TUSCALOOSA, Ala.—Aid for tornado-racked neighborhoods across the South began to arrive in force Friday, as authorities and relief groups brought victims medical care and food—and helped them pick through rubble from the worst storm in decades.

More than a dozen friends congregated at the remains of Donna Chambers's home in Fort Payne, Ala., trying to salvage what they could. A plaid mini-skirt sat on the driveway. A water heater lay in the grass of the front lawn. Two cars sat in what used to be the den.

"Really,...