Flash Flooding possible across parts of northern/central Californiatoday, then central/southern California on Thursday.Heavy Snow spreads into the Sierra Nevada, Great Basin, IntermountainWest and Rockies.Above average temperatures continue across middle third of thecountry.Moisture will focus along a strong surface front entering the West Coasttonight. Heavy to excessive rainfall is expected to impact the southernOregon coast down through central California tonight into early Thursdaymorning. A Slight Risk (level 2/4) of Excessive Rainfall leading to FlashFlooding is in effect for much of the aforementioned areas this evening.The Flash Flooding threat shifts southward into southern California,portions of the Central Valley and the foothills of the Sierra Nevada onThursday as the axis of heavy rain and thunderstorms sags south into ahighly sensitive and/or urbanized corridor. A Slight Risk (level 2/4) ofExcessive Rainfall is in effect for much of the southern California coastfrom Los Angeles County down through the San Diego metro.Heavy Snow will expand from the Shasta Siskiyous to the Sierra Nevadatonight before spilling out over the Great Basin on Thursday andeventually portions of the Intermountain West and Rockies on Friday. Theheaviest snowfall will occur over the Shasta Siskiyous and Sierra where1-3' feet (isolated 4') are expected. Generally 1-2 feet are expected overthe higher elevations of the western mountains with isolated 2'+ amountspossible. Low elevation rain is also expected as there will be warm airassociated with this low pressure system. Cloudy conditions will keeptemperatures cooler than average for much of the West Coast and Southwestover the next couple of days.A strong omega block high will support above average heights andtemperatures across the Central Plains and Mississippi Valley over thenext few days. Low temperatures in the 30s and 40s may break existingrecords tonight over parts of the Central/Southern Plains and Upper/MiddleMississippi Valley. High and low temperature anomalies will be +20-40degrees above average. Elsewhere, snow showers are possible downwind ofthe Lower Great Lakes and interior Northeast over the next couple of daysdue to a low pressure system coming out of Canada. 2-4" of snowfall mayaccumulate over northern Maine by Friday.