CHICAGO – A day after a swarm of deadly tornadoes tore across the South, scientists who study the maddening, unpredictable storms were left with few answers Thursday – other than to say this deadly month will be one for the record books.
Preliminary data from the National Weather Service show that nearly 300 twisters have touched down in April, smashing the existing record of 267 set in 1974.
The outbreak – the deadliest in nearly 40 years – that devastated large swaths of north central Alabama and other southern states appears to follow a historic pattern. Over the decades, the number of tornadoes occurring in the U.S. every year has climbed steadily – from a low of 201 in 1950 to a record high of 1,817 in 2004. Last year’s total was 1,525.
But the apparent increase in storm activity may be as difficult to explain as trying to determine the direction a twister will take as it races across the landscape, experts say.
Greg Carbin, Warning Coordination Meteorologist with the National Weather Service, said more tornadoes, particularly weaker versions, are being reported today than decades earlier. Urban development in the country’s tornado belt and the spread of information technology are factors in the higher number, he and other storm experts say.
So is improved thunderstorm meteorology, including the use of Doppler radar to better pinpoint a severe storm and measure its velocity.
The National Weather Service also has embarked on aggressive storm spotter training. At the same time, the numbers of hobbyists who chase twisters, laptops and cellphones in hand, have swelled along with trained local emergency management officials.
“There’s much better detection, basically,” said Bob Rauber, head of the department of Atmospheric Sciences at University of Illinois, Urbana-Champaign and lead author of the widely-circulated textbook, “Severe & Hazardous Weather.” “It’s not that there are more tornadoes.”
Four essential elements, he said, are necessary for twisters to develop: instability created by warm air at the surface and cold air at higher altitude; increasing wind speed at higher altitude; moisture at lower levels and, typically, a cold front.
All those factors were abundant for the mayhem in Alabama, Rauber and others said.
“If you were writing the book on what you were looking for, all those conditions (for tornadoes) were perfect, in a negative way,” he said.
Particularly distinctive and devastating were the wind shears, the churning winds that kept the storms on the ground for hours, said Morris Weisman, senior scientist at the National Center for Atmospheric Research. The most severe twisters carry winds of 250 miles per hour or more.
Some meteorologists suggest that La Niña, the unusually cold ocean temperatures in the equatorial Pacific, is playing a role in creating a stubborn blanket of cold air in the northern U.S. while warm air is pushing up from the south. The merging of those two systems through the Mid-South and Midwest helps create a powerful jet stream where storms frequently can erupt, according to some meteorologists.
But Rauber and other experts dismissed that theory, contending that La Niña and its counterpart, El Niño, have little if any effect on stormy Midwestern and Southern springs.