Over the next week, tens of millions of Americans will be at risk of severe weather as a colossal storm marches across the country, threatening hurricane-force winds, major flooding, blizzard conditions and a possible tornado outbreak.

A huge storm system crossing the U.S. threatens to unleash tornadoes Friday in the Mississippi Valley, blizzards in the northern Plains and dry, gusty conditions in Texas and Oklahoma that pose a high risk of wildfires.