Severe Weather Targets Mississippi, Tennessee Valleys

Just a day before Christmas Eve, the threat of tornadoes is darkening the pre-holiday mood across parts of the South. In its outlook updated at 10:30 am CDT, NOAA’s Storm Prediction Center (SPC) is projecting a moderate risk of severe weather for Wednesday from eastern Arkansas across northern Mississippi and Arkansas into western Tennessee and Kentucky, with lesser risks over a large surrounding area covering most of the nation between the Mississippi and the Appalachians. In the moderate risk area, long-track tornadoes are a distinct possibility. On Wednesday morning, The Weather Channel’s Dr. Greg Forbes raised the TOR:CON level to 8 across northern Mississippi and Alabama and western Tennessee, calling a tornado outbreak “likely.” According to TWC’s Michael Butler, this is the nation’s first TOR:CON of 8 since April 28, 2014. By late morning, several bands of severe thunderstorms had already developed from Illinois to Arkansas, with scattered storms further to the southeast. Two tornado watches were in effect, and more than two dozen tornado warnings had already been issued. Major tornado outbreaks in December are uncommon but not unheard of. The last "Moderate Risk" forecast for severe weather issued by the SPC in December was in 2013; there were also two "Moderate Risk" forecasts issued in December 2012.


Figure 1. NOAA/SPC severe weather outlook for Wednesday, December 23, 2015, updated at 10:30 am CDT.

A broad channel of warm, soupy air from the Gulf of Mexico has streamed north across much of the eastern U.S. over the last 24 hours. Both temperatures and dew points have risen into the 50s as far north as Detroit and New York and into the low 70s over Louisiana. Later today, warm, humid air feeding into the moderate-risk area will combine with the rich moisture to produce increasingly unstable surface conditions. The instability will not be extreme, but in wintertime even moderate levels of instability are enough to produce tornadic supercell storms if there is strong wind shear present. The latter will be furnished by a sprawling upper-level trough over the western U.S., with jet-stream winds well above 120 mph. A strong upper-level impulse within the trough will shoot northeast across the moderate-risk area later today, providing the dynamics to support widespread severe weather.


Figure 2. WunderMap depiction of jet-stream winds in knots (multiply by 1.15 for mph) at the 300-mb level (around 30,000 feet) projected by the GFS model for 21Z (3:00 pm CST) on Wednesday, December 23, 2015. The sharp kink in the jet centered over eastern Arkansas and western Tennessee will act intensify updrafts and wind shear over the region.

Across the moderate risk area, supercell thunderstorms will pose a particular risk of dangerous tornadoes on Wednesday afternoon and evening. On Thursday, a broad swath of less intense thunderstorms will extend unusually far to the northeast for this time of year. Thunder and lightning could serve as a stand-in for a white Christmas Eve as far north as Albany and Boston. Instability will be somewhat lower on Thursday, and the wind configuration less favorable, so the Day 2 outlook released by SPC on Wednesday morning projects only a marginal risk for severe weather with these storms from the central Gulf Coast to the Washington, D.C., area. As the upper-level trough recharges across the West over the weekend, and another pulse of springlike air moves in, additional severe weather could erupt over eastern Texas and adjoining states.

December can be dangerous for severe weather over the South
Ready access to warm, moist air from the Gulf of Mexico means that severe weather is a risk over the U.S. South even during the cold season. Wednesday’s activity is pushing the typical northern edge of such outbreaks. The most recent significant tornado outbreak (10+ tornadoes) during Christmas week was on December 21, 2013, centered over the Mississippi Delta. Two fatalities were associated with at least 14 twisters that day. The largest outbreak on Christmas Day occurred on December 25, 2012, when more than two dozen Christmas Day tornadoes struck from Texas to North Carolina. On New Year’s Eve 2010, an outbreak with 32 tornadoes included seven EF3s, one of which killed four people in Arkansas (with nine fatalites in all that day). The five deadliest U.S. December tornadoes have been summarized by weather.com, including an F4 twister that killed 11 near Tuscaloosa, AL, on December 16, 2000, and another F4 that took 11 lives at Murphysboro, IL, on December 18, 1957. The most devastating of the lot was the tornado that swept across Vicksburg, MS, on December 5, 1953, killing 38 people and injuring 270.

Record-melting heat still on tap for the holiday weekend
More than a month ago, seasonal prediction models and the record from past strong El Niños pointed toward the chance of a December “warm wave” for much of eastern North America. Those signals are being confirmed in spectacular fashion, as hundreds of daily record-warm highs and lows--and a number of monthly records--are destined to fall over the next several days over much of the United States east of the Mississippi. With overnight temperatures so warm, some record highs for Christmas Eve and Christmas Day may fall at the stroke of midnight! For Thursday (Christmas Eve), WU is predicting record highs on par with typical early-autumn readings, including 56°F in Portland, ME; 68°F in Boston, MA; 72°F in New York, NY; 73°F in Philadelphia, PA; 74°F in Washington, D.C.; and 79°F in Norfolk, VA. A half-hearted cool front will push the warmest air toward the southeast by Friday (Christmas Day), but it’ll still remain unusually mild across parts of the Northeast and downright springlike all weekend further to the south, with 70s from North Carolina to Georgia and 80s in Florida. For more background on the warm weather that’s bathed several continents this month, see our post from last Monday.

Wunderblogger Steve Gregory has a new Wednesday afternoon post, Record Warmth – Severe T-Storms - But Pattern Change on the Way. Another Wunderblogger, Lee Grenci, weighs in today on the "clash of air masses" explanation for severe weather.

Stay safe, everyone!

Bob Henson