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.

The threat of strong tornadoes, damaging hail and high winds is highest in the upper Midwest through Monday night. Severe weather and heavy rain will then stretch from the Great Lakes to the Southern Plains on Tuesday.

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