Rain, Record Warmth Bring Major Flooding Risk to Midwest
December 22, 2013; 5:25 AM
A brief, but major shift in the weather pattern will send warm air northward and will produce a zone of heavy rain in parts of the Central and Eastern states this weekend.
Temperatures will trend upward through the end of the week from the Deep 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 into the 60s along the Ohio River and the Mason-Dixon line bordering Pennsylvania and Maryland.
The combination of warm, moist air flowing over cold ground and snowcover will lead to locally dense fog in some locations. Fog could hinder early holiday travel this weekend.
Enough rain can fall by itself to cause flash, urban and small stream flooding in areas from the northwestern Gulf Coast to the lower Great Lakes and New England.
Where there is still a significant amount of snow remaining on the ground in northern areas, rain combined with warm, moist air can cause that snow to melt rapidly and a substantial rise may result on some rivers.
The situation is being monitored by experts in the National Weather Service and in the commercial weather forecast sector.
The NWS Ohio River Forecast Center is projecting moderate to major flooding along many rivers from southeastern Illinois to northeastern Ohio, including portions of Kentucky.
In the South to part of the Ohio Valley, the rain will be made more intense by strong to severe thunderstorms. The intensity of the thunderstorms will be given a boost by the unusual warmth and the overall strength of the large-scale storm system.
While there is more substantial snow on the ground in much of New England, southeastern New York and southeastern Pennsylvania, AccuWeather Enterprise Solutions expects less rain to fall, when compared to the Ohio Valley states and areas near lakes Erie and Ontario. As a result, flooding problems will tend to be minor, rather than major in the Northeast. However, motorists should be prepared for considerable runoff and street flooding as some of the snow melts.
Rainfall over some snowcovered areas over the Midwest will average 2 to 4 inches with locally 6 inches along the Ohio River. From 1 to 2 inches of rain with locally 3 inches is forecast from northwestern Pennsylvania to northwestern New England. Much less rain is projected farther east in the mid-Atlantic and farther south in New England.
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.