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.

"Tornadoes, some possibly strong (EF2 or greater damage), as well as large hail and damaging winds, are all possible threats," he said. EF2 tornadoes have wind speeds of up to 135 mph, the weather service said.