WeatherFanatics: Rain, Record Warmth Bring Flood Risk to Snowy States in East

By Alex Sosnowski, Expert Senior Meteorologist

December 17,2013; 9:32PM,EST

A brief, but major shift in the weather pattern will send warm air northward and will produce a zone of heavy rain in a large part of the eastern half of the nation this weekend.
Temperatures will trend upward Thursday and Friday from the South to New England.
By the weekend, temperatures may challenge record highs from the Gulf Coast to the Ohio Valley and the mid-Atlantic.
Highs will be in the 70s over much of the South with a few spots flirting with 80 degrees Saturday and Sunday.

Temperatures will reach near 60 degrees along the Ohio River and the Mason-Dixon Line bordering Pennsylvania and Maryland.
The warmth pressing into much of the Midwest and Northeast will lead to snow melt and considerable runoff.
The combination of warm, moist air flowing over cold ground and snow cover will lead to locally dense fog in some locations. Fog could hinder early holiday travel this weekend.

Flooding Risk

As a storm rolls from Texas over the weekend, a swath of heavy rain will reach northeastward across the Ohio Valley states and into the Northeast.
Enough rain can fall by itself to cause flash, urban and small stream flooding in some areas. This includes areas from the Gulf Coast northward to around the Great Lakes and the Northeast.

In the South to part of the Ohio Valley. the rain could be made more intense by strong to severe thunderstorms.
Where there is still a significant amount of snow remaining on the ground in northern areas, heavy rain can cause that snow to melt rapidly and a substantial rise may result on some rivers. This risk is greatest in parts of Missouri, Michigan, Illinois, Indiana, Ohio, Pennsylvania, upstate New York and central and northern New England.
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As long as the 3 to 5 inches of rain expected from the South does not make it in over the areas with snow cover, flooding problems will tend to be minor, rather than major.
Rainfall over some snowcovered areas will average 1 to 2 inches with locally 3 inches possible.
Most areas have less than a foot of snow on the ground, which roughly translates to an inch or less of water.

Snow Load and Isolated Roof-Collapse Concern

In most cases, enough melting will occur and rain will fall to wash away the snow from gable roofs.
However, some flat roofs are at greater risk for trouble, including possible roof collapse.
"The snow may act like a sponge, absorbing the rain and gaining weight in the process," AccuWeather.com Senior Vice President and Forensics Weather Expert Joseph Sobel said. "Prior drifting on flat roofs can be major problem, by causing uneven weight distribution."
One cubic foot of snow weighs about 15 pounds, but less if it is fluffy, and more if it is more dense such as in a a drift.

Record-Challenging Highs Forecast for Sunday, Dec. 22, 2013

Location	Expected High (Existing Record/Date)
New York	59 (63/1998)
Philadelphia	65 (64/1998)
Washington, D.C.	68 (72/1889)
Pittsburgh	63 (67/1949)
Cincinnati	60 (64/1941)
Boston, Mass.	56 (62/1990)
Roanoke, Va.	71 (71/1923)
Charlotte, N.C.	73 (72/1889)
Birmingham, Ala.	68 (71/1990)
Columbia, S.C.	77 (76/2001)
Atlanta, Ga.	68 (70/1889)
Raleigh, N.C.	77 (75/1967)