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How ‘Perfect Storm’ Led to Weekend Twisters

ABC News(NEW MINDEN, Ill.) -- After one of the quietest U.S. tornado seasons in 40 years, Sunday was nature's comeback, with a total of 81 tornado reports in Illinois, Indiana, Kentucky, Missouri, and Ohio.

Illinois was the hardest hit with 43 tornadoes, followed by 23 in Indiana, 13 in Kentucky, one in Missouri and one in Ohio.

According to the National Weather Service's preliminary ratings, New Minden, Ill., in the southern part of the state, was in the swirl of an EF-4 tornado, with winds of at least 166 mph. In Washington, Ill., the tornado was also rated EF-4, with winds from 170 to 190 mph.

According to the climatology of U.S. tornadoes in the Midwest, twisters of such force were unusual for this time of year. In the lower 48 states, the peak of severe weather and tornadoes usually occurs in April and May; November is known as the second peak for severe weather.

On average, the entire United States usually gets hit with 35 tornadoes in November. Only seven tornadoes on average occur in Midwestern states in November, and only one on average strikes Illinois in November. More than 140 tornado warnings were issued Sunday from this storm.

But it was a perfect atmospheric setup: strong jet stream aloft (more than 100 mph), blowing from the southwest to the northeast over the Midwest, which helped to lift the moist air at the surface coming from the Gulf of Mexico, south to north.

When these elements combine, they create "twist" in the atmosphere conducive to tornado development. As a strong cold front moved through this unstable environment, it helped to lift the moist, warm air. The result was violent, large and long-tracked tornadoes, seen more typically in April or May.

Copyright 2013 ABC News Radio

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