Heavy rain and flash flooding threat continues in the Southwest today.Heavy Snow for the Intermountain West mountains through mid-week.Temperatures will continue to run above average from the Plains to theNortheast with record breaking warmth for parts of the Upper MidwestShowers and thunderstorms have expanded into the Desert Southwest ahead ofa deep upper trough that is slowly moving east into the region. Locallyheavy rainfall could lead to scattered instances of flash flooding throughtonight in Southern California and western and central Arizona, especiallyfor the Transverse and Peninsula ranges and Mogollon Rim where terrainwill enhance rainfall. Although rain rates/totals will be trendingdownward across Southern California compared to the last couple of days,the risk for flooding and mud/debris flows remains elevated given the verywet antecedent conditions. For higher elevations, snowfall rates will becoming down for the Sierra Nevada and remain heavy for elevations above7000 feet in the Transverse/Peninsular ranges. Conditions will improvethrough the rest of the week as precipitation becomes lighter and theflash flood threat decreases. As the upper trough moves further inland, precipitation will expand acrossthe Intermountain West through mid-week. Heavy snowfall is forecast formany of the regional mountain ranges over the next couple days, withparticularly high totals expected in the Four Corners region where amountsof 2+ feet will be possible. Other ranges of the Great Basin and NorthernRockies will see totals generally between 6-12", with some locally higheramounts possible. Lower elevation/valley locations across the region willsee a mix of moderate rain and snow showers, although any snowaccumulations are expected to remain limited. Height falls beginning tooverspread the Northern/Central High Plains will also lead to enhanced leecyclogenesis and a developing low pressure/frontal system Wednesday. Thiswill bring increasing chances for wintry precipitation spreading eastwardacross the Northern High Plains as the system strengthens. Some freezingrain and sleet will be possible, as well as for a few inches of snow,particularly for northeastern Montana. Elsewhere, the Midwest, South, and East Coast will remain mostly drythrough mid-week. Upper-level ridging over the central/eastern U.S. willkeep temperatures generally mild and above average from the Plains to theNortheast/Appalachians, with below average temperatures along the coastalSoutheast and west of the Rockies. Highs across the Northern Plains/UpperMidwest in particular continue to remain upwards of 20-30 degrees aboveaverage, with 40s and 50s forecast. Some daily record-tying/breaking hightemperatures will be possible for the Upper Midwest  and Great Lakesregions today through Friday. Precipitation chances will return to theMidwest, South, and East later this week into this weekend as a frontalsystem pushes east.