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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1288. cchsweatherman
11:06 AM EDT on May 28, 2008
Watching the latest RAMSDIS loops, it appears that the low-level circulation has really started to tighten as convection continues to organize and consolidate around the center. Quite an impressive system.

By the way, doesn't the NHC issue a Tropical Weather Outlook every six hours for the Eastern Pacific? Still have no update from the NHC.
Member Since: April 14, 2007 Posts: 8 Comments: 5030
1287. StormJunkie
3:05 PM GMT on May 28, 2008
1284 & 1275

Do we have a model showing that development should be occurring soon?

I refer you to GS's post. imho, he hit the nail on the head as far as what would play out if something develops. Complex situation down there, and it will not lead to instant gratification.
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1286. StormJunkie
3:03 PM GMT on May 28, 2008
GS, sounds very similar to what the models have been hinting at. Very good observation, and I like the comparison pic!
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1284. moonlightcowboy
10:00 AM CDT on May 28, 2008
1275. jphurricane2006 9:59 AM CDT on May 28, 2008
To me the convection in the Caribbean is rotating around a broader low and the center of 90E, if that is the case, then development is even less likely


RAMSDIS VIS loop

JP, that would be my thinking, too.
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1283. Buhdog
2:58 PM GMT on May 28, 2008
Could someone post a couple of links to a couple of model runs...thanks
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1282. kmanislander
2:57 PM GMT on May 28, 2008
Here are a few obs from the S Caribbean.

Buoy 42058 has E winds @ 17 knots

Puerto Cabezas near the Nicaragua/Honduras border has ENE winds @ 12 mph. This is to the SW of the buoy.

Bluefields Nicaragua which is much further S along the coast has N winds @5 mph

These obs are indicative of a surface circulation to the east of Nicaragua and the quikscat pass, that unfortunately missed much of the area, shows significant turning N of Panama.

Pressures are still relatively high though.
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1281. nash28
11:01 AM EDT on May 28, 2008
Barry was under 40-50kts of shear and a boatload of dry air. As I said last night. If there is a strong ULH moving with a Low, it can sheild it from the shear.
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1280. franck
3:00 PM GMT on May 28, 2008
moonlight...think you're right on.
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1278. cchsweatherman
11:00 AM EDT on May 28, 2008
Still no update out from the NHC regarding Invest 90E.
Member Since: April 14, 2007 Posts: 8 Comments: 5030
1277. cchsweatherman
10:59 AM EDT on May 28, 2008
MLC - Just remember, just as quickly as shear can rise, shear can fall just as fast.
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1274. smmcdavid
9:56 AM CDT on May 28, 2008
No kidding... I get so confused sometimes. lol
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1273. moonlightcowboy
9:51 AM CDT on May 28, 2008


Shear may be falling somewhat in the extreme sw and se Carib, but in the northern Carib shear tendency says its increasing 5-20 kts above present high conditions. I just don't see anything developing soon in the Caribbean from a 90E crossing (especially with land interaction) or other cyclogenesis - the latter being more likely, but very slow.
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1272. StormJunkie
2:52 PM GMT on May 28, 2008
Good deal smmc :~)

Just trying to get you in the habit of using the navigational portal instead of the over crowded bookmarks...lol

jp-You always have that link. See previous link ;~)
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1271. IKE
9:54 AM CDT on May 28, 2008
1270. JFLORIDA 9:53 AM CDT on May 28, 2008
Only the main Link Works.


I wondered why I couldn't get it to link...thanks......
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1269. TheCaneWhisperer
2:37 PM GMT on May 28, 2008
There's a lot of 850mb turning down there on both sides. This is day one of what the models we're anticipating, give it time to evolve. Shear is very low in the SW Caribb and EPAC, where development is anticipated. Very little movement will allow many updated shear maps to take place concerning the shear to the north. Which is now caused by a weakness in the ridge in the Western Atlantic.
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1267. smmcdavid
9:52 AM CDT on May 28, 2008
Thanks to everyone else for answering too... SJ just got to it first! Ha ha.
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1265. smmcdavid
9:50 AM CDT on May 28, 2008
Thanks SJ... I actually have that link, but didn't know that's where I need to look. You rock.
Member Since: September 20, 2005 Posts: 31 Comments: 2309
1264. nash28
10:50 AM EDT on May 28, 2008
Correct Kman. ULH is building down in the SW CB as the trough clears out.
Member Since: July 11, 2005 Posts: 190 Comments: 16972
1261. IKE
9:50 AM CDT on May 28, 2008
1257. smmcdavid 9:48 AM CDT on May 28, 2008
Thanks jp. But, can we find out if it's been upgraded to a TD before then? Is it posted anywhere or anything... sorry, this is one thing I'm really not familiar with.


The Navy site will usually update faster...usually .....

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1260. nash28
10:49 AM EDT on May 28, 2008
It's not TD yet. NRL would have it as NONAME. Still 90E.
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1259. IKE
9:49 AM CDT on May 28, 2008
SSD satellite is back working as of 1345 UTC.
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1258. StormJunkie
2:46 PM GMT on May 28, 2008
Morning smmc

That is when the official word would come, but most of the time we get an early alert that something has switched from invest to TD status by using the Navy NRL Monterey Tropical page. The wording of 90E. Invest would change to something like 01E NoName.

Second link under imagery here.
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1257. smmcdavid
9:46 AM CDT on May 28, 2008
Thanks jp. But, can we find out if it's been upgraded to a TD before then? Is it posted anywhere or anything... sorry, this is one thing I'm really not familiar with.
Member Since: September 20, 2005 Posts: 31 Comments: 2309
1256. apocalyps
04:48 PM CEST op 28 Mei, 2008
Will go west out in to the pacific
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1255. kmanislander
2:44 PM GMT on May 28, 2008
good morning everyone

The Caribbean is starting to look interesting. The shear tendency South of 15N has been falling and it looks as if a high is trying to build over the area.
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1252. smmcdavid
9:38 AM CDT on May 28, 2008
When will the NHC post the next outlook? Is that when we would find out if it was upgraded to a depression?
Member Since: September 20, 2005 Posts: 31 Comments: 2309
1251. Tazmanian
7:39 AM PDT on May 28, 2008
wind shear in the gulf is vary low this AM 5 to 10kt if any thing gets in the gulf it will have a good ch
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1250. stormkat
2:37 PM GMT on May 28, 2008
we will see jp...its impossible for something to form in the caribbean in the next week...it just wont happen guys...stormkat
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1249. StormJunkie
2:36 PM GMT on May 28, 2008
Morning all :~)

Good to see everyone.

I see the GFS is still trying to make an Atl side system out of this jumbled area in the Carib.

Cute little swirl out in the Catl as well....
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1248. Tazmanian
7:38 AM PDT on May 28, 2008
. stormkat this be came # 22 on my Ignore
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1247. apocalyps
04:37 PM CEST op 28 Mei, 2008
Will become TD and then pfffff....out.
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1245. IKE
9:36 AM CDT on May 28, 2008
1239. stormkat 9:35 AM CDT on May 28, 2008
ok jp whatever you say....you have your opinion ...i say the convection will be ripped apart by the the strong shear....stormkat


He's talking about 90E...in the east PAC...shear isn't strong there.
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1244. cchsweatherman
10:35 AM EDT on May 28, 2008
The most important thing to remember stormkat is that there is very little to no shear over the circulation right now. It appears that an upper-level high is indeed developing over the system, albeit rather elongated at this time. Current forecasts have this upper-level high building into the Western Caribbean as the upper-level trough in the Caribbean retreats into the Central Atlantic. So, don't rule anything out just yet there stormkat.

I must agree with JP that this is very near Tropical Depression, if not Tropical Storm status right now based upon the latest ADT numbers and satellite imagery.
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1243. Tazmanian
7:35 AM PDT on May 28, 2008
oh look oh is back


run for your lives
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1240. Tazmanian
7:34 AM PDT on May 28, 2008
take a look at the wind shear in the gulf its olny 5 to 10kt this AM
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1239. stormkat
2:32 PM GMT on May 28, 2008
ok jp whatever you say....you have your opinion ...i say the convection will be ripped apart by the the strong shear....stormkat
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1238. smmcdavid
9:33 AM CDT on May 28, 2008
Think it will make depression status today?
Member Since: September 20, 2005 Posts: 31 Comments: 2309

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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.