SAN DIEGO -- Unexpectedly widespread downpours doused the San Diego area Monday, delivering rainfall totals that proved modest but sufficient to snarl traffic for morning commuters.

Though the early autumn showers had been predicted, they came down over a much larger area than anticipated, according to the National Weather Service.

"Some lower pressure along the coast destabilized the air and caused some isolated showers we weren't really expecting," NWS forecast Stan Wasowski said.

Rain began falling in the East County shortly before 6 a.m. and within 90 minutes was pelting Interstate 5, south of State Route 54 in Chula Vista.

About 8 a.m., a cloudburst was drenching eastern San Diego in the area of Interstate 805 and SR-94.

The mild storm also produced scattered lightning. One bolt struck a palm tree in a residential yard near SR-78 and Escondido Avenue in Vista, sparking a small fire.

The California Highway Patrol, meanwhile, logged 48 collisions on the region's rain-slick roadways between midnight and early afternoon. By comparison, 50 to 75 crashes usually occur on local roadways enforced by the CHP over an entire day of fair weather, according to the state agency.

The showers had largely "petered out" by midday, Wasowski said. The highest rainfall reading for the day was a quarter of an inch along the coast of Oceanside, he said.

The cloudbursts dropped 0.12 of an inch of moisture near Cuyamaca Reservoir, 0.02 in Rancho Bernardo and 0.01 in the area of Brown Field airport, just north of the U.S.-Mexico line.