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.

Storms with lightning and hail and at least one observed tornado were moving through the upper Midwest Monday with the potential for strong tornadoes. The National Weather Service said the highest risks — a 4 on a scale of 1 to 5 — were in portions of southern Minnesota,

Multiple rounds of severe thunderstorms are expected across the central U.S. on Monday, some capable of producing damaging winds, large hail and strong tornadoes, forecasters say. According to the National Weather Service,