DALLAS — A destructive winter storm marched across the United States on Wednesday, delivering blizzard-like conditions to the Great Plains hours after tornadoes touched down in parts of Texas, Oklahoma and in Louisiana where a child was killed and his mother remains missing. Five tornadoes were confirmed across north Texas as of Tuesday afternoon based on video and eyewitness reports, but potentially a dozen may have occurred, the National Weather Service in Fort Worth, Texas, reported. The severe weather threat continued into Wednesday for Louisiana, Mississippi, Alabama and the Florida Panhandle, according to the Storm Prediction Center in Norman, Oklahoma.