This weekend, much of the United States will experience cool, fall-like weather, while the South will continue to experience late-summer heat and the Northwest will get a preview of winter. Saturday and Sunday will bring rain and thunderstorms to the southern Plains, Mississippi Valley, Southeast, and Florida. There is a risk of flooding and isolated severe storms in these areas. Snow will accumulate in the Cascades and northern Rockies. High temperatures will range from the 30s in the Northwest peaks to the 90s in southern Texas, Florida, and the Desert Southwest.