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.

Storms with lightning and hail and at least one observed tornado were moving through the upper Midwest with the potential for strong tornadoes.

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.