Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EST Wed Dec 20 2017 Valid 12Z Wed Dec 20 2017 - 12Z Fri Dec 22 2017 ...Heavy rainfall to spread into the Southeast and lower Mid-Atlantic today... ...Light to moderate accumulating snowfall possible across parts of the Intermountain West, the Rockies, and into the northern Plains Wednesday and Thursday... ...Temperatures across the southern tier states will be above normal while much of the Western U.S. should be below normal... Showers and thunderstorms across portions of the Deep South this morning will continue to push eastward today into the Southeast and lower Mid-Atlantic region. The heaviest rainfall is expected along a frontal boundary from roughly northern Georgia into the Carolinas where isolated flash flooding may be a threat. Conditions across the Deep South should dry out by Thursday, but heavy rainfall may return by Friday as another system approaches from the west. Temperatures ahead of this second boundary from the southern Plains to Middle Mississippi Valley on Thursday could be as much as 15 to 25 degrees above average. A deep upper level system pushing through the Western U.S. today will allow for rain and mountain snows across much of the central Great Basin, the Four Corners region, and into the Rockies. Some light to moderate snow accumulations are possible in the mountains where winter weather advisories are currently in effect. This system will also usher in cooler weather across much of the Western U.S. by Thursday with temperature anomalies of 10 to 20 degrees below normal possible from portions of the Southwest to the central High Plains. Accumulating snowfall is also possible across portions of the northern Plains to the upper Great Lakes on Wednesday and into Thursday to the north of the surface low associated with the above mentioned system. A second surface low should develop across the central Plains by Wednesday night which should allow precipitation to spread into portions of the Midwest and the Ohio Valley by Thursday. Much of this should be rain for the middle Mississippi Valley and the Ohio Valley but snow, a rain/snow mix, or even light freezing rain will be possible to the north of a warm front from northern Iowa into Lower Michigan. By Thursday night into early Friday, this wintry precipitation is forecast to move into portions of the interior Northeast and Lower Great Lakes. Santorelli Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php