Another factor was forecasting, which has steadily improved with the arrival of faster, more powerful computers. Scientists are now better able to replicate atmospheric processes into mathematical equations.
In the last decade alone, forecasters have doubled the number of days in advance that weather experts can anticipate major storms, said Bill Bunting of the National Weather Service.
But Bunting, forecast operations chief of the service’s Storm Prediction Center in Norman, Okla., said it was not until Saturday that the atmospheric instability that turns smaller storm systems into larger, more menacing ones came into focus.
Information from weather stations, weather balloons, satellite imagery and radar told scientists that there was more than enough moisture — fuel for storms — making its way northeast from the Gulf of Mexico.
Despite Sunday’s destruction and at least eight deaths, 2013 has been a relatively mild year for twisters in the U.S., with the number of twisters 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 received by the weather service office 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 and National Severe Storm Lab 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 storm lab.
The outbreak occurred 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 tornadoes.
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 storm hit more places before it petered out, affecting more people, but it might have been slightly less damaging where it hit because it was moving so fast, he said.