The Associated Press

LOUISVILLE - Unusual conditions are combining to make Kentucky's summer storms so damaging, meteorologists say.

"Something is going on and we haven't figured it out," said Doug Kluck, climate services program manager for the National Weather Service Central Region Office in Kansas City, Mo.

The type of weather seen in the area in the last two months, especially the intense storms carrying hurricane-force winds, is more commonly seen in March and April.

Meteorologists don't know whether the unusual weather will linger through the summer and fall, Kluck said. Forecasters say El Nino and La Nina aren't to blame.

"Summertime is the hardest time to predict the weather in North America," Kluck said. "There are some aspects to the climate regime we do not know. There just is not this big bubble of hot air over us this summer in the Midwest."

On Tuesday, the jet stream flow helped cooler northwest air collide with a wildly unstable mass of tropical air from the Gulf of Mexico. It formed a storm in northwest Illinois.

Dan Spaeth, a meteorologist with the weather service in Paducah, said the instability in the air built until its release became dangerous.

As the storm tore south, it sparked more than 280 reports of high winds in four states and eight suspected tornadoes in Illinois, one causing 10 injuries. There also were numerous reports of hail in Indiana.

Forecasters call the storm that hit Indiana and Kentucky a derecho, which is Spanish for straight ahead.

Benjamin Schott, a Weather Service meteorologist in Louisville, said storms and straight-line winds on the scale that struck Louisville may occur twice a decade. Similar storms have hit Kentucky in 1991, 1994 and 1996.

"It is something that some people will never see in their lifetime again, but for this area it will happen again," he said.

Kluck, the Kansas City forecaster, said derechos can and likely will come the rest of the summer across the central United States.

"Between the Appalachians and Rockies is where they grow and live," he said.