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 an extreme risk of wildfires.

Local authorities say at least 28 people have been killed in weather-related mayhem that has raked the central United States, demolishing homes, fanning wildfires and wrecking motorists’ vehicles. In Missouri,