Millions in the Upper Midwest face the possibility of a severe thunderstorm outbreak Monday—the start of several days of extreme weather threats across the country this week.

"Tornadoes, some possibly strong (EF2 or greater damage), as well as large hail and damaging winds, are all possible threats," he said. EF2 tornadoes have wind speeds of up to 135 mph, the weather service said.

Monday is probably the peak of this event, but a concerning storm risk shifts eastward Tuesday. Cities in the highest risk for tornadoes include Minneapolis and Rochester in Minnesota and La Crosse in Wisconsin.