Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 235 AM EST Thu Dec 24 2020 Valid 12Z Thu Dec 24 2020 - 12Z Sat Dec 26 2020 ...Heavy rain, flooding/flash flooding, severe weather, and damaging wind gusts will impact the East today into Christmas Day... ...Additional heavy snow is forecast for the Great Lakes region while snow is also possible across eastern portions of the Ohio Valley into the Central Appalachians... A powerful surface low pressure system is forecast to track northeastward into southeast Canada today as another low along its associated cold front takes over and tracks northward across the eastern U.S. today and tomorrow. Along and ahead of the cold front, plentiful moisture from the Gulf of Mexico is forecast to stream in and cause widespread heavy rain and thunderstorms. A combination of factors will lead to flooding and flash flooding with this rain--Slight Risks of excessive rainfall are in place from the Appalachians to the Northeast along with an embedded Moderate Risk in portions of Pennsylvania and New York through tonight, with the risk pushing east tomorrow. First is the highly anomalous moisture leading to widespread rain amounts of 1 to 4 inches. Second is that in some areas, the rain will fall on a deep and relatively fresh snowpack from the nor'easter last week, exacerbating flooding concerns as rain could cause rapid snowmelt. Another concern is the added weight that rain could put on top of snow covered roofs, particularly across regions that still have over a foot of snow on top of buildings from north-central Pennsylvania to Vermont. Thirdly, temperatures will be much warmer than average ahead of the front (with record-setting warm lows likely in the Northeast Friday morning), and the mild temperatures and relatively high dewpoints should also cause rapid snowmelt, filling rivers and streams even more. Another hazard with this system is severe weather, and the Storm Prediction Center has delineated an Enhanced Risk for wind and tornado threats for the eastern Carolinas and far southeastern Virginia through tonight. Non-thunderstorm wind gusts are also a threat, and High Wind Warnings and Wind Advisories are in place for much of the Northeast. On the backside of the strong cold front, heavy snow of several inches should continue today in the Upper Peninsula of Michigan and western Lower Michigan as conditions will be favorable for lake effect snow there. Lake enhanced snow potentially accumulating over a foot is also likely near Lake Erie, with snow amounts of 4 to 8 inches expanding south into eastern portions of the Ohio Valley and into the Central Appalachians. Light freezing rain is also possible in those areas. The strong front will lead to temperatures of 15 to 25 degrees below normal spreading from the central U.S. today into the eastern U.S. Friday. The cold temperatures behind the front could support rapid freezing of any standing water. Elsewhere, increasing precipitation chances are forecast for the Northwest Friday into Saturday as a couple of frontal systems approach. Farther south, there is a Critical Risk of fire weather in place from the Storm Prediction Center for Southern California today. Dry conditions and gusty winds could also cause an elevated risk of fire weather in the Southern Plains. Tate Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php