Late this afternoon, a potent upper level low is spinning above western Minnesota, its cold front buldging across southern Canada and the Great Lakes region and then curving to the southwest, crossing the Upper Mississippi Valley, passing through Missouri and ending in West Texas.
Counterclockwise winds spin around the low, dragging down cool air from Canada to its west; in concert, ahead of the front, warm, moist air is pulled up from the Gulf of Mexico. Along and just behind the cold front showers and thunderstorms (some tornadic) are dropping a swath of rain, showing up as a giant comma on the national radar. The precipitation has been especially heavy here in Missouri as the storms "train" northeastward along the front.
Severe weather is most likely to occur along the southern tip of the comma (in central and west Texas) or east of the low pressure in northeast Minnesota. As the system drifts east, skies will gradually clear and autumn-like air will invade the Heartland; by then, the comma of storms will curve through the Ohio and Tennessee Valleys and stretch southward to the Gulf Coast.