Update, 9:10 a.m. Tuesday: The National Weather Service says its forecast of rain still appears to be on track, with chances of rain increasing during the day Wednesday and precipitation expected Wednesday night and early Thursday. But don't expect a gully washer or a frog choker. The NWS Bay Area office in Monterey says weather models suggest that higher elevations in northern Sonoma County — locations like Cazadero that can see several inches of rain in a day during the kinds of winter storms we might see in a wet winter — might get an inch of rain out of the approaching storm. To the south, we're looking at a quarter-inch or less. Not much more than would wet the bottom of a rain gauge, but we'll take it.
Original post (Monday afternoon): The National Weather Service is saying that it's looking more and more likely that people in the Bay Area will have an excuse to use their umbrellas before the end of January. Forecasters are tracking a storm that weather models show will break through the "ridiculously resilient ridge" of high pressure that has kept virtually the entire state bone-dry for weeks.
The best opportunity for rain in the immediate Bay Area in the next few days appears to be on Wednesday night and Thursday, when the current forecast says there's a 50 percent chance of rain for San Francisco, with that probability higher to the north and slightly lower to the south of the Bay Area.
The Wednesday/Thursday storm won't be a deluge or a drought-buster. Forecasters say the mountains in northern Sonoma County could see about three-quarters of an inch of rain, with areas around San Francisco Bay getting close to a quarter-inch. The California-Nevada River Forecast Center says the upcoming series of storms could bring about 2 inches of rain to the Lake Shasta watershed and as much as 3 inches to the highlands above Lake Oroville. Those are the state's largest reservoirs, and each currently holds just 54 percent of the water they normally store at this time of year.
A modest dumping of snow is on the way, too. NWS forecasters in Sacramento say that as much as 10 inches may fall on locations at 7,000-feet elevation and above. The Sierra snowpack, which typically stores millions of acre-feet of water and then releases it as the spring and summer progress, is at 12 percent its normal level for this point in the season.