SAN FRANCISCO (KRON) -- The National Weather Service issued a flash flood warning on Friday afternoon for San Francisco as heavy downpours dropped inches of rain on the city.

The warning will remain in effect from 12:43 p.m. until 3:45 p.m. as rapid rainfall continues. "Between 0.5 and 1 inch of rain has fallen. Additional rainfall amounts of 1 to 3 inches are possible in the warned area. Flash flooding is already occurring," the NWS wrote.

Local law enforcement agencies reported flash flooding and a "waterfall" on stretches of Interstate-280, including near John Daly Boulevard and Washington Street. Officers blocked off some lanes of I-280 to prevent drivers from trying to cross deep patches of water.

People should avoid walking or driving through flood waters, officials said.

The powerful storm is being generated by an atmospheric river and offshore bomb cyclone.