Widespread showers/ storms will bring the threat of severe weatherand flash flooding from the Southern Plains eastward through the MiddleMississippi/Ohio Valleys Monday...Another Critical Risk of Fire Weather for the Southern High PlainsMonday.Advancing storm system will bring increasing showers and storm chancesto much of the eastern U.S. Tuesday, with another risk of severe weatherin the Ohio/Tennessee Valleys and Mid-Atlantic.Unsettled weather in the West, with lower elevation rain and higherelevation snow, continues Sunday night but will taper off through the dayMonday.A storm system passing through the central and eastern U.S. will bring thethreat of severe weather and flash flooding as well a risk of fire weatherover the next couple of days. A deep, energetic upper-level trough overthe West will begin to shift eastward over the Plains Sunday night intoMonday. Increased height falls will help to deepen/organize an area of lowpressure in the lee of the Rockies over the Central Plains, with increasedflow helping to reinforce a quasi-stationary frontal boundary extendingeastward through the Middle Mississippi/Ohio Valleys and a drylineextending southward through the Southern Plains, with a cold frontapproaching from the west through the Southern Rockies/High Plains.Showers and storms will first expand in coverage Sunday night along thequasi-stationary boundary through the Middle Mississippi Valley eastwardinto the Ohio Valley and Mid-Atlantic as moisture advecting northward fromthe gulf continues to pool along the boundary. Enough shear will bepresent that a few storms may produce some large hail from northernMissouri eastward through central Illinois/Indiana through Sunday evening,with a Slight Risk of Severe Weather (level 2/5) in place from the StormPrediction Center. An isolated threat for flash flooding will exist hereas well, extending eastward into the Upper Ohio Valley, as heavy rain mayhave the tendency to repeat over areas as storms run parallel to theboundary.Another day of return flow from the Gulf as well as the arrival of theupper-level trough from the west on Monday will bring a much broader areaof showers and thunderstorms along the east-west boundary as well assouthward along the dryline through the Central/Southern Plains. Somesupercells are expected to develop along the boundary from southernMissouri west through southeastern Kansas and southward along the drylineinto central Oklahoma by late afternoon/early evening in the presence ofincreasing buoyancy and upper-level shear. The Storm Prediction Center hasissued an Enhanced Risk (level 3/5) of severe weather for the chance ofvery large hail as well as damaging winds and a few tornadoes. Some moreisolated storms will be possible further south into northern Texas, with aSlight Risk in place. A Slight Risk also extends eastward through the OhioValley where some isolated daytime storms and the arrival of stormsgrowing upscale upstream over the Middle Mississippi Valley/Plains willbring the threat of damaging winds, as well as large hail and a fewtornadoes.  By evening, increasing storm coverage and the potential forupscale growth into an organized complex of storms, as well as thetendency for storms to move parallel to the frontal boundary, will lead toa higher chance of locally heavy rainfall totals and some scattered flashflooding, with a Slight Risk of Excessive Rainfall (level 2/4) in place.In addition to the threat of severe weather and flash flooding, verystrong winds upwards of 40 to 50 mph, with gusts as high as 75 mph,combined with very dry conditions behind the dryline over portions of theSouthern High Plains have prompted another Critical Risk of Fire Weather(level 2/3) .