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As deaths in South prove, there's no such thing as 'tornado season'

The five deaths that occurred at the end of November were the first tornado fatalities in the United States since May 9. That was the third longest such streak, and the length of that streak was obviously rare. What is not so rare is the occurrence of tornadoes late in the autumn in the south.

There are times of the year in which tornadoes become more common in different parts of the country, such as April through June in the southern and central plains. But there is no well-defined “tornado season,” and there are no months in which tornadoes simply don’t occur.

Here is the basic monthly climatology for the lower 48:

(National Climatic Data Center)

(National Climatic Data Center)



In late autumn, we often have factors in the south that still favor occasional tornadoes and tornado outbreaks. That has been particularly true this last week, with above-average temperatures across much of the country and the south, and a deep low pressure system passing to the north. A low-level, moist southerly jet off the Gulf was being overridden by a drier, cooler midlevel jet from the west, producing a type of directional wind shear that aids in the formation of supercell thunderstorms with rotation.

The threat of such an outbreak was seen some days in advance by meteorologists and was outlined by the National Weather Service Storm Prediction Center early in the week, on Sunday, Nov. 27. The outbreak occurred Wednesday.

The next day, the Storm Prediction Center widened the area at slight/15 percent risk for Wednesday.



And on the day of the event (the tornadoes occurred mainly late on the night of the 29th/early 30th), SPC raised the ante as indicators grew stronger (see map at left).

By the way, “slight” does not equate to insignificant at all. If someone told you there was a 15 percent probability you’d win in Powerball, you’d be racing to buy a ticket!

In fact, this did turn out to be a well-forecast event. Here are the counties under an SPC Tornado Watch as the outbreak was taking shape and before any of the major tornadoes had begun to form.
ww0508_radarIn the first tornado fatalities, the tornado struck sometime around or shortly after midnight, and the stricken mobile home dwellers had gone to bed, possibly unaware of the Tornado Watch issued hours earlier and the actual Tornado Warning issued before any of the major tornadoes developed.

While most Americans think of the wide-open plains as tornado alley, there is a particularly dangerous aspect in the climatology of tornadoes in the south. In the plains, the land cools after dark, and instability may go down. But the Gulf of Mexico is a heat engine that has an unchanging temperature at night, and it can provide heat energy and water vapor more readily all through the night, when people may be asleep.

When I mention climatology, that does not mean I am trying to tie such an event to global warming, however real it is … and it IS real.

In fact, I participated in a webinar earlier this year with Dr. Harold Brooks, senior scientist at NOAA’s National Severe Storms Laboratory and one of the world’s foremost experts on severe convective storms. The best evidence available from climate models and computer simulations show that as warming continues, we can probably expect more severe non-tornadic thunderstorms and flooding events.

However, the type of wind shear I’d mentioned earlier might occur less frequently. The total number of tornadoes might actually go down, but that there could be an increase in the number of those rare “bad” outbreaks when the shear did come together to produce rotation in thunderstorms. In other words, fewer tornado days in total, but a few more bad tornado days when they do occur.

As Brooks emphasized, none of this is carved in stone. I bring all this up because I don’t believe anyone can demonstrate a tie between this week’s tornadoes and the very real anthropogenic/human induced warming that is ongoing.

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