Heavy rain, flash flooding and severe weather threat will expand intothe central Gulf Coast States, Tennessee Valley and parts of the Southeaston Wednesday, reaching into the Midwest, Ohio Valley and the East Coast byThursday.Critical fire danger remains over the Texas Big Bend and Rio GrandeValley through this evening.A very active weather pattern is currently unfolding throughout much ofthe southern Plains and lower Mississippi Valley as numerous showers andthunderstorms impact the region, with several modes of hazardous weatherpossible. Ample atmospheric moisture content, nearby frontal boundaries,and a supportive upper level environment are expected to continue thethreat of severe weather and flash flooding over the next few days.Through tonight, the bulk of the impactful weather is forecast to remainacross the southern Plains and lower Mississippi Valley, where manylocations have already been dealing with soaking rain and intensethunderstorms over the last 12 hours or so. With more heavy rain on theway, numerous instances of flash flooding are likely overnight fromeast-central Texas to parts of western Mississippi. A Moderate Risk (level3/4) of Excessive Rainfall remains in effect, as well as Flood Watchesextending from Texas to Alabama. Additionally, thunderstorms will becapable of producing a few tornadoes, large to very large hail, and severewind gusts.The disruptive weather is anticipated to expand eastward on Wednesdaythroughout the central Gulf Coast States, Tennessee Valley, and Southeast.Multiple rounds of strong thunderstorms could dump around 2 to 4 inches ofrainfall over a relatively short time period from northeast Louisiana tosouthwest Georgia and the Florida Panhandle. This has prompted theModerate Risk of Excessive Rainfall to be extended into these regions onWednesday. A Moderate Risk (level 4/5) for severe thunderstorms alsoexists on Wednesday from east-central Louisiana to southwest Mississippi,where the potential exists for several tornadoes (a few of which could bestrong), and widespread damaging wind gusts. Residents and visitors areadvised to have multiple ways to receive warnings and never drive throughflooded roadways.This storm system is forecast to become better organized by the latterpart of Wednesday and continue on a northeastward progression over thelower Great Lakes by Thursday night. Showers and thunderstorms will shiftinto the Ohio Valley, Southeast, and Mid-Atlantic, but should become moreprogressive in nature. The greatest threat for damaging wind gustsassociated with strong thunderstorms on Thursday extends from theCarolinas to northern Florida, as well as the Ohio Valley. Heavy rain mayalso lead to scattered instances of flash flooding from thesouthern/central Appalachians to the northern Mid-Atlantic. Lastly, strongsoutherly flow along the East Coast could lead to gusty winds and coastalhazards from the Southeast to the Mid-Atlantic on Thursday.Mostly tranquil weather is forecast elsewhere throughout the Nation asupper ridging builds over the Intermountain West. Rain and snow showersare expected to swing from the Northwest to the northern High Plainstonight as a frontal system quickly moves east and over the northernPlains on Wednesday. Otherwise, gusty winds and low relative humidity willcontinue to create Critical Fire Weather over the Texas Big Bend and RioGrande Valley into tonight, with Elevated Fire Weather continuing intoWednesday.