Threat of tornadoes, severe thunderstorms in South Thursday

It’s been only five days since tornadoes ravaged Oklahoma, and now another severe weather outbreak is brewing. Damaging winds, destructive hail and tornadoes threaten parts of Texas, Arkansas and the Deep South Thursday.

The National Weather Service Storm Prediction Center has declared a level 4 out of 5 risk of severe weather on its forecast maps for Thursday, including the area just to the east and northeast of Dallas and southwest of Little Rock. A much more expansive zone where severe storms are probable — but the risk is not quite as high — extends from Central Texas to western Tennessee.

“A regional severe-thunderstorm outbreak appears likely from parts of central and north Texas, across southeastern Oklahoma and the Arklatex region, and portions of the lower Mississippi Valley,” the Storm Prediction Center wrote Thursday morning. “Severe thunderstorm gusts (some potentially over 70 mph), large to very large hail, and several tornadoes (some strong) are all expected.”

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On Thursday afternoon, the Storm Prediction Center issued a tornado watch for northeast Texas and southeast Oklahoma, including Dallas until 10 p.m. local time, and a severe thunderstorm watch just to the west until 8 p.m.

This same storm system poses the threat of flooding from northeast Texas into the Ohio Valley through Friday where 4 inches of rain could fall.

On the cold side of the same storm, significant snows are possible in northern Illinois, northwest Indiana and Michigan, possibly delivering a plowable accumulation to Chicago. Uncertainty is still high with respect to exactly how much snow will fall and where, but winter storm watches are in effect for the Windy City with rain expected to flip to snow Friday. Additional heavy accumulations are probable in parts of the interior Northeast and New England.

As warm air has surged ahead of the storm, dozens of record high temperatures have been set from Texas to Florida. Temperatures some 15 to 25 degrees above average will continue to be found ahead of the system as heat is scooped north from the southern U.S.

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Falcon Lake, Tex. — near the border with Mexico — preliminarily hit 105 degrees, the earliest-ever 104 degree reading in United States history Wednesday. Indiana and Ohio also saw their earliest-ever 83 and 82 degree readings respectively according to Maximiliano Hererra, a climate historian specializing in extreme temperature records.

A very powerful storm is crossing the country: Five things to know

Before ejecting into the central states, the storm dumped 3 to 5 feet of snow in the Sierra Nevada early in the week and, on Wednesday night, caused Lufthansa Flight 469 from Austin to Frankfurt, Germany, to divert to Dulles Airport in Virginia after encountering severe turbulence over Tennessee. At least seven passengers were injured, during which the plane was reported to go into a brief “free fall.”

Severe thunderstorms on Wednesday produced a spattering of reports of hail from Texas to Indiana and Kentucky while a brief tornado toppled trees and power lines in northwest Alabama. A monster thunderstorm dropped softball-sized hail on Frio County, Tex., southwest of San Antonio. In all, the Storm Prediction Center logged 82 reports of severe weather on Wednesday, more than 60 of those reports were from large hail.

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On Friday, as the storm system sweeps eastward, a third day of severe weather — including a tornado risk — is possible from the Southeast into the Ohio Valley before the system pushes off the East Coast over the weekend.

Thursday’s severe storm threat in depth

Areas affected

The level 4 out of 5 risk zone includes much of the Interstate 20 and 30 corridors in northeast Texas and southwest Arkansas, as well as extreme northwest Louisiana. Mesquite, Tyler and Longview, Tex., as well as Bossier City and Shreveport, La., are in this zone. So are Arkadelphia and Hope, Ark.

A level 3 out of 5 enhanced risk encompasses the entire Dallas-Fort Worth area as well as Arlington, Richardson, Lufkin and Waco, Tex. Little Rock, Memphis and Jackson, Miss. are in the enhanced zone too.

Within the level 2 out of 5 risk area are Houston, Austin and Abilene, Tex., and Jackson, Tenn.

Specific hazards will depend largely on storm mode, or the structure and shape of the thunderstorms that form. That’s something that is genuinely difficult to iron out until thunderstorms begin to bubble up, as it’s unknown the extent to which thunderstorms will interfere or merge with one another.

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If isolated thunderstorms form away from neighboring cells, they’ll be able to tap into the full volatility of the atmosphere — which consists of a dramatic change of wind speed and/or direction with height known as wind shear. That means they’ll rotate, and will be capable of all severe hazards, including damaging straight-line winds over 60 mph, golfball-size or larger hail, and tornadoes. Given the magnitude of spin near the ground, an isolated strong tornado can’t be ruled out. That’s especially true near the warm front.

Then the actual cold front will swing through after, with a line of thunderstorms materializing along it. That line will feature widespread winds of 60 to 70 mph, some hail to quarter size and a number of embedded circulations capable of whipping up quick-hitting spin-up tornadoes. Such QLCSs, or quasi-linear convective systems — aggressive squall lines with transient kinks of rotation along their leading edges — ordinarily produce primarily EF0 or EF1 tornadoes. This time around, however, there are indications that more formidable twisters in the EF2 or EF3 range may accompany the QLCS, highlighting just how dynamic of an event is likely to unfold.

Flood risk

Meanwhile, though tornadoes tend to spawn the more attention-grabbing headlines, a flash flood risk will manifest as well. Flooding causes more casualties annually than tornadoes. Much of northeast Texas, southeast Oklahoma and the majority of Arkansas could see a general 2 inches of rain, as moisture pools along the warm front before the band of cold frontal downpours sweep through. Localized totals of at least 4 inches are possible.

Thunderstorms should develop in an arc from west of Fort Worth to northeast of Dallas near the Red River during the mid afternoon between 2 and 4 p.m. Initial thunderstorms may be supercells before they begin to merge. These will line the warm front, which will be draped southwest to northeast. They’ll ride into southeast Oklahoma and southwest Arkansas along the Interstate 30 corridor.

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During the evening, the QLCS will take shape near or just west of Interstate 35, blowing through Dallas-Ft. Worth around 6 or 7 p.m., then reaching Tyler to Little Rock in the 8 or 9 p.m. window. Storms will be knocking on the doorstep of Memphis around midnight, then may fizzle during the second half of the overnight as they traverse northern Louisiana and Mississippi before redeveloping some in the face of daytime heating Friday afternoon.

The trigger

Inciting the severe weather is a pocket of high altitude frigid air, low pressure and spin nestled within a dip in the jet stream. That upper-air disturbance is blowing east out of Arizona into southwest New Mexico, and will reach Texas during the afternoon.

Ahead of it, warm, moist air is being swirled north by its counterclockwise rotation. That’s wafting Gulf of Mexico mildness and moisture over the southern U.S., contributing to instability, or juice, to fuel thunderstorms.

The jet stream dip, meanwhile, will induce wind shear, encouraging storms to rotate. Some of the momentum aloft will also be mixed down by thunderstorms in the form of strong to damaging wind gusts.

Thunderstorms will roll east as the storm system works into the Midwest on Friday, its cold front trailing to the south.

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