Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Thu Apr 18 2013 Valid 12Z Thu Apr 18 2013 - 12Z Sat Apr 20 2013 ***Severe thunderstorms and heavy rain for the eastern U.S.*** ***Patchy areas of valley rain and mountain snow out West*** A powerful storm system, driven by a deep upper level low and trough moving out of the Rockies into the central U.S., is forecast to continue evolving across the nation's midsection today and then move northeastward into the Great Lakes region. There will be widespread bad weather with this surface low and its frontal boundaries, including the threat of severe thunderstorms for much of the Deep South, Middle Mississippi Valley, and extending northward into the Ohio Valley. Heavy rainfall will also be a concern as deep moisture streaming north from the Gulf of Mexico moves northward across a strong warm front. This will also hold true for the associated cold front that moves eastward. This system is forecast to track into the upper Great Lakes region by Friday morning and then enter Ontario and Quebec by the weekend. With strong low level winds continuing to deliver moisture from the Gulf of Mexico, showers and thunderstorms producing locally heavy rain are expected along the cold front as it approaches the east coast. Back to the northwest across the Upper Midwest, closer to the track of the associated upper low, light to moderate snowfall is likely to be found across parts of the Dakotas, Minnesota, and into the Upper Peninsula of Michigan. In the western U.S., valley rain and mountain snow is expected to spread across the Pacific Northwest into the northern Rockies late Thursday and into Friday as a weak upper disturbance moves across the region. The heaviest precipitation is likely to occur over westward facing mountain slopes. Hamrick Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_wbg.php