The National Weather Service’s Phoenix office issued successive flash-flood warnings for Imperial County, California, on Sunday, extending the alert to 7:15 p.m. Pacific time. Forecasters also posted a severe-thunderstorm warning for the county’s southwestern section as summer monsoon moisture triggered rapidly developing storms capable of producing sudden downpours. Storm cells were also tracking north-west into the Coachella Valley, the San Jacinto Mountains and the Banning Pass, bringing dry lightning and brief but intense rainfall near foothill communities, local weather observers said. Farther north, the annual Burning Man festival in Nevada’s Black Rock Desert faced a separate threat: meteorologists warned of 50-mile-per-hour dust storms and the potential for monsoon-driven thunderstorms, conditions that could hamper travel and outdoor events.