MONTEREY >> Rain from an approaching storm pattern that will have much of the Bay Area under a flood watch this weekend began to fall Friday and is expected to pick up in intensity as the weekend progresses, according to the National Weather Service.

“Everything seems to be on track,” NWS meteorologist Joe Merchant said. He added that for much of Friday and into today, the region would see a “steady batch of rainfall.”

As expected, the heaviest precipitation Friday before 1 p.m. came to areas of the North Bay in Sonoma and Napa counties. Petaluma received about four-tenths of an inch of rain and Santa Rosa received nearly a quarter inch in Sonoma County and Saint Helena received a quarter inch in Napa County.

About five-hundredths of an inch fell on Mount Diablo and .03 inch dropped in Orinda to pace Contra Costa County. Areas of Alameda County had received only .01 inch, and Santa Clara County had not received measurable rain by midday.

Any measurable rain in those counties would be the first since Jan. 3, according to the weather service.

The storm Friday was expected to have more of an influence in the northern part of the region and state, before a second round in the pattern rolls through closer to the central part of the region and state early into next week.

“The second pattern is gonna be a classic atmospheric river-type storm,” Merchant said. “The moderate to heavy rainfall is going to shift south.”

In anticipation of the rain — which remains in the NWS forecast for most of the region steadily through Tuesday — the agency issued a flood watch for Alameda, Contra Costa, Santa Clara, San Mateo, Santa Cruz, Marin, San Francisco, Napa and Sonoma counties late Thursday. The flood watch will be in effect from noon Friday until 10 p.m. Sunday.

According to the weather service, there were be a marginal to slight risk of flooding in the North Bay, in the interior portions of the East Bay and South Bay, and in the Santa Cruz Mountains.

Around 1 to 3 inches of rain are expected across the region by Sunday afternoon, and some higher elevations could see up to 5 inches, according to the weather service.

The weather service urged residents to monitor the forecast and be alert for possible flood warnings.

“Basically from about noon (Friday) throughout the day (today), it’s gonna fall,” Merchant said. “Even though there’s some lull on Sunday, you’re still gonna see light rain across the entire area. Then later Sunday, you’ll see some of the heavier stuff.”

The system also brought a winter weather advisory for the Sierra Nevada mountains, where 12-18 inches of snow above 6,500 feet were expected by Sunday.

The advisory went into effect at 4 a.m. Friday and is scheduled to last until Sunday at 4 a.m.