San Francisco expecting 1.5 inches of rain after North Bay soaking

The entire Bay Area should start to see downpours throughout Thursday, but the brunt of a storm sweeping across the region is expected to hit north counties the hardest with four inches of rain forecast to fall by Friday afternoon, forecasters said.

The precipitation was already being felt early Thursday in Sonoma County and parts of Marin County with the rainfall predicted to move southward into San Francisco at around 10 a.m., said Bob Benjamin, a forecaster with the National Weather Service in Monterey. The rains will be relatively persistent, with a few half-hour breaks, from Thursday until Friday night when isolated showers will move in, he said.

By Friday afternoon, San Francisco is expected to pick up 1.5 inches while areas in Sonoma County could get up to four inches with potentially six in the coastal mountains, according to Benjamin.

Santa Rosa was reported to have received .65 inches of rain while the remote Sonoma County community of Venado had seen 2.32 inches as of 8 a.m. Thursday, he said.

The prospect of stormy conditions prompted the National Weather Service to issue a flash-flood watch for most of the Bay Area.

While the next 36 hours should be especially wet, the conditions will mostly dry out by Saturday morning with a slight chance of showers on Saturday before another storm moves in on Sunday, according to Benjamin.

Hamed Aleaziz is a San Francisco Chronicle staff writer. Email: haleaziz@sfchronicle.com Twitter: @haleaziz