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The meteorology that led to the deadly late-December tornado outbreaks

Warm and cold segments contained within the same storms have generated every conceivable form of severe weather in the central United States over the past week, from blizzard conditions to deadly tornadoes.

On Dec. 26, a Southern Plains cyclone delivered a blizzard, freezing rain, flash flooding, damaging winds and a killer tornado outbreak. The event spawned nine tornadoes, including a monster, EF-4 tornado near Dallas. Eleven people lost their lives that night.

Just three days prior on Dec. 23, a violent tornado outbreak tracked across the South, targeting Mississippi with the most deadly weather. Nearly 40 reports of tornadoes were submitted to the National Weather Service that day, along with over 200 severe wind reports. Two of the tornadoes in this pre-Christmas outbreak had path lengths over 60 miles; one was rated an EF-3 and another an EF-4. Tornadoes killed nine people Dec. 23.

Combined, these outbreaks pushed this month to the deadliest December for tornadoes in 62 years. In fact, 2015 was on track to be the least deadly year on record up until the holiday outbreaks.

[2015 was almost the least-deadly tornado year on record — then December happened]

The weather pattern that triggered these outbreaks featured two very important components: an unusually intense Bermuda high that pushed into the Southeast, and an intense dip in the jet stream — or trough — containing a core of very cold air and rapidly spinning winds.

The Bermuda high pumped unseasonably warm and moist air northward, which acted to destabilize the atmosphere, while the deep trough fostered vigorous ascent and intense cyclogenesis — the formation of low-pressure systems. To enhance the scenario even further, an extreme air mass temperature contrast provided a critical source of energy for the developing cyclones on these days.

Holiday storms cut a path of death and destruction across the country, killing dozens of people and leaving many homeless. (Video: Reuters)

The Dec. 26 tornado outbreak in Texas

The Texas tornadoes were part of a multi-day outbreak sequence spanning Dec. 26-27. On Dec. 26,  severe weather reports were not nearly as widespread as on Dec. 23. The reports that were submitted, however, were of violent and deadly nighttime tornadoes.

[Nature’s startling contrast: Tornadoes torment Texas while a blizzard buries New Mexico]

Mid-latitude cyclones during the winter months feature strongly contrasting warm and cold sides, and each region has its signature weather hazards. On Dec. 26, these hazards created a very colorful watch and warning map for the NWS Dallas forecast region. The day featured just about every type of hazard, save for a plague of locusts: a blizzard, an ice storm, high winds, flash floods, damaging winds and tornadoes.

An EF-4 and an EF-3 were among the tornadoes that day, along with a single EF-2 and five EF-0 tornadoes.

The synoptic weather chart from Dec. 26 shows the key atmospheric players. The strong Bermuda high parked over the Atlantic Ocean circulated warm, moist, unstable air northward, feeding into the warm sector of a surface low pressure system. Meanwhile, fast winds coursed along a pronounced dip in the jet stream, the trough, while sub-freezing air filtered southward into northern Texas.

Some of the cold air aloft overlapped warm air at low levels in the cyclone’s warm sector, creating an explosively unstable atmosphere. The core of fast winds developed both directional turning and increase in speed with altitude (wind shear), an ideal configuration for supercell thunderstorms that spawn nearly all violent tornadoes.

The extreme temperature contrast created a rich source of potential energy that intensifies the parent vortex. Temperatures ranged from 30 degrees across the Texas Panhandle – where a blizzard howled – to 75 degrees in southeast Texas – where supercells and tornadoes rapidly developed.

As in most tornado outbreaks, Doppler radar proves invaluable for locating and tracking supercells and their embedded mesocyclones (3-5 mile diameter vortices giving rise to tornadoes).

The image below is a Doppler screen capture at 6:51 p.m. over northeastern Texas, highlighting the telltale features of a big-league tornado: a hook echo in the precipitation map (upper left), “velocity couplet” (opposing air flows) in the Doppler image (upper right) and a “debris ball” in the polarized radar image (lower left).

The Dec. 23 tornado outbreak in Dixie Alley

Most people are familiar with the concept of “tornado alley,” a diffuse region broadly encompassing the southern and central Plains states. But it turns out that there is no official definition of tornado alley, and in fact there are several alleys and corridors of high tornado frequency across the United States.

Although the Dec. 26 Texas twisters were located in the southern end of the traditional tornado alley, those on Dec. 23 (and a few on Dec. 25 and 27) developed squarely within another deadly tornado hot spot: Dixie alley.

Dixie alley roughly encompasses the South, including Louisiana, Arkansas, Tennessee, Mississippi, Alabama, Georgia and the Florida Panhandle. Published research reveals that when compared to the traditional tornado alley, Dixie alley features stronger and more-violent tornadoes, and greater violent tornado fatalities.

The graphic below illustrates just how dangerous this region is. In terms of annual average number of tornado watches, the familiar tornado alley in the Great Plains is shown centered on Oklahoma. But this region is overshadowed by Dixie alley. The national bullseye for tornado watches, in fact, straddles southern Mississippi and Alabama.

Compared with the Great Plains tornado alley, Dixie alley tornadoes are more likely to occur year-round, with significant probabilities throughout the winter season, ramping up in February into early spring. Dixie alley tornadoes are also more likely to remain active during the overnight, even into the early morning hours.

Thus, unlike the narrow two- to three-month peak in late spring tornado activity across the Great Plains (a well-defined tornado season), the South’s tornado threat is much more diffuse. This makes focused tornado awareness and safety efforts problematic, and strong, violent tornadoes can strike when people are most vulnerable — during darkness, and while sleeping.