Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 400 PM EST Mon Nov 17 2014 Valid 00Z Tue Nov 18 2014 - 00Z Thu Nov 20 2014 ...Subfreezing high temperatures are forecast on Tuesday as far south as the Lower Tennessee Valley... ...Heavy lake effect snows are expected throughout the period... A deep upper trough digging across the northeastern sector of the country will drive well below normal temperatures all the way to the Gulf Coast through mid-week. The latest forecast suggests near 0 degree Fahrenheit readings tonight over the Northern/Central Plains and into the Upper/Middle Mississippi Valleys. By tomorrow afternoon, it appears much of the Mid-Atlantic and Lower Tennessee Valley will struggle to reach the freezing mark as polar air invades the central to eastern U.S. It appears the well below normal temperatures will persist during the next couple of days as a reinforcing shot of modified arctic air accompanies the next cold front. The strong cold front currently moving across the I-95 corridor has spread abundant precipitation up and down the boundary. Showers and thunderstorms will remain commonplace farther south across the Carolinas and southeastern U.S. where the air has become more unstable. Temperatures have been cold enough on the back side of the precipitation shield to spread light to moderate snows to sections of the Ohio Valley, Central Appalachians, Upper Mid-Atlantic, along with the Great Lakes region. The biggest impacts in terms of snowfall will be across the Great Lakes, particularly downwind of Lakes Erie and Ontario. This is where an extended period of heavy lake effect snows are forecast given favorable wind trajectories and very unstable air in the lowest levels of the atmosphere. The WPC winter weather desk suggests snowfall amounts approaching 2 feet downwind of Lakes Erie and Ontario with isolated heavier amounts possible. Farther upstream, amounts of 6 to 12 inches should be more commonplace over the Upper Great Lakes. Conditions should be relatively quiet across the western states given a strong ridge of high pressure building overhead. This will limit any threat of precipitation until around Wednesday morning when the next Pacific system approaches the California/Oregon border. Surface high pressure over the Great Basin will favor offshore flow across California which should keep relative humidity values quite low. Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_wbg.php