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Tornado Alley just had one of its slowest seasons ever

CNN logo CNN 1 day ago By Max Claypool and Judson Jones, CNN
a close up of clouds in the sky © KOCO

After a deadly start to the tornado season in the South, May and June were unusually quiet across the Plains, with near record lows for tornadoes, giving residents of Tornado Alley a rare reprieve.

Usually, central US states are hit by an onslaught of tornadoes in late spring, but this year that failed to materialize.

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Nationally, May saw 140 tornadoes, well below the average of 276, and the fewest in 50 years. And the month's two strong tornadoes, EF2 or greater, were also the fewest in recorded history.

In fact, this year there were more tornadoes in two days in April than in all of May. On Easter Sunday and the early hours of that Monday, 190 tornadoes, including 16 rated EF3 or higher, tore through 10 states, killing 36 people.

Typically, tornado season runs March through June, with peak activity shifting from the Southeast to the Plains after late April.

Tornado activity peaked in the Southeast early in the season. The South is more densely populated than Tornado Alley, which makes tornado outbreaks in this region especially deadly.

"You never want to see tornado outbreaks in the Southeast," said Jeff Frame, a teaching associate professor of Atmospheric Sciences at the University of Illinois.

It seemed that we were headed into a busy season as storm activity shifted to the Plains. But after April, things went quiet.

This storm chaser's photos all began with Bruce Springsteen

Weather systems over the Southeast in May and June caused areas of higher pressure to build to the west over the Gulf of Mexico. These high-pressure areas prevented moisture, a necessary ingredient for storm formation, from making it to the Plains.

"Without the really deep, rich, high-quality Gulf of Mexico moisture, it's really hard to produce a lot of tornadoes," Patrick Marsh, a Warning Coordination Meteorologist at the Storm Prediction Center said.

"It was one unfavorable pattern after another," Frame said. "It's rare to see something like this."

These unfavorable patterns continued into June. As of Tuesday morning, there were 108 tornadoes recorded this month, well below average. To date, that's the fewest since 2016, and the third-fewest since 1988.

The combined number of tornadoes for May and June was 232 -- one of the lowest ever. Now compare that to the 513 tornadoes in May of last year alone. It doesn't take much to see how quiet the two traditionally most active months have been.

The number of hail reports -- another hazard of severe storms -- was also down. They were the lowest in at least a decade and almost twice as low as the 10-year average.

The past decade has been marked by erratic tornado patterns, with huge year-to-year variations and numerous tornado outbreaks sandwiched between long dry spells. The most active years have been characterized by abnormally large or innumerable outbreaks that accounted for most of that year's tornadoes.

One of the quietest years on record was 2018. It was quickly followed by 2019 as one of the most active. This was followed by 2020, which was historically quiet, despite April's burst of activity.

Scientists are unsure whether climate change is playing a role. They aren't even sure if what is being seen is a trend.

"Time will tell whether (recent years are) an anomaly or a trend," Marsh said. "If we had talked in 2018, people probably would have told you it was a trend. Then 2019 happened and people said maybe it was an anomaly, and now we're in 2020 and people are saying maybe 2019 was the anomaly."

a close up of a map © NOAA a close up of a map © NOAA
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