Millions in the Upper Midwest face the possibility of a severe thunderstorm outbreak Monday that could bring rounds of dangerous storms capable of unleashing damaging winds, large hail and strong tornadoes — the start of several days of extreme weather threats across the country this week.

Monday is probably the peak, but a storm risk shifts eastward Tuesday. Cities in the highest risk for tornadoes include Minneapolis and Rochester in Minnesota.

Officials in the Upper Midwest warned of possible power outages and closed some schools early as the storms loomed.