Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EST Tue Dec 13 2016 Valid 12Z Tue Dec 13 2016 - 12Z Thu Dec 15 2016 ...Arctic air mass poised to overspread much of the north central and northeastern U.S... ...Pacific storm system to bring a prolonged period of potentially heavy rain and snow to the western U.S... A frontal system will move from the Mississippi valley to the Appalachians today, bringing areas of snow to the Great Lakes, Ohio valley, and portions of the Mid-Atlantic and Northeast. Farther south, another lingering frontal system will keep showers and a few thunderstorms possible across the southeastern U.S. through early Wednesday morning. Across the northern plains, a cold air mass already in place today will be reinforced by the arrival of a fast-moving arctic frontal boundary on Wednesday. High temperatures today and Wednesday are expected to be 20 to 30 degrees below average from the northern plains to the Upper Midwest. On Thursday, the front will quickly move into the eastern U.S., high temperatures expected to be 15 to 30 degrees below average across the Ohio valley as well as portions of the Mid-Atlantic and Northeast. The passage of the front will also bring renewed lake-effect snow activity downwind of the Great Lakes by Wednesday and Thursday. The West will start off relatively quiet in terms of weather today. A few scattered snow showers are possible along a lingering frontal boundary from the Pacific Northwest to the northern Rockies. By this afternoon, however, a Pacific low pressure system accompanied by an expansive plume of moisture will begin to approach the coast, spreading rain and mountain snow into northern California. Precipitation will continue to expand across the West Tuesday night into Wednesday, spreading into much of the Great Basin, Pacific Northwest, and northern Intermountain West, where colder air already in place will allow precipitation to fall as snow. Warmer air will gradually push into the region as the low pressure system approaches, raising snow levels Wednesday night and Thursday. The rapid progression of the warm front northward could also result in areas of sleet or freezing rain at lower elevations where cold air could remain trapped. The potential exists for significant snow accumulations across portions of the Great Basin, northern Intermountain West, and Pacific Northwest, along with some ice accumulations in valleys. Additionally, flooding rains are possible across much of central and northern California. The threat for heavy precipitation is expected to persist into Thursday as multiple waves of low pressure move along the frontal boundary into the West Coast. Please refer to products issued by your local NWS forecast office for further details specific to your area. Ryan Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php