Tornado Season Intensifies, Without Clear Scientific Consensus on Why

KANSAS CITY, Mo. — All the warning sirens echoing across the Great Plains, Midwest and Southeast this month leave little doubt that the tornado season — which has plowed a trail of destruction through communities from Oklahoma to Wisconsin to Georgia — is off to an unusually busy start.

So far this year, tornadoes have killed 41 people and torn apart countless neighborhoods and, this weekend, one major airport.

Now, as the country braces for several more days of potentially violent weather, meteorologists say the number of April tornadoes is on track to top the current record. There have been, according to preliminary estimates, about 250 tornadoes so far this month and, in all likelihood, more are still to come, said Greg Carbin, the warning coordination meteorologist for the National Weather Service.

“It’s unusual but it does happen,” said Howard Bluestein, a meteorology professor at the University of Oklahoma who specializes in tornado research. “This isn’t a sign that the world is about to end.”

Those same experts note that drawing conclusions about the true size of, or reason for, an increase in tornado activity is difficult because historical statistics are unreliable due to changes in the way storms are tracked and measured.

Although the average number of April tornadoes steadily increased from 74 a year in the 1950s to 163 a year in the 2000s, nearly all of the increase is of the least powerful tornadoes that may touch down briefly without causing much damage. That suggests better reporting is largely responsible for the increase.

There are, on average, 1,300 tornadoes each year in the United States, which have caused an average of 65 deaths annually in recent years.

The number of tornadoes rated from EF1 to EF5 on the enhanced Fujita scale, used to measure tornado strength, has stayed relatively constant for the past half century at about 500 annually. But in that time the number of confirmed EF0 tornadoes has steadily increased to more than 800 a year from less than 100 a year, said Harold Brooks, a research meteorologist at the National Severe Storms Laboratory.

In April 1974, for example, there was a record 267 tornadoes reported, but the actual number that occurred is believed to be closer to 500.

“Today we seem to know about every single tree branch knocked down,” Mr. Carbin said. “We have eyes everywhere, and we have radar and satellite. It would be very difficult for a tornado to sneak through unnoticed.”

Tornadoes form when warm moist air combines with powerful dynamic winds inside a thunderstorm, sending a funnel cloud spinning toward the ground. They are most common in spring, typically peaking in May.

Though scientists believe that climate change will contribute to increasingly severe weather phenomena, including hurricanes and thunderstorms, there is little consensus about how it may affect tornadoes.

It remains unclear, partly because of the lack of historical data and partly because of their unpredictable nature, whether they will increase in number or strength or geographic range.

The large number of tornadoes so far may simply reflect normal variability, said Mr. Brooks.

Those assurances do not mean much to people like Kandice Shaw, a frequent business traveler who arrived at her hometown airport in St. Louis to find most of the windows boarded up and many other signs of storm damage. She worried about the increase in violent weather this spring. “We’ve had nothing but tornadoes,” she said. “I feel like I’m living in the Land of Oz.”