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.

"A regional severe weather outbreak still appears possible across the Upper Midwest, with a threat of strong to intense tornadoes, very large hail, and swaths of damaging winds," said NOAA's Storm Prediction Center (SPC) in an update issued at 7:37 a.m. Monday.