BORREGO SPRINGS, Calif. -- An afternoon thunderstorm dropped as much as 3 inches of rain in the deserts between San Diego and Indio in less than an hour Sunday, stranding an unknown number of motorists on roads near the Borrego Springs resort area.

California Highway Patrol officers and state parks rangers said people were trapped in their vehicles in some of the washes that were flowing across County Highway S-22 between Salton City and Borrego Springs.

That road crosses numerous washes that drain out of narrow mountain canyons in the Anza-Borrego Desert State Park, 95 miles northeast of San Diego.

The National Weather Service issued a flash flood warning for the Borrego Springs area shortly after noon and a worker at the Borrego Springs Airport said heavy thunderstorms raked the area.

State Route 78 was reportedly covered with debris near Scissors Crossing, a road junction south of Borrego Springs.

The afternoon thunderstorm was spreading to the north and east, with rain reported in the Coachella Valley south of Indio.

The storm came as sunny skies spread across the rest of San Diego, including areas near Escondido that received as much as 2 inches of rainfall during intense thunderstorms starting at 6:30 a.m.