Update, 8:50 a.m. Friday: Our rainfall map is updated with storm rainfall totals. The wettest spot — not technically in our nine-county Bay Area, but close enough to count — was Ben Lomond in the Santa Cruz Mountains, with 5.02 inches. Now: Enjoy two or three days of sunshine (or at least dry weather) before the next storm arrives.
Update, 9:35 a.m. Thursday: Above: a map showing 24-hour rainfall totals through 8 a.m. this morning.
The storm is still dumping plenty of rain across the region, mostly from the Golden Gate south to the Santa Cruz Mountains, and by all appearances will be slow to exit. The North Bay and central Bay Area should see an end to rainfall by early afternoon. The South Bay and adjacent mountains could stay wet until late afternoon or early evening. Dry weather is expected Friday and through the weekend, with rain returning late Sunday and Monday.
The highest rain amounts — in northern Sonoma County, the East Bay Hills and Santa Cruz Mountains — were around 3 inches over the past 24 hours. That’s a little lower than expected — though the wettest parts of the Santa Cruz range will pick up more during the day today.
The storm passed without triggering flash flooding or other problems in the extensive North Bay burn zones.
Update, 7:20 a.m. Wednesday: The possibility of prolonged heavy rain arriving with the Bay Area’s first major storm of the season has prompted the National Weather Service to issue a flash flood watch for the extensive swaths of the North Bay burned in last month’s wildfires.
As expected, light rain began falling overnight in the North Bay, with totals generally under a tenth of an inch by dawn Wednesday. The exceptions: Dillon Beach with .13 of an inch and Bodega Bay with .11.
As the storm intensifies later in the day, rain could fall at a rate of half an inch per hour in some spots, the weather service says. Flash floods could occur in the burn scars left behind by the Atlas (Napa County), Tubbs (Napa and Sonoma counties), Nuns (Sonoma and Napa) and Pocket (Sonoma) fires. Forecasters singled out the devastated Fountaingrove neighborhood in northeast Santa Rosa as an area of special concern.
The watch is a notice that flash flooding could occur and touch off debris flows in the affected areas. The watch issued Tuesday night is in effect from 4 p.m. Wednesday through 3 a.m. Thursday.
Forecasters also issued a separate flood watch for southeastern Mendocino County, again due to concern about the effect of heavy rain on areas that burned last month near the communities of Redwood Valley and Potter Valley.
In the Sierra Nevada, meantime, a winter storm warning is in effect through Friday morning. The current forecast calls for 5 to 11 inches of snow at elevations above 7,000 feet, with as much as 30 inches at 9,000 feet and above.
Original post: If weather forecast models are to be believed, the greater Bay Area is in for its first real regionwide soaking on Wednesday and Thursday. For bonus points, the storm is also forecast to bring high winds to hills and ridges around the bay.
The weather system, which will blend a cold storm moving south from the waters south of Alaska with a surge of warm, wet air flowing across the Pacific from the southwest, is also expected to bring a deluge to the Feather River watershed above Oroville Dam and unload heavy snow on the higher elevations of the Sierra Nevada.
The National Weather Service’s reading of the forecast models as of Tuesday morning is that rain will start along the northern Sonoma County coast early Wednesday morning and move slowly south toward the Golden Gate. The central Bay Area and South Bay aren’t expected to see significant rainfall until after the evening rush hour, with the highest rain rates late in the evening and early Thursday morning.
Rainfall amounts are expected to range from 3 to 5 inches in the North Bay hills and the Santa Cruz Mountains, with anywhere from a half-inch to 1.5 inches at lower elevations.
The rain will be accompanied by strong southerly winds — 20 to 30 mph, gusting to 50 mph — at higher elevations throughout the region. Those winds should pick up in the North Bay hills early Wednesday afternoon and sweep south across higher ridges and peaks through early Thursday morning.
Outside the Bay Area, the storm is expected to bring as much as 3 feet of snow to the higher reaches of the Sierra and the state’s northern mountains.
And forecast maps show a heavy precipitation bull’s-eye in the Feather River, with as much as 6 inches of rain in the rugged county above Oroville Dam. That’s of note because the level of Lake Oroville, the immense reservoir behind the dam, will be closely monitored during the coming wet season.
Construction crews recently wrapped up the first season of a massive project to rebuild the dam’s main spillway, which failed dramatically last year, and are still working on strengthening an eroded hillside that serves as the facility’s emergency spillway.
The California Department of Water Resources says the half-completed main spillway will be able to handle high flows this season, if necessary. At the insistence of federal regulators, DWR has also moved to lower the level of Lake Oroville to make plenty of room for any incoming runoff over the next seven months.
On Tuesday morning, the agency reported the lake’s surface level at 689 feet — 124 feet below the spillway inlet and about 160 feet below the level at which the managers would be required to begin releasing water to make room for heavy runoff from Feather River tributaries above the reservoir.