The National Weather Service Storm Prediction Center on Tuesday highlighted a Level 3 of 5 “increased” severe weather risk from Southeast Louisiana to the Florida Panhandle. Nearly 4.5 million Americans are in that zone, including residents of New Orleans; 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, stretch from eastern New Mexico to western Ohio. Up to 8 inches of snow is likely to fall from the fast-moving storm, which should be slipping into the Great Lakes by the end of Wednesday.
The weather so far this month has been abnormally active with severe thunderstorms, but relatively calm across the eastern United States regarding winter weather. Since the beginning of January, the Storm Prediction Center has recorded 138 reports of tornadoes, compared to a monthly average of about three dozen.
The continued mild weather in the east, while favorable for severe thunderstorms, has proved detrimental to snowfall. New York, Philadelphia, Baltimore and D.C. have yet to record measurable snowfall, and Boston sits at a paltry 4.3 inches — compared to an average of more than 20 inches at this point in the season.
Thunderstorms are expected to sweep across central and northern Texas and perhaps Oklahoma Tuesday morning. They must be elevated, or rooted in warm air flowing up and over a shallow cold rim. As a result, they are unlikely to pose a major tornado threat to begin with.
A line of thunderstorms with damaging straight-line winds and embedded tornadic circulations is likely to form over a warm corridor toward the coastline. There may also be a few isolated rotating supercell thunderstorms ahead.
Early afternoon storms are predicted to blow through Houston and Galveston, Tex. There is a small chance of an isolated strong tornado from New Orleans toward Mobile.
On Monday morning, a low pressure area was spread over Las Vegas. It should dive southeast over Chihuahua and Coahuila, Mexico, before crossing into South Texas near Brownsville and then working northeast through Tuesday.
The low then crosses northern Louisiana en route to the Mississippi Valley. Since lows in the Northern Hemisphere rotate counterclockwise, this means that the wind for the system is coming from the south. That will pull a warm, moist air mass from the Gulf of Mexico northward, but it could be as little as 100 miles north of the actual coastline. How far north it travels will determine the northern extent of the tornado risk.
At the same time, there should be a dip in the jet stream overhead with a sharp change in wind speed and/or direction with altitude. Surface winds should be streaming in from the south – then southwest at mid-levels and south-southeast high up. That “wind shear” will make it easy for thunderstorms that span multiple layers of the atmosphere to rotate.
At the back of the system, cold air wrapping south is expected to convert precipitation to snow in parts of northern Texas, Oklahoma, northwestern Arkansas, southeastern Missouri, southern Illinois, central Indiana, western Ohio and Southern Michigan.
The strip of snow will be narrow, but considerable accumulations are expected in the heart of the tyre. 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 metropolitan areas, such as St. Louis; Fort Smith, Ark.; Indianapolis; and Dayton, Ohio, are included in winter storm watches.
Snow is expected in the Sooner State Tuesday morning and will last for about 18 hours in most places.
Precipitation should begin as rain in northwestern Arkansas Tuesday night, transitioning to heavy, sleet as the atmosphere cools. West Tennessee may see a little snow Tuesday evening, and Illinois, Indiana and Ohio are likely to wake up to snow on their doorstops Wednesday morning.
From Wednesday through Thursday, snow is likely to reach the interior northeast, with most of the rain along the east coast.
