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Midwest storms: Better forecasts, church services saved lives (video)
Despite Sunday's destruction, 2013 has been a relatively mild year for twisters in the U.S., with the number of tornadoes running at or near record lows.
So far this year, there have been 886 preliminary reports of tornadoes, compared with about 1,400 preliminary reports usually sent to the weather service by mid-November.
Similar slow years were 1987 and 1989.
An outbreak like the one that developed Sunday usually happens about once every seven to 10 years, according to tornado experts at the National Weather Service's Storm Prediction Center in Norman, Okla.
There were similar November outbreaks in 1992 and 2002, with the 1992 one being even bigger than this year's, said top tornado researcher Harold Brooks at the National Severe Storms Laboratory, also in Norman.
The storms erupted because of unusually warm moist air from Louisiana to Michigan that was then hit by an upper-level cold front. That crash of hot and cold, dry and wet, is what triggers twisters.
Like most November storms, this one was high in wind shear and lower in moist energy. Wind shear is the difference between winds at high altitude and wind near the surface.
Because it was high in wind shear, the storm system moved fast, like a speeding car, Brooks said. That meant the system hit more places before it petered out, affecting more people. But in places where it hit, the system may have been slightly less damaging because it was moving so fast, he said.
About 90 minutes after the tornado plowed through Washington, rain and high winds slammed into downtown Chicago, prompting officials at Soldier Field to evacuate the stands and order the Bears and Baltimore Ravens off the field. Fans were allowed back to their seats shortly after 2 p.m., and the game resumed after about a two-hour delay.
Babwin reported from Chicago. Associated Press writers Seth Borenstein in Washington, D.C., and Tom Murphy in Kokomo, Ind., contributed to this report.