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Developing storm to bring tornado threat to Gulf Coast, and snow to north

Heavy snow is expected from New Mexico to Ohio

A high-resolution weather model simulation of severe storms and snow associated with the larger-scale system on Wednesday morning. (WeatherBell)

A soon-to-develop storm system over the south-central United States is about to unleash a blast of winter for some but severe storms more typical of springtime for others. There is a growing risk for tornadoes over parts of the Gulf Coast, while residents on the storm system’s cold side — not that far to the north — brace for a plowable snowfall.

The National Weather Service Storm Prediction Center has highlighted a Level 3 out of 5 “enhanced” risk of severe weather from southeast Louisiana to the Florida Panhandle on Tuesday. Nearly 4.5 million Americans are within that zone, including residents of New Orleans; Baton Rouge; Gulfport, Miss.; and Mobile, Ala.

On the storm system’s cold side, a moderate snowfall is expected. Winter storm watches and warnings, as well as winter weather advisories, span from eastern New Mexico to western Ohio. Up to 8 inches of snow is likely out of the quick-hitting storm, which should slip into the Great Lakes by late Wednesday.

The weather so far this month has been anomalously active as far as severe thunderstorms, but comparatively tranquil in the eastern United States with regard to wintry weather. Since January began, the Storm Prediction Center has logged 138 reports of tornadoes compared with a monthly average of about three dozen.

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The persistent mild weather in the East, while favorable for severe thunderstorms, has proved pernicious for snowfall. New York, Philadelphia, Baltimore and D.C. have yet to record any measurable snowfall, and Boston sits at a paltry 4.3 inches — compared with an average of more than 20 inches by this point in the season.

Southern severe storms

What to expect

Thunderstorms are expected to rumble across central and northern Texas and perhaps Oklahoma on Tuesday morning. Those should be elevated, or rooted in warm air riding up and over a shallow lip of cold. As a result, it’s unlikely they’ll yield much of a tornado threat early on.

Toward the coastline over a corridor of warmth, a line of thunderstorms with damaging straight-line winds and embedded tornadic circulations is likely to form. There may also be a few isolated rotating supercell thunderstorms ahead of it.

Storms are predicted to roll through Houston and Galveston, Tex., during the early afternoon, reach the Golden Triangle of southeast Texas and southwest Louisiana by evening, and probably make it to New Orleans around midnight. There’s a low-end chance of an isolated strong tornado from New Orleans toward Mobile.

The setup

On Monday morning, a low-pressure system was spread over Las Vegas. It should dive southeast over Chihuahua and Coahuila, Mexico, before crossing South Texas near Brownsville and then working northeast into Tuesday.

The low will then pass over northern Louisiana en route up the Mississippi Valley. Since lows spin counterclockwise in the Northern Hemisphere, that means winds ahead of the system will come from the south. That will draw a warm, humid Gulf of Mexico air mass northward — but it might make it only 100 miles or so north of the actual coastline. How far north it rides will determine the northern extent of the tornado risk.

At the same time, a dip in the jet stream should be sweeping overhead with a robust change in wind speed and/or direction with height. Surface winds should flow in from out of the south — then southwest at the mid-levels, and south-southeast high aloft. That “wind shear” will make it easy for any thunderstorms spanning multiple layers of atmosphere to rotate.

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Snow potential

On the backside of the system, cold air wrapping southward is expected to flip precipitation over to snow across parts of northern Texas, Oklahoma, northwest Arkansas, southeastern Missouri, southern Illinois, central Indiana, western Ohio and southern Michigan.

The strip of snow will be narrow, but within the heart of the band, decent accumulations are expected. In southern areas, most people can expect 3 to 6 inches, with 4 to 8 in the most heavily impacted parts of the Midwest.

Wind is not expected to be an issue. Several major metro areas, such as St. Louis; Fort Smith, Ark.; Indianapolis; and Dayton, Ohio, are included in winter storm watches.

Snow is expected to arrive in the Sooner State on Tuesday morning and last about 18 hours in most places.

Precipitation should begin as rain in northwest Arkansas on Tuesday evening, flipping to a heavy, wet snow as the atmosphere cools. Western Tennessee could see a bit of snow Tuesday night, and Illinois, Indiana and Ohio will probably awaken to snow on their doorstop Wednesday morning.

By Wednesday into Thursday, snow will probably reach the interior Northeast, with mostly rain expected along the East Coast.

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