Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 301 AM EST Sat Dec 11 2021 Valid 12Z Sat Dec 11 2021 - 12Z Mon Dec 13 2021 ...There is a Slight Risk of severe thunderstorms over parts of the Ohio, Tennessee Valleys, and parts of the Southeast through Sunday morning... ...There is a Slight Risk of excessive rainfall over parts of the Pacific Northwest and parts of Northern California through Sunday morning... ...Heavy snow over the Upper Great Lakes through Saturday afternoon; heavy snow for the Cascades, Sierra Nevada Mountains, and Northern Rockies... An area of deep low pressure over the Great Lakes will move quickly into Southeastern Canada by Saturday evening. The system will produce heavy snow over the Upper Great Lakes through Saturday afternoon. Rain will start over most of Michiganâ€s Lower Peninsula on Saturday morning that will change over to snow by Saturday afternoon. Rain will also develop over parts of the Northeast and change over to snow overnight Saturday before ending over Northern New England by Sunday morning. The snow will result in reduced visibility and hazardous driving conditions. Moreover, showers and thunderstorms will develop along and ahead of the cold front from the Ohio Valley to the Lower Mississippi Valley. Some of the thunderstorms will be severe. Therefore, the SPC has issued a Slight Risk of severe thunderstorms over parts of the Ohio, Tennessee Valleys, and parts of the Southeast through Sunday morning. The hazards associated with these thunderstorms are frequent lightning, severe thunderstorm wind gusts, hail, and a few tornadoes. Showers and thunderstorms will also develop over parts of the Mid-Atlantic on Saturday. The threat for severe thunderstorms ends on Sunday morning as the front moves offshore. In the meantime, the thunderstorms will contain areas of heavy rain. Therefore, the WPC has issued a Marginal Risk of excessive rainfall over parts of the Southern Appalachians, Ohio/Tennessee Valleys into the Lower Mississippi Valley through Sunday morning. The associated heavy rain will create localized areas of flash flooding, affecting areas that experience rapid runoff with heavy rain. The excessive rainfall threat ends on Sunday as the front moves off the East Coast. Meanwhile, a front moves onshore over the Pacific Northwest and moves inland to the Northern Plains by Monday. On Saturday, a plume of moisture will stream into parts of the Pacific Northwest and Northern California and slowly move southward over the coast to Southern California by Sunday evening into Monday. The system will produce rain and higher elevation snow over the Northwest, with snow extending inland to the Northern Rockies. Overnight Saturday, the rain and higher elevation snow will expand into parts of Northern California and Central California Sunday night into Monday. During this long-duration event, the snow will be heavy over the Cascades, Sierra Nevada Mountains, and Northern Rockies. The snow will result in reduced visibility and hazardous driving conditions. Furthermore, the rain will be heavy at times along the coast. Therefore, the WPC has issued a Slight Risk of excessive rainfall over parts of the Pacific Northwest and Northern California through Sunday morning. The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, and small streams the most vulnerable. Similarly, the WPC has issued a Slight Risk of excessive rainfall over parts of Central California from Sunday into Monday morning. Likewise, the associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, and small streams the most vulnerable. Ziegenfelder Graphics are available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php