An atmospheric river of moisture is slated to come ashore Sunday. A low-pressure system will tap into its warm, southerly winds by the evening and raise showers across the Bay Area.
Baron/LynxAfter a calm week, California’s weather pattern is about to take a more active turn, with stormy weather expected to arrive by the end of the weekend.
A ribbon of strong, humid winds high above the ground will flow between Hawaii and California throughout the weekend. This so-called subtropical jet stream is expected to reel in large amounts of moisture toward the West Coast, fueling a storm system that’s slated to bring wind and rain impacts as early as Sunday.
Calm before the storm
Mostly calm conditions are expected just before the storm system approaches the coast. A warming trend Friday will raise temperatures to the upper 60s throughout the Bay Area, with lower 70s possible inland. Winds will start out light Friday and Saturday, allowing pollutants to pile up in the Sacramento Valley and Bay Area. The Bay Area Air Quality Management District issued a Spare the Air alert for Friday, and air quality concerns are expected to linger into Saturday morning.
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The National Weather Service issued a coastal flood advisory for Friday that covers the shoreline along San Francisco Bay.
Baron/LynxResidents strolling along San Francisco Bay will also want to watch for high tide Friday afternoon. The National Weather Service issued a coastal flood advisory for the shoreline that includes San Francisco, Alameda and Vallejo. Tidewaters could spill onto low-lying areas, causing minor flooding on beaches and nearby roads. These conditions will taper off by Saturday.
As the tide recedes, warmer and muggier winds will increase over the Bay Area on Saturday afternoon. Daytime temperatures are expected to run 3 to 5 degrees above average for this time of the year in places like Santa Rosa and Livermore. In San Jose, the mercury could reach the lower 70s by Saturday, putting at risk the daily record of 72 degrees set back in 1998.
Northern Californians may want to take advantage of these warm and fairly quiet conditions. Weather enthusiasts will notice a drop in pressure on their barometers Sunday afternoon as conditions turn stormier.
Inclement weather
Stronger rounds of winds will arrive out of the south Sunday, clearing out particulates and eventually introducing the wettest weather since March from Sunday night to Monday morning. Bands of rainfall could bring up to half an inch of rain to communities along the bay shoreline. Downpours and 30 mph gusts will introduce an inch of rain to Sonoma and Marin counties and the delta region from Vallejo to Sacramento.
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Model rendering of a deep area of low pressure off the coast of California on Sunday evening. This system will quickly approach the Bay Area overnight.
Baron/LynxMore widespread rainfall is expected Monday and Tuesday as the storm system raises intermittent showers and isolated thunderstorms. The coastal mountains along Highway 1 will bear the bulk of these impacts, including the Santa Cruz Mountains and highlands of the North Bay. That said, moderate rainfall and a stray thunder shower or two isn’t out of the question for San Franciscans and Oaklanders. Look for an additional half-inch to three quarters of an inch of rain across the Bay Area by Tuesday, with an additional inch possible along the immediate coastline.
These upcoming showers and winds are slated to affect road conditions across the region between Sunday and Tuesday, so plan for longer commute times and weather-related delays to bus services.
The weather is expected to briefly improve by the middle of the week as dry air returns to Northern California. But the strong jet stream and bursts of moisture aren’t going away soon. El Niño conditions are expected to crank up over the next couple of weeks, meaning more storms are likely through the end of the year.
Weekend breakdown
San Francisco: Warm and humid weather is on tap for San Franciscans this weekend. Look for partly cloudy skies, light winds out of the south and daytime temperatures reaching the upper 60s downtown and mid-60s on the west side.
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Cloud cover will gradually increase Sunday as drizzle falls on Twin Peaks and the hillsides of the city in the evening. These drizzles will eventually turn to light rain overnight and into Monday morning, so plan for slick conditions.
East Bay: Alameda and Contra Costa counties can expect balmy conditions throughout the weekend, with daytime highs peaking in the mid- to upper 60s along the bay shore and lower 70s east of the Caldecott Tunnel in Tri-Valley area cities like Walnut Creek, Dublin and Pleasanton.
By Sunday, the air will be primed for cloudier skies and drizzle on the Oakland and Berkeley hills. Intermittent rounds of light rain are expected to fall by the evening, with heavier rainfall possible in the delta region. Watch for slick roads and ponding on sidewalks overnight.
North Bay: Visitors to the vineyards in Napa, Sonoma and Marin counties are in for a warm and muggy weekend, with daytime highs expected to reach the upper 60s by Saturday. Nighttime lows are slated to fall to the lower 40s, so plan for a big temperature swing. Similar conditions are on tap for Solano County, while light winds out of the south raise daytime temperatures to the mid-60s for San Rafael, Vallejo and Tiburon.
The weather takes a turn on Sunday as light rain returns to the Sonoma and Marin shoreline. These showers will become more widespread in the evening, evolving into moderate rain conditions for much of the region overnight.
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Pacific Coast and Peninsula: Those along Highway 1 from Daly City to Pacifica can expect occasional fog in the morning hours Friday and Saturday, followed by sunshine and warm daytime highs in the mid-60s. Temperatures across the rest of San Mateo County are slated to rise to the upper 60s this weekend, including in San Bruno, Millbrae and Redwood City.
Cloudier conditions return Sunday afternoon. As winds begin to blow out of the south, expect occasional gusts to 30 mph across most of the Peninsula. Light rain is expected in the evening, becoming heavier at times overnight and into Monday morning.
South Bay and Santa Cruz: Partly cloudy skies and light winds out of the south are expected across the Santa Clara Valley and Monterey Bay shoreline this weekend, with daytime highs reaching the mid- to upper 60s. A couple of spots like downtown San Jose, Morgan Hill and Gilroy can expect temperatures in the lower 70s Saturday afternoon.
These warm and balmy conditions will continue into Sunday, though cloud cover is likely to increase by the evening. Look for light rain and occasional gusts to 30 mph overnight and into Monday morning.
Reach Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave
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December 15, 2023 at 07:01PM
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Storm is headed to California. Here's when to expect rain - San Francisco Chronicle
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