Over the weekend, 19 people were killed in violent thunderstorms that spawned multiple tornadoes across the South. Over 350 reports of severe weather were logged by the National Weather Service, clustered over Louisiana, Mississippi, Alabama, Georgia and Florida.
The stormy region has been dubbed “Dixie Alley” and statistics point to a sobering reality: It accrues on average more annual tornado warnings, greater numbers of strong and violent tornadoes, and more tornado fatalities than its cousin “Tornado Alley” in the Great Plains.
In Dixie Alley, tornadoes occur more frequently in winter and at night, when they are especially deadly.
Wintertime and nighttime tornadoes are possible in the South because the region abuts the Gulf of Mexico. The warm, wintertime waters breed unstable air masses. Low-pressure systems draw these unstable plumes northward, so the arrival of unstable air is not necessarily tied to the season or time of day.
Jet stream winds are particularly powerful during the winter months, howling across the southern United States at high speeds. These powerful belts of wind create strong wind shear (changes in wind direction and speed with altitude), which helps build and sustain violent storms.
The overlap between strong shear and warm, unstable air plumes often sets up along the Gulf Coast states — a recipe for the rotating thunderstorms we call supercells and their tornado progeny.
Very early in the morning Saturday, a strong tornado — preliminarily rated EF3 — struck Hattiesburg in southern Mississippi.
As the image above shows, the weather map was rather unimpressive at first glance: Multiple warm fronts extended along the Gulf Coast, and a small, weak area of low pressure (called a mesolow) helped to draw a plume of unstable, Gulf air northward. Interestingly, there was no large, intense, cyclonic storm present.
However, the core of the jet stream aloft was surging along the Gulf Coast, and a separate jet of intense, low-level winds (noted in the figure as “50+ kt LLJ”) created a very intense wind shear. Additionally, an approaching ripple in the jet stream (called a shortwave) was beginning to draw the air upward over Mississippi.
The combination of unstable air, uplift and shear was ripe for rotating thunderstorms.
The image below is a radar snapshot from the early morning on Jan. 21. A compact cluster of intense thunderstorms erupted in the vicinity of the mesolow (red “L”). Magenta arrows point out individual supercells that began to stream into southern Mississippi. One of these spawned the Hattiesburg tornado.
Early the next morning of Jan. 22, the pattern of unstable air and powerful wind shear shifted eastward. Now southern Georgia was under the gun.
The image above shows the familiar weather elements: warm front, mesolow and a low level jet of 40-50 knots. The strength of the wind shear and rotational tendency of the air were both quite striking.
The figure below shows a crop of supercell thunderstorms that blew up along this warm front boundary. Note also the surge of warm, unstable air on southerly winds off the Gulf (small orange arrows).
During the late afternoon and evening of Jan. 22, a larger storm complex began to rapidly organize and deepen. The strengthening low pressure swept a surge of cool, dry air over the Southeast, creating a cold front (image shown below). An intense squall line of thunderstorms developed along the front.
Out ahead of this line, across central Florida and southeastern Georgia, numerous supercells erupted (image below). One of these cells created a destructive, wedge-shaped tornado over Albany, Ga.
This tornado outbreak did not rack up many dozens of tornadoes, as is common in the spring. Its geographic focus was unusually compact, and the parent storm complex was somewhat diffuse; it lacked a strong, central, organizing low-pressure region. But the number of fatalities (19) was considerable.
Contributing to the high death toll was the nighttime arrival of many tornadoes, when people were asleep and most vulnerable. Unfortunately, this is one of the more challenging attributes in Dixie Alley — how to effectively warn the public.