Heavy rain, flash flooding and severe weather threat emerging acrossthe Southern Plains into the Lower Mississippi Valley throughTuesday & Wednesday.Critical fire danger shifts into Texas Big Bend by Tuesday.Tuesday will fair little better across the Southern Plains as theupper-level low brings cooling aloft and with continued warm, moist airoff the Gulf and dry air from the Mexican Plateau in the low tomid-levels; very unstable weather conditions will exist again across muchof Texa...