A towering wall of dust swept across the Phoenix metropolitan area late Monday, reducing visibility to almost zero and draping the city in near darkness. The National Weather Service issued a dust-storm warning as winds approached 70 mph and visibility reportedly fell below 50 feet. The storm knocked out electricity to roughly 57,000 customers at its peak, according to PowerOutage.us, and downed trees and traffic lights across Maricopa County. Police and transportation officials urged motorists to pull off roadways until conditions improved. Operations at Phoenix Sky Harbor International Airport were suspended for about an hour, triggering a ground stop that forced more than 40 inbound flights to divert and contributed to over 300 delays recorded by FlightAware. Airport officials said heavy rain that followed the dust storm damaged a section of the roof at Terminal 4, but no injuries were reported. Forecasters said the event was one of the largest monsoon-season haboobs so far this year. The National Weather Service put the chance of additional storms in the Phoenix area at 40 percent on Tuesday before drier conditions are expected to return.