A dynamic storm system bringing Severe Weather and Excessive Rainfallimpacts to the Southeast this afternoon will transition to a strongnor'easter for parts of the Mid-Atlantic and New England by Tuesday.New storm system to arrive across the Northwest by the middle of theweek with areas of locally heavy rain and mountain snowfall.A significant winter storm associated with a strong upper-level trough andassociated closed low will cross through the Ohio Valley tonight, whichwill continue to interact with enough cold air to produce some areas oflocally heavy snowfall across portions of the Ohio Valley to the north ofthe low track. Light accumulations are expected across the Tennessee andOhio Valleys tonight.Farther south and east across the Southeast, and on the warm side of thestrengthening area of low pressure crossing the Tennessee/Ohio Valley, thenorthward advance of moisture and instability from the Gulf of Mexico andits interaction with a strong frontal zone will continue to produce someheavy showers and thunderstorms across the Carolinas and Georgia earlythis evening before pushing into the Atlantic. There's a Slight Risk ofExcessive Rainfall leading to Flash Flooding over portions of southernSouth Carolina into central/southern Georgia to account for this threat.This storm system will arrive across the Mid-Atlantic states tonight andwill transition to a strong nor'easter Tuesday morning as low pressureexits off the Mid-Atlantic coast near the Delmarva and ejects welloffshore of southern New England by Tuesday night. The concern willrefocus back to winter weather impacts as moisture surging northward aheadof the low center encounters sufficient levels of cold air for a swath ofheavy accumulating snowfall. There continues to be an unusual amount ofuncertainty with this forecast, particularly with where the axis ofsnowfall will develop and occur. The latest guidance suggests heavysnowfall occurring from southern New England down into southernPennsylvania/New Jersey. Generally, between 6-12 inches are probable forportions of the aforementioned areas. The nor'easter will bring strongwinds to the region on Tuesday which coupled with the heavy snowfall coulddamage trees and power lines. The strong winds will also bring a threatfor coastal flooding.A new storm system meanwhile will arrive across the Northwest by themiddle of the week which will bring areas of heavy rainfall to the coastalranges of the Pacific Northwest, and heavy snowfall for the higherelevations of the Cascades. This snowfall threat will also extend eastwardinto the northern Rockies as Pacific moisture streams inland.