A string of thunderstorms generated a dust storm in the Phoenix area, and 60 mph winds downed trees and damaged property, forecasters said. The National Weather Service said wind gusts toppled trees at least a foot thick and tore the roof off a house, The Arizona Republic in Phoenix reported. Officials said several areas reported hail. Electricity was knocked out for nearly 5,500 customers, utility officials said. Officials said the the storm system\'s winds blew out vehicle windows and caused minor injuries. Blowing dust Thursday reduced visibility to less than a quarter-mile, the Republic said. The dust storm grounded traffic at Phoenix Sky Harbor International Airport for about 40 minutes, airport spokesman Julie Rodriguez said.