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.

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,