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.

ST. LOUIS — An elementary school south of St. Louis was damaged by a possible tornado as severe storms rolled through the region on Tuesday. The storm caused damage to Potosi Elementary School and left debris from a nearby building on the school’s parking lot.