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

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

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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1238. smmcdavid
9:33 AM CDT on May 28, 2008
Think it will make depression status today?
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1237. cchsweatherman
10:32 AM EDT on May 28, 2008
Watching satellite loops for Invest 90E reminds me of a sprawling low that we had sitting over the Yucatan late last hurricane season that had moisture inflow from the Pacific, Gulf of Mexico, and the Caribbean all at the same time. It seems like 90E is churning up a large expanse of atmosphere in the Eastern Pacific and Western Caribbean. This could be one factor that could help it survive a trek across Central America.
Member Since: April 14, 2007 Posts: 8 Comments: 5169
1235. groundswell
2:29 PM GMT on May 28, 2008
05 28 1250

ENE 19.4 23.3
From NWS bouy in sw caribbean-now have a wind switch from east to northeast & increasing
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1233. stormkat
2:29 PM GMT on May 28, 2008
jp dont get to excited its not going to happen...the shear is to strong..stormkat
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1231. smmcdavid
9:27 AM CDT on May 28, 2008
Good morning... what's going on?
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1230. stormkat
2:24 PM GMT on May 28, 2008
i see nothing developing in the sw caribbean...the shear is forecast to increase and that will tear whatever is trying to develop apart...its a hostile enviroment so dont get to excited..the season has just started..concentraste in the pacific..stormkat
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1228. guygee
2:21 PM GMT on May 28, 2008
Disagree, the discussion for some late blobless night, tho...
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1227. Ivansrvivr
2:21 PM GMT on May 28, 2008
1222. It sure looks organized on the the sat if you look at both basins together. If it makes it into the Caribbean it could be a beautiful rainmaker for Florida and the S.E US toward the middle of next week.
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10:17 AM EDT on May 28, 2008
1109. HIEXPRESS 1:46 PM EDT on May 24, 2008
Another option, just as likely says the M8B model:
We got enough rain to put a damper on the fires here in FL, so I think we should cancel the TS we had on order (which was backordered anyway due to shear and/or not being able to get a big enough cluster of storms together to keep out the dry mid-level air. See all the arcs?

Broad low can rain out some of the energy starting to build up down South, & the rest can transport North efficiently enough without all the dramatic cyclonic turning.

Has it rained out all the juice yet? It better do something fast if it's going to, because the dry air is coming back. GTG BBL
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1224. IKE
9:20 AM CDT on May 28, 2008
RAMSDIS satellite is working.......


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1223. Ivansrvivr
2:16 PM GMT on May 28, 2008
Carbon..Carbon..Carbon...Plants are made of Carbon. Animals are made of Carbon. We are made of Carbon. This planet is made of carbon. That is why we are CARBON BASED LIFE FORMS! If the ultimate goal to stop using carbon then we should either plan to evacualte earth or go extinct. Enough about carbon. Lets talk tropics.
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1221. nrtiwlnvragn
10:14 AM EDT on May 28, 2008
8AM multi-model spaghetti plots. Would post an image but the site does not allow it.
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1219. IKE
9:07 AM CDT on May 28, 2008
1213. Patrap 9:07 AM CDT on May 28, 2008
GOES-12 (3-Channel)Gulf and Tropics (Updated every ~1/2 hour) Link

Thanks for that view...this is starting to look impressive on the Caribbean side.
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1217. Patrap
9:07 AM CDT on May 28, 2008
Thanx for that source data nrtiwlnvragn.
Member Since: July 3, 2005 Posts: 454 Comments: 144446
1215. Patrap
9:07 AM CDT on May 28, 2008
Cue the Jaws, Movie Theme.....
Member Since: July 3, 2005 Posts: 454 Comments: 144446
1214. nrtiwlnvragn
10:07 AM EDT on May 28, 2008
The following rules apply for tropical cyclones passing from one basin to another: Retain
the name if a tropical cyclone passes from one basin into another basin as a tropical cyclone; i.e. advisories are continuous. An unnamed tropical depression will also retain its number (e.g. Tropical Depression Six-E remains Tropical Depression Six-E) if it crosses into another area of responsibility. For unnamed tropical depressions moving from west to east across 180°, CPHC will use the associated Joint Typhoon Warning Center’s (JTWC) number and indicate JTWC in parentheses following the number. For named systems, CPHC will use the RSMC Tokyo name and provide the associated JTWC number in parentheses.
Within a basin, if the remnant of a tropical cyclone redevelops into a tropical cyclone, it is assigned its original number or name. If the remnants of a former tropical cyclone regenerate in a new basin, the regenerated tropical cyclone will be given a new designation.

National Hurricane Operations Plan
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1213. Patrap
9:04 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: 454 Comments: 144446
1212. Ivansrvivr
2:00 PM GMT on May 28, 2008
Good Morning Storm. What do you think of the continuing westerlies in the E-Pac. Is it a long term trend or just associated with a very broad low pressure that would tend to congeal.

"....Overestimating the strength of the Bermuda High"

Look at how rapidly moisture has drawn northward into the Caribbean sea. 2 days ago it was totally moisture starved. The fact that moisture is moving northward like that would indicate that the Bermuda High is starting to influence the Tropical Atlantic.
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1211. guygee
1:58 PM GMT on May 28, 2008
GS - It seems that the newer studies have trended towards a higher estimate of total carbon emissions due to tropical deforestation. That is what I meant. The research shows that the carbon emissions from human-caused deforestation (land-use change) is roughly constant over time, but that due to natural causes such as droughts and wildfires fluctuates from year to year, as would be expected.
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1210. FLWeatherFreak91
10:02 AM EDT on May 28, 2008
1206. fmbill 9:59 AM EDT on May 28, 2008
If a storm gets a name in the EPAC, and successfully moves into the Caribbean, does it get a new name or does it keep the EPAC name?

As long as its winds stay above 39 mph then it will keep its name.
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1209. thalos
2:02 PM GMT on Mayo 28, 2008
These three days it has almost rained what falls usually in one month, there was drought to the north of my country, but with this wave of low pressure it has been reverted the situation a little, we wait that it benefits the inhabitants of the north, greetings from Costa Rica....
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1208. IKE
8:57 AM CDT on May 28, 2008
SSD satellite isn't updating...looks like a large mass of convection coming off of SA...must be that tropical wave....that'll add fuel to the fire.
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1207. nash28
9:58 AM EDT on May 28, 2008
NOGAPS has stayed with the most EWD solution for days now.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
1206. fmbill
1:57 PM GMT on May 28, 2008
If a storm gets a name in the EPAC, and successfully moves into the Caribbean, does it get a new name or does it keep the EPAC name?
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1205. Buhdog
1:57 PM GMT on May 28, 2008
The whole thing is puffing out some serious tops! I was thinking that none of us in Florida should ask for this to come our way. We do this anytime there are clouds anywhere within 1,000 miles or us or more. Guess what? We never seem to get the rain...and the odds of this blob forming and coming north... then being "just west" of us for the most rain.... is just's just a big let down. I am going for reverse psych.....It WILL NEVER GET HERE!!! (I hope this works)
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1204. FLWeatherFreak91
9:57 AM EDT on May 28, 2008
1200. melwerle 9:57 AM EDT on May 28, 2008
Have a great day to the beach with the many more days till school is back in session?!!!??

However many days it is until mid August lol
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1203. Tazmanian
6:57 AM PDT on May 28, 2008
WOW take a look at the wind shear in the gulf this AM its olny 5 to 10kt
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1202. FLWeatherFreak91
9:57 AM EDT on May 28, 2008
Thanks guys. So it seems the nogaps is back on with a stronger system, althoughy sheared to the east. Is this correct?
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1200. melwerle
1:56 PM GMT on May 28, 2008
Have a great day to the beach with the many more days till school is back in session?!!!??

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1199. melwerle
1:55 PM GMT on May 28, 2008
Hey can find them all on one site -

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1198. weathersp
9:47 AM EDT on May 28, 2008
It seems to me that the models are over estimating the power of the bermuda high...
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1:52 PM GMT on May 28, 2008
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1195. Ivansrvivr
1:51 PM GMT on May 28, 2008
Panama Canal radar loop show continuing westerly flow over E-Pac. that would favor eventual development in W.Caribbean if at all.
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1194. FLWeatherFreak91
9:51 AM EDT on May 28, 2008
I'm currently in class so could someone post the link to the GFS and NOGAPS please. (Or prefferably the site by FSU that shows all of them)
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1193. Tazmanian
6:49 AM PDT on May 28, 2008
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1192. guygee
1:50 PM GMT on May 28, 2008
GulfScotsman - Yes, correct. Seems to be a lot of debate, but the trend in time is towards the higher side.
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1191. Tazmanian
6:48 AM PDT on May 28, 2008
WOW take a look at the wind shear in the gulf this AM its olny 5 to 10kt
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1190. guygee
1:46 PM GMT on May 28, 2008
Panama Canal Authority Time Lapse Loop Radar Images
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1189. jhicok
8:45 AM CDT on May 28, 2008
The Parkersburg, IA tornado of 5/25/08 was officially rated as an EF5 yesterday. The Des Moines office of the NWS has pictures and radar imagery of the storm.
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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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