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Fall is known as a ‘second severe season’ for weather. Here’s why.

While spring is most active for severe thunderstorms, a second surge of activity accompanies autumn.

Homes destroyed by a tornado as seen from an aerial view on Dec. 12, 2021, in Mayfield, Ken. (Joshua Lott/The Washington Post)

Residents of the Plains and Deep South know to hold their breath during the months of April and May. That’s when the clashing seasons routinely bring swarms of severe thunderstorms, with bouts of destructive hail, damaging winds and tornadoes terrorizing the landscape. But autumn can offer a sneaky flurry of dangerous weather, and it’s known as a “second season” for strong to severe thunderstorms and tornadoes.

Tornado deaths trending down, but last December's outbreak shows enduring danger

High-end events aren’t as common as during the spring months, but tornadoes in the late fall and wintertime can be every bit as dangerous. In fact, there’s research to suggest tornadoes in this second severe season may be stronger in some instances, quicker-moving and able to cover more ground.

In the near term, meteorologists are already tracking the threat of some severe weather over the Corn Belt and Mid-South next week.

What are the ingredients for severe weather?

Severe weather requires two main ingredients: fuel (or “juice” for storms) and spin. The former, called CAPE, or Convective Available Potential Energy, is the energy that helps air parcels to ascend. CAPE is ordinarily greatest on warm, humid days. In the autumn and winter, it’s typically more meager than spring and summer.

Spin is needed to sculpt any updraft into a rotating storm. That comes from shear, or a change of wind speed and/or direction with height. In the presence of shear, any cloud that grows tall enough to span multiple layers of atmosphere will begin to twist, in some cases rotating like a barber pole. These clouds can become supercell thunderstorms capable of producing baseball-sized hail, hurricane-force winds and tornadoes.

Supercells result when a rotating thunderstorm can exist in isolation. Other times, storms explode along a boundary and merge, becoming a squall line. That happened last Dec. 15, when a line of violent thunderstorms brought 90 to 100 mph winds and a whopping 120 confirmed tornadoes to the Midwest, namely eastern Nebraska, Iowa and southeastern Minnesota.

Five days prior, a high-end severe weather outbreak produced an EF4 tornado that razed most of Mayfield, Ky., claiming 57 lives during its 2 hours and 54 minute rampage.

What happens in the fall?

In the fall, the jet stream strengthens and shifts farther south. The jet stream is a river of swiftly moving winds in the upper atmosphere. As winter’s chill builds in the polar regions to the north, it pushes the jet stream toward the equator as it “follows” the steepest north-to-south temperature contrast.

That means wind shear climbs markedly in the autumn. While CAPE is tough to come by, in the periodic instances that it is available, thunderstorms often become tornadic. The stronger jet stream translates to higher-end tornadoes (a greater proportion of “significant” tornadoes reaching EF2 strength or greater), which tend to move more quickly, because storm systems translate west to east at a swifter speed.

Late October, November and December tend to see an uptick in tornado events. According to the website U.S. Tornadoes, November has an average of 56 tornadoes. December averages 27 twisters — though that comes with enormous variability, since some years have hardly any, and others feature outbreaks.

There are some indications that human-induced climate change is also amplifying the extent to which insurances of warm, moist air can ride northward in the cool season, bolstering the availability of CAPE. That may have been a contributing factor to last year’s December severe weather outbreaks.

What’s the forecast for next week?

On Sunday, a strong shortwave, or pocket of high-altitude cold air, low pressure and spin, will dive over the western U.S. By Monday, it will pass over the Four Corners. That will enhance upward motion ahead of it. The jet stream, meanwhile, will be ripping overhead.

Meanwhile, that upper-air disturbance will enhance the development of a surface low pressure, ahead of which a sliver of acutely warm, moist air will ride northward over the Corn Belt. The National Weather Service Storm Prediction Center has already highlighted an area of level 2 out of 5 “slight risk,” which includes extreme eastern Nebraska and the Loess Hills of Iowa, as well as the Missouri River. That means places like Omaha and Council Bluffs, Iowa, could be in line for storms.

“[The] primary risk will likely be locally damaging wind gusts from late afternoon into the evening hours, within a narrow zone of [shower and thunderstorm activity] near/just ahead of the advancing cold front,” wrote the Center.

Then on Tuesday, a secondary surface low could develop over Oklahoma and move northeast, potentially bringing additional strong to severe storms somewhere in the Mississippi Valley. Confidence on that second episode, however, is low.

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