April sets record for tornadoes in US

April 30, 2011 at 12:00a.m.

Chicago Tribune

CHICAGO

A day after a swarm of tornadoes tore across the South killing hundreds, 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 cell phones 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.

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 said.