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How the multi-state December tornado formed

How the multi-state December tornado formed
Twisted metal and chunks of concrete crushing vehicles, are all that is left of the Amazon Hub in Edwardsville, Ill., on Saturday. Photo by Bill Greenblatt/UPI | License Photo

The deadly tornado outbreak of Friday spanned multiple states in the Southeast and left hundreds of miles of devastation. With tornado outbreaks of this caliber typically occurring between February and June, many are left to wonder how such a devastating outbreak could occur in December.

AccuWeather's Director of Forecast Operations, Dan Depodwin, said the outbreak that occurred in the Southeast was "a very rare situation."

"We talk about these types of events with numerous tornadoes...several violent with what appears to be violent EF4 or maybe EF5 tornadoes. Those typically occur in the spring time in the February through June time frame in most cases," said Depodwin.

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Severe weather can happen this time of year, but Depodwin says it's typically confined to the Gulf Coast region and sometimes a bit further north compared to the outbreak that just occurred in the Southeast.

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"I think what really surprised me...and struck most of the weather community about this outbreak yesterday was how far north they came and how violent the tornadoes were," he said.

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An average of two dozen or so tornadoes are reported in the United States during the month of December. The multi-state outbreak is shaping up to be much more than average and much farther north than usual. In addition, the number of fatalities that the outbreak may have caused is exceptionally rare, as the month of December rarely sees tornado fatalities at all.

The unusual warmth in the north that preceded the outbreak was one of the main ingredients that drove it to occur. It's typically not so warm this time of year for that region, so outbreaks are usually unable to form.

"I think the warm December certainly played a role. We haven't had a lot of cold fronts make their way all the way down into the Gulf of Mexico yet so that can sometimes help cool off the sea surface temperatures," said Depodwin.

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The Gulf of Mexico being at or above normal this time of year can further aid in the transport of warmer air northward, which is what helped fuel the multi-state outbreak seen Friday.

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As the warmer air from the gulf moved north, a very strong low pressure area was moving out of the Rockies and into the Great Lakes region. This pulled the warm and moist air north.

Depodwin said the four main ingredients for severe weather were prevalent to create the outbreak.

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"You need moisture, which we had from the Gulf of Mexico. You need instability, rising air. You need colder air aloft, we had that. You need some type of lifting mechanism, a cold front in this case. And then you need some type of turning in the atmosphere or windshield, as we call it," he explained.

All four ingredients have to come together perfectly to have a strong outbreak, as they did on Friday.

"This sort of reminded me of a March or April type setup, certainly something we don't see often at all. In December...we talk about December here. We don't typically have a lot of warm air," he said.

As for later this month, the potential for severe weather continues to exist. Next week will see a storm eject from the Rockies, bringing more warm moist air north. That air will collide with colder air to the north, creating a severe weather threat.

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The spots to watch for that potential severe weather outbreak will include portions of northern Missouri, Iowa and potentially parts of southern Minnesota.

"If you live in any parts of the middle of the country here or the Mississippi Valley in the next week or so, we have to keep an eye on one or two chances for severe weather. And then, as we head more into January, we typically see a low in the severe weather before it ramps back up," said Depodwin.

As for the possibility for another outbreak like last Friday night, Depodwin says it's unlikely.

"There can certainly be more tornadoes this month, though," he said.

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