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Wet and windy Christmas storm bringing severe weather to Southeast, potential for heavy snow in parts of Great Lakes

Total precipitation forecast through Friday morning. Nearly all of the five-plus inches of rain along the Gulf Coast is expected to fall before Wednesday morning, while much of the precipitation in the western Great Lakes — including Illinois and Michigan — will likely fall as snow. (NOAA WPC)

Flooding rains and severe thunderstorms are expected in the Southeast on Tuesday as a potent winter storm, which could bring heavy snowfall to parts of the Midwest, builds north from the Gulf Coast.

Severe thunderstorms are rumbling across the Southeast on Tuesday as the much-anticipated Christmas storm begins to take shape near the Gulf Coast. While strong winds and hail are the largest thunderstorm threat, tornadoes are still possible in well-organized storms.

A tornado watch is in effect for parts of the Gulf Coast from southern Louisiana to the Mobile, Ala., area until 6 p.m. central time Tuesday. Tornado warnings were necessary Tuesday morning for strong, rotating thunderstorms in southwest Louisiana.

The wet winter storm has prompted a flash flood warning for parts of the Gulf Coast, including southern Alabama, southwest Georgia and the Florida Panhandle, until Wednesday evening. Widespread rainfall totals of three to five inches are expected in this region Tuesday and Wednesday, with the potential for localized totals exceeding six inches.

Airport delays due to low clouds, poor visibility and/or heavy rain are likely at major hubs from Atlanta to New York. On Tuesday morning, delays were already over an hour in Philadelphia, Newark and at La Guardia Airport in New York City.

Looking beyond Tuesday, both the U.S. model and the European model forecast the winter storm to strengthen rapidly as it tracks north into the Great Lakes. The cyclone is expected to drop from a pressure of about 1001 millibars Tuesday night to 978-979 millibars Wednesday night, which nearly constitutes a meteorological “bomb” cyclone — or a storm that drops at least 24 millibars in 24 hours.

While far from a sure thing, the rapid deepening of the storm enhances the probability of an intense snow band setting up on the western side of the storm, most likely from Illinois into Michigan. On Tuesday morning, the National Weather Service in Chicago — though still with some uncertainty — was communicating the potential for snowfall rates of one to two inches per hour in the heaviest snow bands, which could lead to white-out conditions.

However, winter storm watches have not yet been posted in Illinois or Michigan due to the remaining uncertainty in the storm’s track. West of the surface low pressure center, heavy snow is likely, while east of the center, precipitation will fall mostly in the form of rain. Thus, the location of the low as it tracks north is critically important for the forecast across the Great Lakes states.

“Unfortunately,” writes the Weather Service in Gaylord, Mich., “this is a case where literally a 20 or 30 mile difference in low level cyclone track will mean the difference between a cold rain, heavy wet snow, or nothing. Good luck!”

East of the cyclone center (Ohio, Kentucky and East), the main threats will be from heavy rain and strong winds. Periodic rainfall, which could be heavy at times, will be slow to move up the East Coast this week, lasting into Wednesday night for the Washington, D.C., area, and through Christmas morning and possibly afternoon for New York City and Boston.

As for the potential for severe weather in the Mid-Atlantic and Washington, D.C., this week, severe weather expert Jeff Halverson thinks the chances are low. “Isolated thunderstorms are possible on Christmas Eve, into early Christmas morning, ahead of a vigorous cold front,” Halverson said. “Locally strong wind gusts may occur within a few cells.  However, both the instability and wind shear remain on the weak side … so organized severe weather is not anticipated.”

Strong winds will accompany the storm as it intensifies over the Great Lakes. A high wind watch is in effect for areas east of lakes Erie and Ontario. The National Weather Service in Buffalo is asking people in these locations to be prepared for winds of 25 to 35 mph, with gusts up to 50 or 60 mph. These gusts would be strong enough to bring down branches and small trees, leading to power outages.

Additional wind warnings or advisories are likely forthcoming from Ohio into the Northeast, where winds are expected to be similar in strength.