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In the Deep South, ‘tornado season’ runs all year

Tornadoes in December aren’t as rare as you might think. In fact, they’re quite common.

Cathie Morris, right, tells Red Cross volunteer Laurie Howell that her sister was killed by one of the April 2011 tornadoes not far from where they’re standing in front of Morris’s house. The home was damaged by Monday’s storms in Limestone County, Ala. (Jeronimo Nisa/The Decatur Daily/AP)

At least three are dead following an outbreak of more than two dozen tornadoes — some rated EF3 strength — that battered parts of the Deep South on Monday. In addition to the damage, the storms left many scratching their heads, wondering whether tornadoes in December are unusual. They’re not overly rare — in fact, it’s the contrary. “Tornado season” for the southern United States runs most of the year.

As of Wednesday morning, at least 27 tornadoes had been confirmed across Louisiana, Mississippi and Alabama following Monday’s violent storms. Among them were three EF3 tornadoes on the 0 to 5 Enhanced Fujita (EF) scale, near Alexandria, La.; Laurel, Miss., and Sumrall, Miss.

Tornado outbreak sweeps across Deep South, leaves three dead

If we look at an average across the country, it’s true that tornado activity peaks during the spring and dwindles thereafter. But that’s across the nation as a whole. If we look at Gulf Coast states, however, the trends are a little different.

The first observation is an enhancement across our three big central Gulf states — Louisiana, Mississippi and Alabama — during November. This is the true “second season” across the Deep South and Interstate 20 corridor.

The storms that produce tornadoes — supercells — feed off a combination of ingredients. Among them is a temperature clash. During the spring months, much of the central United States cooks up big-time boomers as air masses meet. To the north and west, winter retreats; meanwhile, summer’s nascent warmth begins to surge northward in punctuated charges. That steep difference in temperature contributes to the instability that fuels severe thunderstorms.

In the wintertime, cold overtakes most of the Lower 48. The exception is along the Gulf Coast, where the reliably toasty waters keep adjacent land relatively mild well into everyone else’s traditional “cold season.” The greatest contrast occurs when late autumn cold fronts plow south.

According to U.S. Tornadoes, November is the third most-active month of the year for Louisiana, which they say averaged 5.2 tornadoes per November between 1991 and 2015. April narrowly clinched the top spot at 5.8 tornadoes. In fact, you’re more likely to be hit by a tornado in Louisiana during December, January or February — meteorological winter — than in June, July or August.

For Mississippi and Alabama, it’s a similar story. The summertime, despite having the most thunderstorms, is the quietest. Spring is most active, but late autumn/winter aren’t too far behind.

Why? Thank the jet stream. During the summertime, the upper-level jet stream meanders north, all the way up toward Canada. Warmth builds in to its south. That might make for more pop-up “air mass” thunderstorms, fueled by heat. Portions of Mississippi and Alabama can see upward of 50 to 60 days with thunder per year. But they’re ordinary thunderstorms. To whip up supercells, or those nasty rotating thunderstorms, you usually have to introduce some jet stream dynamics.

When the jet races overhead or passes nearby, its strong winds aloft boost wind shear. That’s a change in wind speed/direction with height. Wind shear is a necessary ingredient in setting storms spinning. The jet stream also “carries” the most energetic disturbances that can brew feisty thunderstorms.

In the winter, the jet stream moves south, back over the Lower 48. That’s why you’re much more likely to get a big coast-to-coast storm system pass overhead during the wintertime than the summer. With a jet stream close by and the region of greatest temperature clash near the Gulf Coast, it comes as no surprise we see tornado reports there in the winter months. (That threat zone shifts north during the spring months, arriving in “tornado alley” over the Plains by April or May as summertime warmth, and the jet stream, builds northward.)

The National Weather Service Storm Prediction Center maintains a climatology of where severe weather is most probable at different times throughout the year. It’s based on storm reports from 1982 to 2011. You’ll notice that the bulk of all severe weather to have occurred during this week in their three-decade window has historically been where Monday’s outbreak struck. That’s not a coincidence.

This is all nothing new. The Deep South has been dealing with fall and winter tornadoes longer than Louisiana, Mississippi and Alabama have been states.

Monday marked the 19th anniversary of a devastating F4 tornado that killed 11 as it ravaged southeast Tuscaloosa, Ala., on Dec. 16, 2000. An EF2 tornado hit Mobile, Ala., on Christmas Day 2012. Alexandria, La. — which saw an EF3 tornado on Monday — also got a deadly F4 on Dec. 3, 1953. Columbia, Miss., got an EF2 on Monday; it also had a deadly EF3 on Dec. 23, 2014.

So to clarify: In the Gulf states, tornado season runs just about all year round, taking somewhat of a breather during the summer. Saying “tornadoes don’t happen in the winter” is flat-out wrong — be wary of claims to the contrary.