Surface observations and satellite imagery late Friday showed ingredients for a major storm system phasing over the Plains. Warm, moisture-rich air was streaming northward across Texas, while a powerful, 140 mph jet stream approached from the southwest. On Saturday, low pressure over Kansas is forecast to intensify as it moves toward the upper Mississippi valley. To the north, winter storm warnings have been posted. At Minneapolis, where the season’s snowfall is already more than 17 inches above normal, new snow accumulations of up to a foot are expected. In the storm’s warm sector, an outbreak of severe thunderstorms is anticipated to impact areas as nearby as southern Illinois. Waves of rain, with a few embedded thunderstorms are to sweep across the Chicago area Saturday afternoon and evening. Wind gusts may top 40 mph over the weekend.