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Those scary “swarms” of tornadoes that have swept across the nation’s midsection over the last 10 days are manifestations of one of the most discernible changes in tornadic weather in recent decades.

That’s according to one of the nation’s foremost experts on tornadoes, Harold Brooks, a senior research scientist with the National Severe Storms Laboratory, which is funded by the National Oceanic and Atmospheric Administration.

Nearly every day since May 17 — when a line of tornadoes touched down in southwest Nebraska — some part of the Great Plains and Midwest has been under threat of tornadoes. At least six people have died, one in Iowa, three in Missouri and two in Oklahoma.

During that period, 248 tornadoes had been reported to the National Weather Service. (Tornado reports are just that — sightings or damage — and must be verified to weed out duplicates and false reports. From a starting point of 288 reports between May 17 and Sunday evening, the “whittled down” number stood at 248.)