The first significant rainfall of the season is expected Monday, bringing up to one inch of rain to the North Coast in the first of three storms expected to pass through the region next week, the National Weather Service said.
The area could receive as much as three inches of precipitation before the week is out, and snowfall is expected in the Sierra, as well, affecting mountain passes as early as Monday, forecasters said.
Bay Area temperatures also should cool significantly after a late season heat wave this past week, with day-time temperatures in Santa Rosa peaking in the mid-60s for most of next week, the weather service said.
Some cooling and partly cloudy skies were expected Friday and this weekend. But the most dramatic change was to begin late Sunday, with cold temperatures bringing light rain overnight and steady rain anticipated during the Monday morning commute, the weather service said.
"We're expecting the rain to be focused initially in the North Bay," NWS meteorologist Mark Strudley said.
The North Bay area should receive between a half inch and one inch of rain, with a quarter to a half inch falling in the Bay Area, Strudley said.
Forecasts for later in the week are still tentative, but weather models suggested a second system arriving at mid-week and a third toward the end of the week, Strudley said.
With most of the season's abundant wine grape harvest already brought in, the shift in weather was expected to produce few problems for local growers, Nick Frey, president of the Sonoma County Winegrape Commission, said Friday.
Less than 20 percent of the crop was expected to remain on the vines by Monday, and most of that would be cabernet, which should withstand the rain, Frey said.
A little zinfandel and chardonnay may remained unpicked, as well, but only small amounts, he said. The harvest already has been so good, the impact should be limited, he said.
"So I think while we'd prefer we have another week of dry weather where we could pretty much finish up harvest, if we don't get that I think it (the rain) should move through pretty quickly and damage should be pretty minimal," Frey said.
In the Sierra, meanwhile, a system coming out of Alaska was expected to bring much colder temperatures, gusty winds, rain and snow, with precipitation arriving as early as Sunday but the heaviest rain and snowfall expected Monday night through Wednesday, forecasters said.
Snow and rain were possible, however, from Sunday through Thursday, with snow levels as low as 5,500-6,500 feet north of Interstate 80 and down to 6,500-to-7,500 south of Highway 80, the National Weather Service said.