.There's a Moderate risk of excessive rainfall along coastal southernCalifornia today......Heavy Snow likely over Sierra Nevada and Shasta Siskiyous inCalifornia......Warming trend expected to expand across the Great Plains throughmidweek...A potent upper-level low will continue directing subtropical moisture overmuch of California today. Heavy coastal and low elevation rain,thunderstorms, heavy mountain snow and high winds are all expected fromthis Atmospheric River event. Excessive Rainfall leading to Flash Floodingis possible for much of the state today. A Slight Risk of ExcessiveRainfall (level 2/4) is in effect from Humboldt to Orange County as wellas over parts of the Sacramento Valley and along upslope portions of theSierra. A Slight Risk of Severe Thunderstorms is in effect for parts ofthe Sacramento Valley where an isolated tornado will be possible. Atargeted Moderate Risk of Excessive Rainfall is in effect for SantaBarbara and Ventura where heavy rainfall will focus, especially overelevated terrain. Heavy Snow will blanket the Sierra Nevada and ShastaSiskiyous today as well, with those mountains forecast to receive 1-3 feet(isolated higher) of snow by Tuesday morning.The moisture feed into California will weaken considerably and sag southon Tuesday, but the threat of Flash Flooding will persist across southernCalifornia, in particular. Upslope flow into the Transverse ranges willsupport a renewed threat of Flash Flooding. A Slight Risk of ExcessiveRainfall is in effect from Humboldt down through San Diego County onTuesday mainly due to sensitive soils from today's heavy rain. Up to afoot of additional snow accumulations with locally higher amounts arepossible over the Sierra and Shasta Siskiyous on Tuesday. Damaging windgusts should continue into Wednesday morning before gradually weakening.Elsewhere, lake effect snow will come to an end this afternoon across theLower Great Lakes. Upper level ridging with embedded shortwave energy willsupport a warming trend acrosss the Great Plains and Mississippi Valleythrough midweek when anomalous temperatures are expected to reach theirpeak for the week. High temperatures will be between 15-25 degrees aboveaverage by Wednesday over much of the central U.S..