Climate Change Opens the Calendar to More Opportunities for Severe Weather

As a former meteorologist and on-air talent in multiple markets, I’ve seen a lot of weather patterns. You don’t often hear “December” and “tornado outbreak” in the same sentence. December, in fact, is typically the month with the fewest twisters in the U.S. — all the better for a safe Santa sleigh ride. But with a pair of devastating tornado outbreaks that produced a whopping 157 recorded twisters, last month became the most active December for tornadoes since NOAA began keeping records in 1950. The previous record of 98 tornadoes was set in December 2002.

A tornado outbreak is a storm system that unleashes six or more tornadoes. Many of the outbreaks this December sent tornadoes spiraling across Nebraska and Iowa, spawned alongside what’s known as a derecho — a long-lived and powerful line of thunderstorms that causes severe wind gusts over hundreds of miles. These kinds of storms require a large swath of warm, moist air along with sufficient wind shear, where wind direction and strength change with altitude. These dynamics typically come together a few times a summer — it’s highly unusual to see them in winter.

Some of these were also “long-track tornadoes,” with one that started in northeast Arkansas and remained on the ground through the Missouri bootheel and northwest Tennessee for 80 miles. That same cell then produced another long-track tornado across Kentucky for 167 miles, which devastated communities such as Mayfield, where we saw countless pictures and videos across social media and the nightly news. If we loosely define long-track tornadoes as those on the ground for 25 miles or more, they account for only about 2.5% of all twisters. Most twisters last less than 10 minutes and have an average damage path of about 3.5 miles.

It’s fair to say these kinds of long-lived twisters are uncommon any time of year, but they are essentially unheard of during what is usually a cold, uneventful month for severe weather.

Unusually warm temperatures were one of the main drivers for these impactful severe weather events across much of the U.S. in December. Severe storms thrive off warmer conditions, higher humidity, and wind shear. While those ingredients are abundant in the springtime, they’re not typically found very often during the winter months.

To get severe weather outbreaks like this in December, you need to hit the bullseye on a dartboard. But what if we were to make that target bigger? Instead of the size of a quarter, how about the size of, say, a cookie? (Peanut butter would be my preference.)

Could you still miss hitting the dartboard entirely? Sure — and I speak from experience. But the odds of landing a bullseye are much higher.

This is what’s happening in our warming climate. Although we traditionally think of spring as the most active time of year for severe weather in the United States, the warm and moist air needed for severe storms and tornado development will likely be present more frequently year-round, increasing the odds for severe weather setups in the winter.

Compared to extreme weather events like heat waves, extreme cold, and severe flooding, we understand less about the connections between climate change and tornadoes directly. Severe weather events are also much smaller — an individual thunderstorm or tornado is dwarfed by a region-wide heat wave, drought, or large flooding event. And, of course, linking any individual storm to climate change is particularly complicated.

As the odds of severe weather setups increase throughout the calendar year, we should expect more frequent wintertime severe weather events in our warming world.