds. This wind pattern recently ended, and the SSTs quickly returned toward normal in these regions. Consistent with past La Niña episodes, and with the ongoing tropical multi-decadal signal, above-average temperatures are expected to return in the central and western MDR as the summer progresses.
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875. IKE
11:52 AM CDT on May 22, 2008
Entire 12Z GFS model run....Link

Is it gonna happen? I say yes.......
Member Since: June 9, 2005 Posts: 23 Comments: 37858
874. cchsweatherman
12:42 PM EDT on May 22, 2008
GFS Model 276 Hours Out
Member Since: April 14, 2007 Posts: 8 Comments: 5169
873. groundswell
4:41 PM GMT on May 22, 2008
Raining here in Ocala-we really need it. Looks like the blob in the SW caribbean is moving north for a sneak attack on Florida!
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872. cchsweatherman
12:38 PM EDT on May 22, 2008
Just an update on the latest GFS model output, through 216 hours, the GFS continues it on the same path and development as in the previous five runs. Still rather lopsided looking system. At the 216 hour, the model has it still far from South Florida, but nearing Western Cuba (about 150 miles to the south).
Member Since: April 14, 2007 Posts: 8 Comments: 5169
871. weathermanwannabe
12:37 PM EDT on May 22, 2008
870. cchsweatherman 12:35 PM EDT on May 22, 2008........Cool; looks like the Florida "blob" is rapidly losing it's convection so it does not seem like we will get as much rain as some were hoping for.....
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870. cchsweatherman
12:32 PM EDT on May 22, 2008
868. weathermanwannabe 12:30 PM EDT on May 22, 2008
863. cchsweatherman 12:10 PM EDT on May 22, 2008 Besides that, I'm not scheduled to work all week until Saturday, so that is why it may seem that I'm on 24/7

But, make sure to take a break every now and then and to squeeze in some food and a shower before Saturday....This blog can become rather addicting....Lol


I've actually been diagnosed by a psychiatrist with OCD for weather. I can't get over the urge and fascination no matter how hard I try, but I'm disciplined enough not to allow it to take control of my life. FYI I took a shower this morning after my brother woke me up for a good reason; he got a 1400 out of 1600 on his SAT. Highest I ever got was a 1320.

Member Since: April 14, 2007 Posts: 8 Comments: 5169
869. AndyN
4:31 PM GMT on May 22, 2008
I thought this was particularly interesting in the NOAA 2008 Hurricane Season outlook:

Presently, La Niña is indicated by below average sea-surface temperatures across the central and east-central equatorial Pacific. La Niña is dominating the atmospheric convection and low-level winds in these regions as well, with suppressed convection over the central and eastern Pacific and enhanced convection over the western Pacific. There has been a tremendous tropics-wide response in the upper-level (200-hPa) atmospheric winds to this anomalous convection, with easterly anomalies extending across the eastern tropical Pacific Ocean, the tropical Atlantic Ocean, and northern Africa. If these anomalies persist through the summer, they would reinforce the multi-decadal signal and increase the probability of an above-normal and even hyperactive season.

In the latest ENSO Diagnostics Discussion released 8 May 2008, NOAA forecasters stated that La Niña has weakened since February, and that a transition to ENSO-neutral conditions is possible during June-July just prior to the peak months of the Atlantic hurricane season. This evolution is typical for La Niña, which often dissipates during the late spring or summer.

There is considerable spread and uncertainty among the climate models regarding how strong the La Niña influence will be on the Atlantic hurricane season. Most models are predicting ENSO-neutral conditions (neither El Niño nor La Niña) during the summer, with sea-surface temperature anomalies in the central and east-central equatorial Pacific between -0.5oC to 0.5oC. However, most of these models have historically shown little-to-no skill at this time of the year.

Historically, we have almost never (only once in 100 years) seen a La Niña transition from its current present strength to an El Niño by ASO. Therefore, there is a likelihood that the current La Niña patterns of tropical convection and winds will persist and affect the hurricane season, even if La Niña dissipates.

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868. weathermanwannabe
12:25 PM EDT on May 22, 2008
863. cchsweatherman 12:10 PM EDT on May 22, 2008 Besides that, I'm not scheduled to work all week until Saturday, so that is why it may seem that I'm on 24/7

But, make sure to take a break every now and then and to squeeze in some food and a shower before Saturday....This blog can become rather addicting....Lol
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867. MasterForecaster
4:16 PM GMT on May 22, 2008
Thanks cchs your information is always appreciated. I hear BCC is a nice place I might look at FAU or FIU I hear they have great weather related programs.

It'd be weird to have a tropical system threaten us at the END of school instead of the beggining...we'll chat later about that possible system developing but now its off to American History X.
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866. cchsweatherman
12:12 PM EDT on May 22, 2008
Now the GFS starts developing the system within five days as it begins forming at 108 hours out (or 4.5 days).
GFS Model 108 HRS
Member Since: April 14, 2007 Posts: 8 Comments: 5169
865. zoomiami
4:12 PM GMT on May 22, 2008
This question is for someone more technologically advanced then I am - I was reading the blog on my blackberry this morning, but couldn't figure out how to refresh the screen. Anyone know how to do this?

Thanks
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864. Patrap
11:10 AM CDT on May 22, 2008
The Climate Prediction Center outlook calls for considerable activity with a 65 percent probability of an above normal season and a 25 percent probability of a near normal season. This means there is a 90 percent chance of a near or above normal season.

The climate patterns expected during this years hurricane season have in past seasons produced a wide range of activity and have been associated with both near-normal and above-normal seasons. For 2008, the outlook indicates a 60 to 70 percent chance of 12 to 16 named storms, including 6 to 9 hurricanes and 2 to 5 major hurricanes (Category 3, 4 or 5 on the Saffir-Simpson Scale).
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.