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.)

More than three-fourths were concentrated in just five days. Brooks said it’s likely, once all the numbers have been vetted, that three or four of those days experienced “swarms” — 25 or more tornadoes in an outbreak. That’s as many as typically happens in a year, nowadays.

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Back in the 1970s, there might be one day a year with 25 or more tornadoes. In recent years, that’s jumped to four days in a year, “a really large change,” Brooks said.

In the last 15 years, there haven’t been fewer than two days a year with tornado swarms, he said.

Other changes occurring with tornadoes are a drop in the number of days when they occur, big swings year-to-year in numbers, and a slight eastward shift in occurrence.

“We have fewer days with tornadoes, but more big days,” Brooks said.

In the 1970s, there were about 150 days per year in which an EF-1 or stronger tornado occurred. Now that’s down to less than 100 days. An EF-1 tornado has wind speeds of 86 mph to 110 mph. At those wind speeds, a tornado can strip off roofs, overturn or significantly damage a mobile home and otherwise damage homes.

Another change is large swings, from year-to-year, in numbers of tornadoes. Sometimes there are “tornado drought” years, and then others see significant numbers.

Additionally, there’s been a statistically noticeable shift eastward in tornadoes — away from Tornado Alley. States like Nebraska, Kansas, Oklahoma and Texas are seeing fewer tornadoes, overall, while there’s an increase in states like Illinois, Indiana, Kentucky, Tennessee, Missouri, Arkansas, Alabama, Mississippi and Louisiana. Brooks cautions that residents of Nebraska and other states in Tornado Alley shouldn’t relax their guard, because the risk remains real.

What hasn’t changed, though, is the overall number of damaging tornadoes — on an annualized basis, he said.

Connecting these changes in tornadoes to global warming — or finding some other explanation — is difficult, Brooks said. And although scientists are unable to draw a direct line between the increased variability in tornadoes and global warming, that’s what climate models say will happen in a warming world.

While the science on tornadoes remains unsettled, there’s a clear trend with severe weather, Brooks said. People can expect more damaging windstorms and probably more hailstorms as the planet warms.

Overnight tornadoes are rare, but as the past two nights have shown, they can be deadly

A crucial factor in tornado formation is something called “wind shear,” which refers to changes in wind speed and direction, from the ground up into the atmosphere. Researchers believe that changes in wind shear are the driving factor in changes in tornadic weather.

Something that has been somewhat unusual this month has been the day-after-day worry about storms and tornadoes.

Such a prolonged stretch of storms — severe weather lasting a week or two over a broad area — typically only occurs about once or twice a decade, Brooks said.

The Omaha metro area is at risk of severe weather through Tuesday night, and maybe again Wednesday afternoon. But by Thursday, there should be at least a brief break from stormy weather.

Nancy Gaarder helps cover public safety and weather events as an editor on The World-Herald's breaking news desk. Follow her on Twitter @gaarder. Phone: 402-444-1102.

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