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.

One of the most active days of severe weather so far this season is ahead for the upper Midwest early this week, including a risk of strong, long-tracked tornadoes.

"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.