The tornado season of 2008: climate change to blame? And, tropical update
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
Missing House, if found call Dennis and Carla
Nice structure on upercell east of Pratt, Kansas. Photo copyright Mike Theiss.
Reader Comments
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Which graphic is that?
GFS) still show some development in the western
Caribbean, however have pushed this back until the
first week of June. The NOGAPS brings a Tropical
Storm up to the Yucatan Channel at 144 hours out, and
the GFS brings the system over SE FL late in the first
week of June. I cannot rule this out completely at
this time, as some of the computer models do reduce
wind shear around that time to marginal to slightly
favorable levels close to the end of that time frame.
As far as any development in the western Caribbean
within the next few days...it is slim at this
time...as with the EPAC system so close, the outflow
from the upper levels will have a tendency to stifle
anything in the western Caribbean. I will continue to
monitor this area over the next week.-StormW
jp, you say the voice of reason, but I think this is exactly what GS and myself were referring to. So now I am even more confused as to what we are debating?
Link
That is PDT not EDT, 11AM PDT - 2PM EDT
From the Mobile,AL morning discussion...long-term.....
"The airmass will be fairly capped off through
much of the weekend. We bring the slight chance probability of precipitation back into the
picture in the early next week as a weakness is projected to form on
the east side of the upper ridge...giving a somewhat greater
potential for any mesoscale system developing along the southern
edge of the stronger midlevel westerlies to come down the
northwesterly flow aloft into Mississippi and Alabama."
I will not be surprised to see 90E come ashore on the W. Coast of Costa Rica within the next 24 hrs, with the possibility on a new surface low center forming in the SW Caribbean. What happens after that remains to be seen (a bridge too far for my tastes).
BBL.
That area is a slow roller and it will just have to fester for a while if anything is to come of it.
I'm thinking 90E will strengthen somewhat, possibly get named, eventually moving n, wnw, nw, and then dissipate with land interaction.
How all of us learn. Heck sometimes I play devil's advocate just so I can make sure there is nothing I am missing on the other side of what I perceive to be going on.
Always a informative read!
Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.
Should be interesting but are you prepared for Hurricane season?
This shows the MJO pulse weakening.
The E-Pac/Caribbean system looks to be splitting apart now. There is a definite circulation in the E-Pac but there are hints of a circulation in the SW Carib. Topography will assist anything developing in the SW Carib.
Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.
That last part of your post depends upon how developed the tropical system would be at the time it would enter the GOM. Dry air is real damaging to developing systems, but if a system has already developed, it usually can produce its own moisture and dry air will not be a great inhibiting factor.
I hope this is not a dumb question. I am trying to learn from all of you. What is the MJO pulse and what effect does it have on the tropics?
Shear is very low in the GOM but not nearly as low as atmospheric moisture content. The moisture that has filled in the Caribbean over the last 48 hrs. would have to spread northward into the GOM for a tropical system to have a chance to develop there.
...Amen!
The MJO is interesting. It can create additional moisture that aids in tropical development this time of year. I'll post this from the CPC and a couple of links so you'll get a better understanding. I'm still learning about it myself.
What is the Madden-Julian Oscillation (MJO)?
The Madden-Julian Oscillation (MJO) is a tropical disturbance that propagates eastward
around the global tropics with a cycle on the order of 30-60 days. The MJO has wide
ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric
circulation, and surface temperature around the global tropics and subtropics. There is
evidence that the MJO influences the ENSO cycle. It does not cause El Nino or La Nina,
but can contribute to the speed of development and intensity of El Nino and La Nina
episodes.
MJO defined and characteristics.
MJO conditions and forecasts
I blogged on it a few weeks ago here.
Thanks for taking the time to do all of that. It is greatly appreciated. I have some studying to do on my lunch hour!
I'm enjoying the reading and can now interject my comments and opinions. The tornado footage last week was amazing and appreciate those that posted the links.
BTW, to my eye it looks like we've got some splitting of the low(s) in the EPac and SW Carib. But I have to agree with 456, it appears that the total "draw" is to the Carib of all the energy. Time will tell.
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