Winter Storm Leon Roars Through the South


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Freezing rain scenario. Image courtesy of NOAA

Freezing rain scenario. Image courtesy of NOAA

Like its predecessor Kronos, the latest weather-channel-named storm, Leon, will be modest by the standards of northerners used to snow and ice.

Southern drivers, unaccustomed to such conditions, will be slippin’ and slidin’, and power outages are likely where the precipitation falls as freezing rain.

The Conditions That Produce Freezing Precipitation in the Deep South

The cold air mass that covers the eastern United States, courtesy of the polar vortex, has even invaded the deep south, pushing through most of Texas, all of Louisiana, Mississippi, Alabama, and Georgia, and northern Florida.

Meanwhile, warm and humid air from the Gulf of Mexico (water temperature 60 degrees or higher) is moving northward. Where the cold air has wedged underneath the warm, a wide variety of precipitation types is possible.

The Various Forms of Frozen Precipitation

The clouds that produce precipitation in the winter over the United States are almost always high enough to be colder than freezing, and precipitation in the clouds starts as snow. As the snow falls, it can reach the ground in that form, even if the temperature near the ground is above freezing — the snowflakes simply don’t have time to melt.

If the warm air is deep enough, the snowflakes will melt and fall as rain.

The interesting scenario occurs when the cold air cannot be dislodged at the surface and the warm air rides up over it. This can produce two types of precipitation: sleet and freezing rain. Sleet occurs when the snow melts in the warm air and then freezes in the cold air; sleet is simply frozen raindrops.

But when the layer of cold air is shallow, the raindrops can survive the short trip through sub-freezing air and freeze on contact with the ground, trees, and power lines. The resulting ice is hazardous to travel, large tree limbs, and utility company property.

Winter Storm Leon Will Produce Every Kind of Precipitation

The precipitation from Leon will fall along a path from southeast Texas through the deep south and into North Carolina and Virginia, in the zone where the warm and cold air fight for supremacy. In the extreme south of the region, only rain will fall; at the northern fringe, up to 10 inches of snow is possible, especially in North Carolina and Virginia where the storm will tap moisture from the Atlantic Ocean; other areas will see a mixture of rain, snow, freezing rain and sleet.

The Worst of Winter Storm Leon

Bands of freezing rain are likely from southeast Texas through southern Louisiana and along the Atlantic coast from southern Georgia, through coastal South Carolina, to extreme southeast North Carolina, including the cities of Savannah, Charleston, and Wilmington. The situation is exacerbated by the large number of bridges over wetlands in those areas. Bridge surfaces tend to freeze even when the ambient temperature is above freezing.

How Long Will Leon Last?

The worst of Leon should depart Texas and Louisiana today and the rest of the northern Gulf tonight. The storm will wind down in the Carolinas and Virginia on Wednesday, and by Thursday all that will be left of Leon will be some cleaning up to do.

Will the Winter of 2014 Ever End?

Computer forecasts indicate that a substantial change in the overall weather pattern is in the works. It’s a song we’ve heard before this winter and the band never seems to produce the desired music. This time, however, most of the forecast models predict that the omega block in the western Pacific will erode and the polar vortex will relax its grip on the eastern United States.

The general outlook for February is for above normal temperatures across the south, with moderation to near normal temperatures in the northeast and only slightly below average in the midwest.

I can hear Northeners saying “From your keyboard to God’s ears.”

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