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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1188. FLWeatherFreak91
9:45 AM EDT on May 28, 2008
Thanks weathersp... Why is it that despite the continuing westward movement the models still insist on a northerly path? This thing has done nothing but gone east since it was named an invest.
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1186. guygee
1:45 PM GMT on May 28, 2008
Maybe somebody posted this already:

Panama Canal Authority radar

(Last updated 28 May 08 13:30)
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1184. weathersp
9:42 AM EDT on May 28, 2008

I think it really depends. If it hit's an area with mountains then forget about it going into the Caribbean. If it slips through either near panama or up by Mexico then maybe just maybe something resembling an area of low pressure could make it through.
Member Since: January 14, 2007 Posts: 17 Comments: 4140
1183. cchsweatherman
9:40 AM EDT on May 28, 2008

That was a rough drawing and I agree that it should be further north and slightly further west. Based upon my analysis, I have the center at 9.0N, 87.0W just like Storm. Made a mistake on Paint when I drew the center.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
1180. FLWeatherFreak91
9:42 AM EDT on May 28, 2008
(especially the west coast that didn't get in on last weeks rains at all)

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1179. NEwxguy
1:41 PM GMT on May 28, 2008
Don't forget the season has started in the EPAC
Member Since: September 6, 2007 Posts: 896 Comments: 16202
1178. FLWeatherFreak91
9:41 AM EDT on May 28, 2008
So are we thinking that this Storm will eventually cross into the Caribbean despite the models' predictions?
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1177. weathersp
9:40 AM EDT on May 28, 2008
Click the magic buttons and all your problems will go away..
Member Since: January 14, 2007 Posts: 17 Comments: 4140
1175. Ivansrvivr
1:38 PM GMT on May 28, 2008
1172. If you lived in south Florida and understood how dry it has been here you would be very understanding of folks wishing for an early season tropical system to pass just to our west and dump copious rains on our parched areas. (especially the west coast that didn't get in on last weeks rains at all)
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1174. LSU
1:39 PM GMT on May 28, 2008
Then you must have missed the now-deleted above comments about it being TS Alma later today.
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1171. guygee
1:35 PM GMT on May 28, 2008
Thanks homegirl! That is a great view.

I need to catch up on the backposts.
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1168. weathersp
9:35 AM EDT on May 28, 2008
I think 90E is going to have a CDO in the next 6-12 hours if it keeps wrapping like this.
Member Since: January 14, 2007 Posts: 17 Comments: 4140
1167. groundswell
1:31 PM GMT on May 28, 2008
convection increasing in sw caribbean & lifting north in spite of high shear. Possible swell pump up the channel? Improbable but if development persists, there would be sliver of a glimmer of hope for Gulfsters.
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1165. Patrap
8:32 AM CDT on May 28, 2008
GOES-12 (3-Channel)Gulf and Tropics (Updated every ~1/2 hour) Link
Member Since: July 3, 2005 Posts: 437 Comments: 134804
1164. weathersp
9:31 AM EDT on May 28, 2008

Well that helps! Thanks Homegirl! (and NHC)
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1163. nrtiwlnvragn
9:27 AM EDT on May 28, 2008
1153. jphurricane2006

Past coordinates as shown below:

EP 90 2008052812 1 CARQ -24 97N 898W
EP 90 2008052812 1 CARQ -18 93N 895W
EP 90 2008052812 1 CARQ -12 93N 890W
EP 90 2008052812 1 CARQ -6 96N 885W
EP 90 2008052812 1 CARQ 0 100N 880W
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1162. homegirl
1:25 PM GMT on May 28, 2008
Hey Guygee...I don't know if you saw it...but I posted earlier that NHC has a floater on 90E

EPAC Floater 2: Link
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1161. guygee
1:22 PM GMT on May 28, 2008
I was looking for Pacific views of 90E so I neglected to check the regular Atlantic SSD site, where the GOES West Atlantic Imagery gives a good view of 90E with reasonable update frequency and timeliness.

It it interesting to see on the WV loop how the Central CONUS/Mexican DLH is separated from a similar WATL high center by a ULL that is pulling the ULH over 90E towards the N and NE attm. The overall pattern seems to be one of slow retrogression.
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1158. nash28
9:19 AM EDT on May 28, 2008
WU main page has it moving ENE.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
1155. Ohio91
1:13 PM GMT on May 28, 2008
Wow im really suprised looks like we could have TS Alma forming in the next few days, mabye even today.
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1151. IKE
8:08 AM CDT on May 28, 2008
Just going by what he/she posted..."DIRCUR = 65DEG SPDCUR = 4KT"
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1149. IKE
8:05 AM CDT on May 28, 2008
1143. nrtiwlnvragn 8:00 AM CDT on May 28, 2008
8AM spaghetti maps are up on WU.

Those models are already's moving at 65 degrees or ENE....
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1148. moonlightcowboy
8:04 AM CDT on May 28, 2008

Nothing will develop in the Caribbean as long as shear is 30-50kts. Some relaxing in the next 24 hrs in the extremes swCarib, but elsewhere increases another 5-15 kts.
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1146. weathersp
9:03 AM EDT on May 28, 2008
Mornin Gryffindor! Its a Invest and its a spinin around..
Member Since: January 14, 2007 Posts: 17 Comments: 4140
1144. Weather456
8:37 AM AST on May 28, 2008
I have updated my blog:

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1143. nrtiwlnvragn
8:59 AM EDT on May 28, 2008
8AM spaghetti maps are up on WU.
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1141. weathersp
8:58 AM EDT on May 28, 2008
Intresting EXTRAP..
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1139. moonlightcowboy
7:51 AM CDT on May 28, 2008

Possible TC now in the sfc map.
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1138. floridastorm
12:49 PM GMT on May 28, 2008
while 90E is developing
the countries of central america will
get heavy rain and mudslides
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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