Heavy to Excessive Rainfall impact the West Coast this week, whileHeavy snow to blankets the Sierra Nevada, Intermountain West and Rockies.Above average warm air expands from Great Plains into MississippiValley.Tonight, a low pressure system moving across the Ohio Valley will spreadsome light rain showers over the region through portions of theMid-Atlantic. Snow showers and a wintry mix are possible over the CentralAppalachians of western Virginia, eastern West Virginia and western NorthCarolina. Some light snow may also work its way into eastern Pennsylvania,northern New Jersey, southern Upstate New York and New York City throughearly tomorrow morning. Little to no accumulations/impacts are expectedfrom this winter weather.Meanwhile, tonight, a strong surface low pressures system, supported by adeep upper-level low will arrive over the West Coast. This system willtransport deep moisture into the West over the course of the next fewdays. Heavy rainfall and scattered thunderstorms are possible from thesouthern coast of Oregon down into Los Angeles County. A Slight Risk ofExcessive Rainfall leading to Flash Flooding is in effect for portions ofthe southern Oregon coast down through central California on Wednesdaydue, in part, to the potential presence of instability generating higherrainfall rates over vulnerable surfaces. Heavy mountain snow (6000-7000')kicks off over the Shasta Siskiyous and Sierra on Wednesday afternoon.Once the upper trough moves ashore and the surface cold front moves intothe Great Basin on Thursday, snow levels will drop to about 4000-5000'leading to lower elevation heavy snowfall potential in the Sierra. Totalsnowfall accumulations will range between 1-3 feet by Friday, whenconditions are expected to improve. The focus for heavy rainfall willshift into southern California on Thursday, enhanced by a digging troughaloft, an embedded surface wave and attendant cold front at the surface.Rain and snow showers spread into the Great Basin, Intermountain West, andRockies on Thursday. Meanwhile, a low pressure system moving acrossOntario and Quebec will produce light snowfall over portions of northernUpstate New York/New Hampshire, Vermont and Maine beginning Thursdaymorning.An omega block high with embedded shortwave energy will continue tosupport increased heights over the Central U.S.. Warm temperatureanomalies ranging between 20-40 degrees above average will expand acrossthe Great Plains and Mississippi Valley through the end of the week as aresult. The potential for widespread record low temperatures will pick uplater in the week over the Northern/Central Plains and Upper/MiddleMississippi Valley. Lower heights in the West will generate coolertemperatures around 10-20 degrees below average later this week and intothe weekend.