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Some 600 tornadoes are believed to have struck the United States in April, the most in any single month since records started being kept decades ago.
The carnage from last week’s twisters in the South — at least 341 deaths — was the worst since March 18, 1925, when 747 died in storms that raged through Missouri, Illinois, and Indiana.
Ohio’s 17 tornadoes for 2011 already have surpassed the state’s annual average of 15.
And tornado season has barely begun.
All of the above seems to suggest that Ohio, Michigan, and the rest of the Midwest — which traditionally takes the greatest wrath from tornadoes — are in for a pounding as 2011’s season of violent weather peaks this month.
But, according to one of the nation’s top tornado experts, that’s not necessarily the case.
“What happens early in the season is not a good indicator to what happens later in the season,” said Harold Brooks, a meteorologist who specializes in tornado research at the National Severe Storms Laboratory in Norman, Okla. The lab is operated by the federal government’s National Oceanic and Atmospheric Administration.
Jet streams “change every week or so,” he said, explaining that the one in the South has been unusually strong.
“We don’t know what will continue into the future. It’s beyond our ability to predict,” Mr. Brooks said.
The United States typically has 1,300 tornadoes a year, killing 65 people.
Each state has one expert designated as its official climatologist, and Jeffrey Rogers, an Ohio State University professor of geography and atmospheric sciences, is Ohio’s.
He agreed that tornadoes are one of nature’s hardest things to predict but said Ohio could be headed for trouble if the jet stream in the South hangs together as it makes its way north.
Tornadoes typically strike between early April and late June, although an occasional one occurs outside those months.
They are a complicated collision of warm and cool air masses, plus a variety of other atmospheric conditions, as cold air moves out and warm air moves in.
But they are such a fluke that experts say the mere presence of the right conditions doesn’t guarantee they’ll form.
They are hard to distinguish from one another, in part because they traverse great distances quickly — often in the night or when vision is obstructed by thunderstorms — and sometimes last for only seconds.
By the numbers
Consider this: More than 750 tornadoes were reported in April. Government climatologists assume some were unconfirmed or were reported multiple times.
The National Severe Storm Laboratory’s estimate of about 600 for April is really just a ballpark figure, based on experience and a formula applied to what’s been reported.
State Emergency Management Agencies have until the end of June — 60 days past the last day of the month of the April storms — to report their findings.
Though the actual figure is still weeks away, climatologists say they feel confident that this April’s number will at least surpass the single-month record of 530 tornadoes, set in May, 2003, Mr. Brooks said.
The previous high for any single April on record was 267, set in 1974.
Some reports have tried to correlate tornadoes to La Nina weather events, in which South Pacific water near the Equator becomes cooler than normal.
That changes the dynamics of the atmosphere across the globe for months at a time.
The federal Climate Prediction Center said last month that La Nina conditions were weakening but could continue to affect weather for months.
Mr. Brooks noted that 1974 was a La Nina year. The Associated Press reported that a series of twisters killed 315 people in 11 states in April of that year.
“La Nina probably plays a role,” Mr. Brooks said, adding that it affects the position and strength of the jet stream.
“That said, it’s not a really good predictor. It’s like you’re weighting a coin so that it’s heads 55 percent of the time, not 50-50,” Mr. Brooks said. “It appears [La Nina] has an impact, but there are years it has more impact than others.”
The United States has more tornadoes than any other country because of its geography: It is the only place where warm air from the Equator and cool air from Canada collide with dry air from the Rocky Mountain region.
As difficult as it is making cause-and-effect correlations between La Nina events and tornadoes, it’s even more difficult pinning the blame on climate change.
Tony Del Genio, a climatologist at NASA’s Goddard Institute for Space Studies in New York City, is lead author on a groundbreaking 2007 report about climate change and storm intensity.
He told The Blade in an interview last year that tornadoes are “really beyond the edge of our understanding of things.”
Mr. Del Genio and other scientists have said they can safely predict more storms, more hurricanes, more violent weather in general as the Earth’s climate warms.
But they draw the line at tornadoes because of their complexity and the energy needed to create them.
The formation of tornadoes requires hot, humid air near the ground plus a cool-air mass above.
They also need, among other things, strong wind velocity at higher altitudes, known as wind shear, to get them spinning.
Many of those ingredients are likely to increase in abundance as the Earth’s climate warms over the next 50 to 100 years.
But, as Mr. Del Genio noted, a more frequent convergence of tornado-inducing atmospheric conditions doesn’t necessarily correlate to more twisters.
All it does is improve the odds.
Close to home
Mr. Brooks has noted that the Great Lakes region is especially difficult for long-range predictions because of the fickle relationship between its enormous bodies of water and the air masses hovering over them.
A warmer climate will no doubt bring more humid and unstable air masses, which tornadoes need to form. But wind shear, or gusts, may not be as strong in the future, Mr. Rogers said.
“The big issue is tornadoes are events that last at best a few seconds,” Mr. Rogers said. “It’s a big difference in time scales. That’s what makes it hard to link the two.”
Chris Weiss, a tornado expert at Texas Tech University, said that “trying to relate a climate signal to a specific weather event is always dangerous.”
Information form The Blade’s news services was used in this report.
Contact Tom Henry at: firstname.lastname@example.org or 419-724-6079.