Severe weather and isolated flash flooding mostly likely over theMidwest into the Ohio Valley in early Friday with thunderstorms extendingsouthwest into Texas.Wet snow and wintry mix over the central High Plains on Saturday asheavy rain threat develops and expands over Texas.Unseasonably warm across much of the South and Southwest as cold airfrom Canada pours south into the Great Plains.A relatively progressive weather pattern is forecast to establish acrossthe nation as we head into the weekend.  This progressive pattern willkick a relatively slow-moving low pressure system further out into theAtlantic while allowing a cold air mass from Canada to pour southward intothe Great Plains.  A low pressure wave currently developing along theboundary of the cold air mass over the Midwest will bring an enhancedthreat of severe thunderstorms across the Midwest tonight, moving into theOhio Valley early on Friday.  A slight chance of severe thunderstorms isforecast to extend farther southwest into central Texas near the trailingportion of the cold front.  In addition to the severe weather threat, theheavy rain associated with the thunderstorms may result in flash floodingconcerns at some places for aforementioned areas.By Friday, the cold front is forecast to move steadily east toward theEast Coast.  Showers and thunderstorms will likewise move farther east outof the Ohio Valley and into the Appalachians, reaching into the interiorMid-Atlantic, Southeast, and New England by Friday evening.  The severeweather threat is expected to be not as high on Friday for these areas. Saturday will see these showers and storms steadily exiting the East Coastas the cold front pushes off the coast into the Atlantic.  However,interior New England will see the showers linger due to the arrival of areinforcing cold front from Canada.As a large dome of high pressure system associated with the Canadian coldair mass settles into the Great Plains, the upslope dynamics will set upthe opportunity for wet snow to develop over the central High Plains onSaturday, mixing with rain during the day.  Meanwhile, as an upper troughmoves farther east into northern Mexico on Saturday, the associateddynamics will support an expanding area of rain over the southern Plainsas the trough interacts with moist air returning from the Gulf of Mexico. It appears that the threat of heavy rain threat will expand througheastern Texas by the end of the forecast period on Saturday evening.Much of the western U.S. will remain dry through the next couple of daysbut the next Pacific system will likely break the drought later onSaturday as rain reaches the Pacific Northwest.  Forecast hightemperatures will remain rather warm across southern and southwesternportions of the country through Saturday but the cold front will dispelsome of the warmth across the Mid-South, Midwest, Great Lakes, andeventually the Northeast with the passage of the cold front.  In contrast,cold air will gradually push southward through the Great Plains reachinginto central Texas by Friday.