Cold, wet weather is on the way early next week, when a series of winter-like storms are expected to bring rain to the North Coast and snow to the Sierra, the National Weather Service warned Thursday.
The change is expected to be so significant that the weather service has issued a "special weather" alert for most of Northern California, in part so anyone traveling to the high country can be forewarned about winter conditions, National Weather Service meteorologist Steve Anderson said.
In Sonoma County "it will be on the order of 30 degrees cooler than (Wednesday) by the middle of next week," Anderson said Thursday.
An onshore sea breeze cooled down the region Thursday, when temperatures peaked in Santa Rosa at 75 degrees — already a significant drop from a high of 97 one day earlier.
Temperatures were forecast to fall another 10 degrees or so through the weekend before the rain arrives Monday or early Tuesday.
Forecasters expect a quarter- to a half-inch of rain north of the Golden Gate Bridge on Monday as the first front passes through the area.
Another, likely stronger wave of cold, wet weather with high winds and heavy rain was expected to come through the region Wednesday into Thursday, Anderson said.
"It's a winter-like storm, early — about three weeks early," Anderson said. "Typically we see our first one toward the end of October."
The looming storm is an added stress for winemakers around Sonoma County, where harvest got off to a late start because of the cool summer.
Only 20 percent of the county's $400 million grape crop will be harvested by the end of this week, leaving the vast majority of the crop exposed, estimates Nick Frey, president of the Sonoma County Winegrape Commission.
Vineyard managers are attempting to navigate a delicate balance between the need to get grapes off the vine before it rains, and the desire to let them ripen as long as possible.
"Everybody wants to pick the day before the rain. And it's not possible," said Glenn Alexander of Bacchus Vineyard Management, which farms about 600 acres.
The main concern is that cool, wet weather could damage grapes by creating conditions for botrytis, or "bunch rot," Frey said. If the rain is followed by wind that dries things out, it might not be so bad.
"You just don't want rain during harvest if you can avoid it," Frey said. "It's always a concern."
Growers in Lake County also were scrambling to get grapes off the vine, especially growers with large operations, said Greg Hanson of Hanson Ranch Vineyards.
"They're going to really push the crews they have to get as many grapes as they can this week and this weekend, before the rain," Hanson said. "Hopefully it won't happen."
In the Sierra, snow was expected above 7,000 feet in some areas during the mid-week storm, the National Weather Service said.
Travelers routed through trans-Sierra passes were advised to carry snow chains.
The weather service reminded low-landers to take steps to prepare for the onset of rainy weather — for instance, clearing leaves that might otherwise clog storm drains.
Motorists also should be ready for oil-slickened roads made all the slippier by rain, forecasters said.
But just as quickly as the weather changes this weekend, it should shift back later next week to more seasonally appropriate conditions, with sunny skies and temperatures rebounding into the upper 70s, Anderson said.