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.

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.