Iowa seeing rash of tornadoes in 2011

Like hard-hit states to the south, Iowa has seen more than its usual share of tornadoes this season, although without the death toll.

"They don't usually hit urban areas, because we don't have as much urban areas to hit," State Climatologist Harry Hillaker.

This year's 37 Iowa tornadoes outnumber the 33 recorded over all of 2010, Hillaker said, "although last year was a very quiet year."

"We've had two very quiet years in a row, but this year's off to an active start," he said.

Twisters that hit Mapleton in Monona County April 10 and Lenox in Taylor County May 11 were pr0bably the most destructive so far this season, but an April 9 tornado in Pocahontas County was the most powerful.  That tornado,  an EF-4 on the five-level rating scale, with winds up to 200 mph, destroyed a home west of the town of Pocahontas.

"If that would have hit an Ames or an Ankney, it would've been a different story," Hillaker said.

The nation's stormy spring is typical for a waning La Nina, Hillaker said. The La Nina weather pattern is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific.

Nationwide, the National Weather Service has confirmed 947 tornadoes this year, with more than 500 deaths. That makes this the deadliest tornado year since 1953 and the seventh-deadliest on record.

Since 1980 Iowa has averaged 48 tornadoes a season, most between March and July. This year's pace is more than double the usual rate, Hillaker said. The state saw 120 tornadoes in 2004, 105 in 2008.

Otherwise, this spring averages out as...average, although it may not seem that way.

"It's been extremely variable," Hillaker said. "We haven't spent much time with normal temperatures. We spent half the time pretty warm and half the time really cool."

Hillaker noted the state has already recorded a 100-degree temperature, at Jefferson May 10. That's the earliest 100 degrees since 1980.

Through Thursday, the statewide average from 150 recording sites is "basically the middle of the road," less than a half-degree cooler than normal, Hillaker said. Precipitation has also been "almost exactly normal."

Climatologists classify spring as March through May, although traditionalists may prefer the June 21 solstice as the first day of summer.

Waning La Nina summers "have very, very slight tendency historically to be a little cooler," Hillaker said. "A little drieer for Iowa, but it's not much more than a coin flip."