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

By: Dr. Jeff Masters , 5:07 PM GMT on May 27, 2008

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


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

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

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

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

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


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

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


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

I'll have an update by Wednesday afternoon.

Jeff Masters

New Hartford (snp4u)
Missing House, if found call Dennis and Carla
New Hartford
New Hartford (snp4u)
car pile up
New Hartford
Supercell near Pratt, Kansas (MikeTheiss)
Nice structure on upercell east of Pratt, Kansas. Photo copyright Mike Theiss.
Supercell near Pratt, Kansas

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1088. stoormfury
11:12 AM GMT on May 28, 2008
a very pwerful wave to exist the coast of africa


Link
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1087. Ivansrvivr
11:11 AM GMT on May 28, 2008
JP, It is possible that the low level center and mid level support are moving two different directions. Looking at it in IR loop, 90E looks like it is moving eastward in the last few frames.
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1086. Stormchaser2007
7:13 AM EDT on May 28, 2008
chaser i still think there is an elongated trough, if anything is spinning in the SW Caribbean, it isnt at the surface

I know that theres no surface low, I was implying that the Caribbean spin is stronger than 90E...
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1084. Ivansrvivr
11:07 AM GMT on May 28, 2008
456 development in that area would recurve very early but would be strange no matter what it's track is. That "Blob" over africa is probably just thunderstorms. Often when they look very strong over Africa, they fall apart once they hit the ocean. It looks like the CV season will be super busy this year.
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1083. Stormchaser2007
7:09 AM EDT on May 28, 2008
Well if theres no low in the Caribbean then theres something thats gotta be spinning down there.
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1081. IKE
6:06 AM CDT on May 28, 2008
The extended, 6-10 day and 8-14 day outlook, is for above average rain for most of Florida.
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1080. Stormchaser2007
7:06 AM EDT on May 28, 2008
But the Caribbean low would have to survive this first....
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1079. Weather456
7:01 AM AST on May 28, 2008
Good Morning,

90E is near depression status and becuz of its close proximity to the SW Caribbean, low or no low, it cannot breath effectively becuz of 90E. Also, the GFS and CMC is showing development of the coast of africa this weekend. I am updating my blog.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
1078. Ivansrvivr
10:56 AM GMT on May 28, 2008
System in the E-Pac looks like it has started an eastward drift. if that continues, it may tend to relocate to the other side of Central America where topography supports cyclonic turning of surface winds. The overall tropical moisture has moved northward throughout the Caribbean rapidly. If the E-Pac system emerges somehow into the E-Carribean or dissipates, the low in the Caribbean would have a decent chance to develop or merge with the W-Pac low. Any development in that area would be a huge rainmaker for Florida down the road most likely.
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1077. IKE
6:01 AM CDT on May 28, 2008
1076...OK
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1076. all4hurricanes
10:58 AM GMT on May 28, 2008
I'm referring to the wave off on Africa, the small band attached to it and the direction it's facing makes it look like it's spinning clockwise which is typical in the southern hemisphere.
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1074. IKE
5:58 AM CDT on May 28, 2008
I only see one low on satellite...in the eastern PAC...I see no spin in the Caribbean.
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1073. extreme236
10:57 AM GMT on May 28, 2008
JP, before I go, the NHC does seem to have taken off the low from their sfc maps in the Caribbean however they still forecast one to develop in 24 hours.
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1072. all4hurricanes
10:56 AM GMT on May 28, 2008
IKE i agree that it looks like a TS but it looks like it's spinning the wrong way! I think conditions are favorable immediately offshore.
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1071. IKE
5:56 AM CDT on May 28, 2008
1069. all4hurricanes 5:56 AM CDT on May 28, 2008
IKE i agree that it looks like a TS but it looks like it's spinning the wrong way! I think conditions are favorable immediately offshore.


What do you mean by..."the wrong way?" You mean it's movement TOWARD the coast?
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1069. all4hurricanes
10:54 AM GMT on May 28, 2008
IKE i agree that it looks like a TS but it looks like it's spinning the wrong way! I think conditions are favorable immediately offshore.
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1068. extreme236
10:54 AM GMT on May 28, 2008
The NHC believes the system will drift East/NE over time...be back later.
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1067. Stormchaser2007
6:54 AM EDT on May 28, 2008
This thing has a good chance of developing....
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1065. IKE
5:51 AM CDT on May 28, 2008
1061. Thundercloud01221991 5:42 AM CDT on May 28, 2008
this thing looks like a TS already


Yeah it does.
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1064. Stormchaser2007
6:50 AM EDT on May 28, 2008
WOW!!
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1063. extreme236
10:44 AM GMT on May 28, 2008
New EPAC Discussion:

...SPECIAL FEATURES...

A BROAD AREA OF SURFACE LOW PRESSURE DOMINATES THE SW CARIBBEAN
AND THE E PAC ALONG THE COAST OF CENTRAL AMERICA. DURING THE
LAST SEVERAL HOURS THE OVERALL PATTERN OF THIS AREA HAS BEEN
SUGGESTING THAT A SURFACE LOW HAS BECOME MORE IDENTIFIABLE IN
THE VICINITY OF 09N88W WITH SCATTERED MODERATE TO STRONG
CONVECTION OCCURRING WITHIN 300 NM SE AND 210 NM SW QUADRANTS.
SCATTERED STRONG CONVECTION IS WITHIN 90 NM N AND NE
QUADRANTS...AND ALSO WITHIN 45 NM EITHER SIDE OF A LINE FROM
07N80W TO 08N84W.

NWP MODELS CONTINUE TO SUGGEST THAT THIS LOW PRESSURE WILL BE
PRESENT IN THIS AREA FOR THE NEXT 2 TO POSSIBLY 3 DAYS AS IT
DRIFTS ENE. THE LOW HAS ACQUIRED THE POTENTIAL TO DEVELOP INTO
A TROPICAL CYCLONE OVER THIS PERIOD AS THE UPPER LEVEL PATTERN
HAS BECOME MORE FAVORABLE...AND IN ADDITION THE APPROACHING
TROPICAL WAVE MENTIONED BELOW MAY ADD THE NEEDED LOW LEVEL
VORTICITY TO FURTHER HELP SPIN UP THE LOW DURING THE NEXT 48
HOURS. THE MODELS...FOR THE MOST PART...TRY TO BRING THE LOW
OR WHAT IS LEFT OF IT IN A GENERAL NLY TRACK ACROSS OR NEAR THE
REMAINDER OF CENTRAL AMERICA LATE FRI AND THROUGH THE UPCOMING
WEEKEND.

OF NOTE...THE GRADIENT S OF THE E PAC LOW PRESSURE IS ALREADY
SUPPORTING SW-W WINDS IN THE RANGE OF 20 TO 25 KT WITH SEAS
8-11 FT. QUIKSCAT DATA FROM YESTERDAY EVENING SHOWED WINDS IN
THE 20-25 KT RANGE WITH DIRECTION FROM SW TO W WITHIN THE BROAD
LOW PRES AREA BETWEEN 81W-93W.

THESE CONDITIONS ARE EXPECTED TO PERSIST S OF THE LOW THROUGH 48
HOURS WITH THE FOOTNOTE THAT THEY MAY HAVE TO BE
ADJUSTED UPWARD IF LOW CONTINUES TO GET BETTER ORGANIZED. THE
MAIN THING TO CONSIDER FOR TIME BEING IS THE HEAVY RAIN POTENTIAL
EXPECTED OVER MUCH OF CENTRAL AMERICA FROM NICARAGUA TO PANAMA
AT LEAST DURING THE NEXT 48 HOURS.
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1062. TerraNova
6:41 AM EDT on May 28, 2008
Morning everyone.

Steering currents would take 90E towards Costa Rica or Nicaragua; I don't know how this would go westward unless I'm missing something. Right?

I wouldn't put much faith on the models, especially regarding the recent runs. The computer guidance is having problems deciding which low will dominate over the other and this is creating discrepancies during the longer range forecasts. GFS suppresses the Caribbean disturbance despite it showing favorable conditions as it moves to the northwest.
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1061. Thundercloud01221991
10:42 AM GMT on May 28, 2008
this thing looks like a TS already
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1060. IKE
5:38 AM CDT on May 28, 2008
According to the latest GFS(6Z), the ridge will break down in about a week allowing this system to finally head north or NNE toward south FL.

Link
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1059. BahaHurican
6:38 AM EDT on May 28, 2008
1057. leftovers 6:31 AM EDT on May 28, 2008
I have not seen too many EPac disturbances develop this close to Latin America. Most develop south of Mexico so?


It's not that unusual at this time of year. Eward movement into Costa Rica is less usual, though.
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1058. IKE
5:32 AM CDT on May 28, 2008
Just about all of the models I've looked at take this entire mess WNW to NW for the next several days...winding up around the Yucatan peninsula to the BOC.
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1056. extreme236
10:23 AM GMT on May 28, 2008
The SFC maps from the NHC suggest a possible tropical cyclone from 90E on the 24 and 48 hour maps then on the 72 hour map I looked at, it showed the Caribbean low stationary in the Gulf of Honduras. It looks like the tropical wave that will soon interact with the Caribbean low will kick it NW.
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1055. extreme236
10:19 AM GMT on May 28, 2008
T1.0/1.0 90E -- East Pacific Ocean
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1054. extreme236
10:10 AM GMT on May 28, 2008
Caribbean system seems more organized and 90E is more organized. Once the Caribbean system moves NW perhaps both could develop?
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1053. extreme236
10:09 AM GMT on May 28, 2008
NHC has a 1006mb SFC low in the Caribbean.
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1052. BahaHurican
5:16 AM EDT on May 28, 2008
I'd seen some references to the tornado incidence before. Seems like there is quite a bit of support for this. Not surprisingly, there is also a slightly greater incidence of TCs in the ATL basin during La Nina years as well.
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1051. reasonmclucus
8:49 AM GMT on May 28, 2008
There is a study indicating that the cooler temperatures associated with the la Nina portion of the el Nino la Nina Southern Oscillation is associated with larger tornado outbreaks and stronger tornadoes.
Mark C. Bove of the Center for Ocean-Atmospheric Prediction Studies (COAPS) in a paper published in 1999 noted a connection between ENSO and tornadic activity in the U.S. La Ninas were associated with tornado outbreaks and strong tornadoes. The tornado that wiped out Greensburg, Kansas, last May occurred while the current la Nina was developing.

http://www.coaps.fsu.edu/papers/impacts_enso_tornadic_activity/

According to Bove, "The results indicate that El Nio events reduce tornadic activity in the southern plain states, while El Viejo [la Nina] events increase tornadic activity in the Ohio River Valley and Deep South. Results further show that El Nio inhibits the chances of multiple tornado outbreaks, while La Nia facilitates large tornadic outbreaks and produces more devastating tornadoes."

A 1996 report for the Defense Technical Information Center noted increased freezing rain in the western U.S. during la Nina years as compared to el Nino years.

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1050. BahaHurican
4:48 AM EDT on May 28, 2008
This is about the most active the ATL in our area has been for ages. That northern portion of the Twave is producing some impressive convection with the diurnal max. . .

ATLANTIC...
AN UPPER RIDGE IS OVER THE FAR W ATLC INTO THE E GULF OF MEXICO
WITH A WELL DEFINED UPPER LOW NEAR 31N66W EXTENDING A SHEAR AXIS
S OF THE UPPER RIDGE ALONG 25N70W TO OVER CUBA NEAR 22N78W. A
REMNANT FRONTAL BOUNDARY IS BEGINNING TO LIFT BACK N SLOWLY AS A
WARM FRONT THAT EXTENDS THROUGH 32N64W TO THE BAHAMAS NEAR
23N75W DISSIPATING TO OVER CUBA NEAR 21N78W. CLUSTERS OF
SCATTERED SHOWERS/ISOLATED THUNDERSTORMS ARE S OF 23N BETWEEN
70W AND THE FLORIDA STRAITS INCLUDING THE TURKS AND CAICOS.
SIMILAR ACTIVITY IS ASSOCIATED WITH THE UPPER LOW WITHIN 90 NM
OF THE FRONT N OF 27N. AN UPPER RIDGE EXTENDS FROM THE BROADER
UPPER RIDGE OVER THE CARIBBEAN ACROSS THE VIRGIN/LEEWARD ISLANDS
NE TO BEYOND 32N50W. THE N PORTION OF THE TROPICAL WAVE
APPROACHING THE LESSER ANTILLES HAS BROKEN OFF AS A SURFACE
TROUGH EXTENDING FROM 23N61W TO 28N57W WITH SCATTERED SHOWERS/
THUNDERSTORMS COVERING THE AREA FROM 23N-28N BETWEEN 57W-62W.
BROAD UPPER TROUGH IS IN THE E/CENTRAL ATLC EXTENDING FROM AN
ELONGATED UPPER LOW NEAR 32N34W SW INTO THE TROPICAL ATLC TO
NEAR 13N48W WITH A SECOND UPPER TROUGH OVER THE FAR E ATLC E OF
20W OVER THE CANARY ISLANDS AND WESTERN SAHARA AFRICA. THE E AND
CENTRAL ATLC IS DOMINATED BY A SURFACE RIDGE N OF 15N BRIDGING
THE FRONT IN THE W ATLC ACROSS THE NE GULF OF MEXICO ANCHORED
BY A 1028 MB HIGH NEAR 33N45W. THE RESULT IS CALM WEATHER FOR
THE ATLC E OF 55W.


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1049. BahaHurican
4:36 AM EDT on May 28, 2008
JMA's summary of Nakri:

STS 0805 (Nakri)
Issued at 06:50 UTC, 28 May 2008

Scale -
Intensity -
Center position N15°30'(15.5°)
E136°50'(136.8°)
Direction and speed of movement NNW 15km/h(7kt)
Central pressure 985hPa
Maximum wind speed near the center 25m/s(50kt)
Maximum wind gust speed 35m/s(70kt)
Area of 30kt winds or more Wide 260km(140NM)


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1048. BahaHurican
4:29 AM EDT on May 28, 2008


So this is how NHC is resolving the "two lows" development of the various models. I notice that by Sat the EPac low is gone; I wonder if the movement of the Twave into the area disrupts the development or simply acts to draw it towards the coast . . .
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1047. BahaHurican
4:22 AM EDT on May 28, 2008
TWO is set for an update at 5 a.m. Wonder if we're going to see an increased risk?
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1046. BahaHurican
4:16 AM EDT on May 28, 2008
Latest visible of Nakri:



It's looking pretty impressive.

From JWTC

1. WESTERN NORTH PACIFIC AREA (180 TO MALAY PENINSULA):
A. TROPICAL CYCLONE SUMMARY:
(1) AT 280000Z TROPICAL STORM (TS) 06W (NAKRI) WAS LOCATED
NEAR 15.4N 137.1E, APPROXIMATELY 605 NM SOUTH-SOUTHWEST OF IWO TO,
AND HAD TRACKED NORTHWARD AT 05 KNOTS OVER THE PAST SIX HOURS.
MAXIMUM SUSTAINED WINDS WERE ESTIMATED AT 55 KNOTS GUSTING TO
70 KNOTS.
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1045. BahaHurican
4:02 AM EDT on May 28, 2008
Morning everybody.

Interesting to notice the moisture is spreading across the basin. The blowup east of the Bahamas / north of the Lesser Antilles looks impressive. Looks like much of the dry air has been shifted for a while.
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1044. moonlightcowboy
2:45 AM CDT on May 28, 2008
G'morning, Hades.

If this thing goes east it's got to deal with land interaction(even briefly) and 20-50 kts shear.
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1043. HadesGodWyvern (Mod)
7:40 AM GMT on May 28, 2008
National Hurricane Center - Miami, Florida

Tropical Cyclone Outlook (0600z 28May)
==================================
A broad area of low pressure is centered a couple hundred miles west of Costa Rica.. and is producing widespread showers and thunderstorms activity that extends west-southwest from central america for several hundred miles.

Gradual development of this system is possible and it could become a tropical depression within the next couple of days as it drifts eastward or northeastward. Regardless of development..locally heavy rains are expected over portions of Panama, Costa Rica, and Nicaragua during the next day or two.. These rains could cause life-threatening flash floods and mudslides.

Tropical Cyclone Development Potential
=======================================
MODERATE (20-50%)
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1040. moonlightcowboy
2:31 AM CDT on May 28, 2008
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1039. moonlightcowboy
2:25 AM CDT on May 28, 2008
90EINVEST.25kts-1006mb-9.0N-89.0W
Member Since: July 9, 2006 Posts: 184 Comments: 29610

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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.

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