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A storm system soon to develop over the south central United States is about to unleash a blast of winter for some, but strong storms more typical of spring for others. There is an increasing risk of tornadoes over parts of the Gulf Coast as residents on the cold side of the storm system — not that far north — prepare for plowable snowfall.
The National Weather Service’s Storm Prediction Center highlighted a Level 3 of 5 “elevated” risk of severe weather from southeast Louisiana to the Florida Panhandle on Tuesday. Nearly 4.5 million Americans are in this area, including New Orleans residents; Baton Rouge; Gulfport, Miss.; and Mobile, Ala.
Moderate snowfall is expected on the cold side of the storm system. Winter storm watches and warnings, as well as winter weather advisories, extend from eastern New Mexico to western Ohio. Up to 8 inches of snow is likely to fall from the fast-moving storm, which is expected to enter the Great Lakes by late Wednesday.
The weather so far this month has been anomalously active in terms of severe thunderstorms, but relatively quiet in the eastern United States in terms of winter weather. Since the beginning of January, the Storm Prediction Center has recorded 138 tornado reports, compared to a monthly average of about three dozen.
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Prolonged mild weather in the east, while favorable for severe thunderstorms, proved detrimental to snowfall. New York, Philadelphia, Baltimore and D.C. have yet to record measurable snowfall, and Boston is at a paltry 4.3 inches — compared to an average of more than 20 inches so far this season.
Thunderstorms are expected to develop across central and northern Texas and possibly Oklahoma on Tuesday morning. They must be raised or rooted in warm air moving up and over a shallow lip of cold. As a result, they are unlikely to produce much of a tornado threat to begin with.
A line of thunderstorms with damaging straight-line winds and embedded tornadic circulations is likely to develop toward the coast over a warm corridor. A few isolated rotating supercell thunderstorms are also possible ahead of it.
The storms are expected to pass through Houston and Galveston, Texas, in the early afternoon, reach the Golden Triangle of southeast Texas and southwest Louisiana by evening, and possibly reach New Orleans around midnight. There is a slight chance of an isolated strong tornado from New Orleans to Mobile.
A low pressure system moved over Las Vegas on Monday morning. It should dive southeast over Chihuahua and Coahuila, Mexico before crossing South Texas near Brownsville and then work northeast by Tuesday.
The low will then move over northern Louisiana on its way up the Mississippi Valley. Since lows rotate counterclockwise in the Northern Hemisphere, this means that winds ahead of the system will come from the south. This will draw a warm, moist air mass from the Gulf of Mexico northward — but may only make it 100 miles or so north of the actual coastline. How far north it moves will determine the northern extent of the tornado risk.
At the same time, the downdraft should extend overhead with a steady change in wind speed and/or direction with height. Surface winds should come in from the south—then southwest at mid-levels and south-southeast aloft. This “wind shear” will facilitate the rotation of any thunderstorms spanning multiple layers of the atmosphere.
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On the backside of the system, cold air enveloping the south is expected to turn precipitation into snow in parts of northern Texas, Oklahoma, northwestern Arkansas, southeastern Missouri, southern Illinois, central Indiana, western Ohio, and southern Michigan.
The snow band will be narrow, but decent accumulations are expected in the heart of the band. In southern areas, most people can expect 3 to 6 inches, with 4 to 8 in the hardest-hit parts of the Midwest.
Wind is not expected to be a problem. Several major metro areas, such as St. Louis; Fort Smith, Arkansas; Indianapolis; and Dayton, Ohio, are placed on a winter storm watch.
Snow is expected to arrive in the Sooner State Tuesday morning and last until about 6 p.m. in most locations.
Precipitation should start as rain in northwest Arkansas Tuesday night, changing to heavy, wet snow as the atmosphere cools. West Tennessee could see some snow Tuesday night, and Illinois, Indiana and Ohio will likely wake up to snow on their doorstep Wednesday morning.
Snow is likely to reach the inland Northeast Wednesday through Thursday, with mostly rain expected along the East Coast.
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