.Severe weather and isolated flash flooding for the Great Lakes/OhioValley last evening; threat shifts to the Ohio and middle Mississippivalleys and Southern Plains Thursday......Warm to hot temperatures across the southern U.S.; chillier weatherspreading southeast from the northern Rockies/Plains...Showers and thunderstorms will continue to develop into the evening alongand ahead of a cold front now advancing eastward across Lower Michigan andthe Ohio Valley.  Some of these storms may become strong to severe,producing damaging winds, large hail, and a few tornadoes.  In addition tosevere weather, locally heavy rainfall and isolated flash flooding areconcerns as well.Later this evening, storms are forecast to develop from the central Plainsinto the lower Missouri Valley.  These storms too may become strong tosevere, with damaging hail expected to be the primary threat. As the system in Plains moves further east, storms are likely to redevelopfrom the Ohio Valley into the southern Plains on Thursday.  Severe weatherwill once again become a concern, especially across portions of the midMississippi into the lower Ohio valleys.  The Storm Prediction Center hasissued an Enhanced Risk (level 3/5) for portions of the region, notingthat strong to severe storms capable of damaging winds, and possibly a fewtornadoes, are likely tomorrow.  While widespread heavy rains are notexpected, locally heavy accumulations resulting in isolated flash floodingare possible. Temperatures across the southern U.S. will remain warm through the period,with highs in the upper 80s and 90s common from the Southwest to theSoutheast over the next couple of days.  Much cooler temperatures nowcentered over the northern Rockies and Plains will spread southeastward,lowering temperatures across much of the Midwest and Ohio Valley by Friday.