With this month's tornado tally already up to about 600, April 2011 has hit the record books as the busiest calendar month for twisters since the beginning of modern record keeping in 1950, even though May and June are usually the most active months for tornadoes.
It has also been one of the most lethal. Wednesday's torrent of tornadoes killed over 280 people, making it the sixth deadliest tornado-day in American history, said Harold Brooks a research meteorologist at NOAA's National Severe Storms Laboratory in Norman, Okla. The last time that many people died from tornadoes in an entire year in this country was 1974. The average annual death toll is about 60.
Beyond the dramatic YouTube videos and the devastating stories of lives lost, the recent swell of tornado news has left many people wondering what's driving the sudden swirl of destruction. Does global warming have anything to do with it? And how much more extreme is it going to get?
For better and for worse, experts say, these are questions that they can't answer with certainty. Climate change doesn't act in any particular way on the level of tornadoes. What's more, the frequency of tornadoes in one month does not predict how many twisters will strike in the months that follow.
Instead, this month's turn of events is a result of a sort of perfect atmospheric storm. Unusually strong winds have been persistently blowing eastward out of the southwest, leading to a layer of relatively cool and dry air lingering aloft above the Midwest and southeast. Beneath that layer, stalled cold fronts have pushed a mass of warm, moist air further north than usual.
This particular mixture of conditions -- with cool, dry winds consistently gusting above warm, moist air -- has produced an environment that is ripe for the formation of tornadoes.
"It's sort of your classic combination that can produce large death tolls," Brooks said. "Large tornadoes that are very strong are moving through populated areas in a part of the country that has a high mobile home population. It's a combination of factors that we fortunately don't see very often."
A weakening La Niña may explain much of this April's damaging nature, said Paul Douglas, meteorologist and founder of Weather Nation, a weather outsourcing company in the Twin Cities, Minn. The natural and cyclical phenomenon, which begins with cooler than normal waters in the eastern equatorial Pacific, has energized the jet stream. For late April, overhead winds are now blowing faster and further south than usual.
Meanwhile, water temperatures in the Gulf of Mexico are high for this time of year, which has helped push more warm, moist air into the lower atmosphere. There is also currently an unusually sharp temperature gradient between a colder than normal north and a hotter than average south.
Together, these phenomena set the stage for supercell thunderstorms, along with large and violent tornadoes, Douglas said. The La Niña springs of 1974, 1999 and 2008 likewise featured an excessive number of destructive tornadoes.
Climate change may play a small role, Douglas said, as research suggests that a warming atmosphere makes extreme weather events more likely to occur. Still, as tempting as it may be to blame global warming for this year's wild twisters, there is no conclusive data to make the link between a long-term global trend and a single day's events.
In the age of Twitter and YouTube, he added, people are far more likely to hear about tornadoes than they were a generation ago. That only exacerbates the sense that twisters are happening more often.
"It's human nature to want a smoking gun, a simple explanation to a very complex atmospheric challenge," Douglas said on Thursday -- one day after the record-setting suite of storms. "Climate change may be one of the many contributing factors, but La Niña was probably a much more significant player in yesterday's historic outbreak."
"Tornadoes require a very specific menu of weather ingredients," he added. "And yesterday, all those factors converged on the Deep South."
As for what's to come, no one can say for sure. When it comes to tornadoes, Brooks said, meteorologists can look only as far as a few days ahead. The good news is: Short-term conditions look much calmer than they've been over the last few days. But after that, it's anyone's guess.
"Essentially, if you look at any time year and see how many tornadoes have occurred from the beginning of the year until now and you try to predict how many tornadoes will occur for the rest of the year, the correlation is basically zero," Brooks said. "It's unfortunate, but we just can't do that."
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