Just ahead of an “atmospheric river” storm that is expected to soak the Bay Area with as much as 6 to 8 inches of rain through Wednesday, light precipitation began falling Monday morning north of the Golden Gate with much heavier downpours expected later in the day and through Wednesday.

Through Monday at 12:30 p.m. rainfall totals in the North Bay ranged from a few hundredths of an inch to a few tenths, according to the National Weather Service. Venado in rural Sonoma County was the wettest location with 1.35 inches.

Tracking the #AtmosphericRiver – here is the satellite🛰️ derived imagery showing the moisture plume heading to California. This #AtmosphericRiver will bring flooding concerns to the region. Monitor the weather and NWS alerts over the next 2 days. #cawx pic.twitter.com/InJ2Fjo25a — NWS Bay Area (@NWSBayArea) February 25, 2019

Periods of moderate to heavy rainfall and strong, gusty winds are expected to impact the region later Monday, moving south to San Francisco and possibly the peninsula in the afternoon and evening, according to the National Weather Service.

The South Bay may remain dry during the daylight hours Monday but could see rain move into the area in the evening or overnight, according to Roger Gass, a meteorologist with the weather service.

“It’s still one of those tricky forecasts to nail down when the rain will begin,” Gass said.

Forecasters expect the storm to be focused on the North Bay, where San Rafael could see 6 to 8 inches of rain. The Peninsula and East Bay may see around 2 to 3 inches of rain through the end of the storm, while San Jose and much of the South Bay may only see about half an inch to an inch total.

The heavy rain will bring a potential for widespread flooding, and the weather service has issued a flood watch for Sonoma, Napa and Marin counties in effect from Monday afternoon through late Wednesday night. Flooding is also possible on the Guadalupe River in Santa Clara County.

Once the rain starts to increase in the North Bay, it’s “going to continue’ through the duration of the storm, Gass said.

“They’re going to see the bulk of the rainfall throughout this event,” Gass said.

An atmospheric river will impact the region today through Wednesday morning. Expect periods of moderate to heavy rainfall, widespread flooding from the Santa Cruz mtns into the North Bay, and gusty winds. #cawx pic.twitter.com/s5k62QtcLr — NWS Bay Area (@NWSBayArea) February 25, 2019

The Russian and Napa rivers are forecast to reach moderate flood stage Wednesday night or Thursday morning.

Meanwhile, a flash flood watch is in effect for the rest of the Bay Area from Monday afternoon through late Tuesday night, including the East Bay Hills and the Diablo Range, the Peninsula, and Santa Clara Valley, including the Santa Cruz Mountains.

In the Sierra Nevada, where some ski resorts have already recorded record amounts of snow in February, several feet of new snow are expected through Thursday morning, according to the weather service. A winter storm warning will be in effect for the Greater Lake Tahoe area from Monday morning until Thursday morning.

On Thursday, the statewide Sierra Nevada snow pack was at 144 percent of its historical average for this time of year, up from just 69 percent on New Year’s Day. As of Monday morning, the California Dept. of Water Resources had not yet posted updated measurements.

The “atmospheric river” storm is warm, water vapor-saturated air coming up from the Pacific region near Hawaii, mixing with a “potent” cold front of strong winds from the north, according to Steve Anderson, a meteorologist with the National Weather Service in Monterey.

“The North Bay is going to have sort of a jump start on the rain, and the more intense, heavier rain is focused up there,” said Steve Anderson, a meteorologist with the weather service.

Rainfall totals across Northern California for the water year that began Oct. 1 are near or above average for this time of year, including Redding (25.30 inches, or 107 percent of its historical average), San Francisco (17.94 inches, 104 percent), Oakland (13.96 inches or 95 percent), and San Jose (11.66 inches, 108 percent.)

File photo from the last storm, a driver runs into some flooding at the corner of Emeline Avenue and Grant Street on Wednesday morning, Feb. 13, 2019 in Santa Cruz. The region's wet weather is expected to continue through Saturday. (Anthony L. Solis -- Santa Cruz Sentinel)

File of a young boy leaping over a puddle as parents and children cross the street as they leave Chinatown's Lincoln Elementary School in downtown Oakland, Calif., on Tuesday, Jan. 15, 2019. Heavy rain and flooding is expected tomorrow. (Jane Tyska/Bay Area News Group)

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File of the closed 7th Street at Frontage Road after flooding in West Oakland, Calif., on Thursday, Feb. 14, 2019. (Jane Tyska/Bay Area News Group)



File of the levee along the Novato Creek that was breached during the deluge on Thursday, flooding a field near Highway 37. (Justin Sullivan/Getty Images)

File photo- With a big storm on the way Sidekicks on San Anselmo Avenue has started to prepare for potential flooding in San Anselmo, Calif. on Wednesday, Jan. 16, 2019. (James Cacciatore/Marin Independent Journal)

File photo-Wednesday's storm caused flooding at Felix and Neary streets. (Luis Barragan -- contributed)



Anderson said the weather system’s warm air will keep temperatures mild during the storm.

“You can throw off that extra blanket on the bed,” he said, noting overnight temperatures will range between 45 and 55, and they will hover round 60 in the daytime. “You’ll be able to feel the difference in the air…a more humid, tropical feeling.”

Well, here we go again! Several feet of new snow is forecast for the high Sierra with 2-3 feet for the #Tahoe Basin. This will be a marathon, not a sprint, as totals accumulate through early Thursday. Expect difficult/impossible travel over Sierra at times. #februburied pic.twitter.com/nsAiBtVE7M — NWS Reno (@NWSReno) February 24, 2019

And while the rain will be lingering here, the Sierra will see at least 2 to 4 feet of new snow, with 4 to 8 feet above 7,000 feet, the weather service said.

Winds could be as high as 60 mph in the area, with gusts over 140 mph for the Sierra ridges, and “periods of white-out conditions are likely,” the warning said, making travel “very difficult to impossible.”

“Not a time to be on the roads or the mountains,” Anderson said.

By Wednesday evening, much of the rainy weather should be cleared out of the Bay Area, but Anderson said wet weather could return as soon as the weekend.

Staff writer Jason Green contributed to this report.