The American weather model’s 250 mb (contours) view at around 20,000 to 40,000 feet, with wind speeds (shaded) through this weekend and into early next week. The west-east flow will still hover over California, but a break in the jet stream over Japan and the central Pacific Ocean is a sign that the persistent west-east flow pattern that’s brought so much rain and snow to the Golden State is beginning to loosen up. By Diaz, Gerry / Weatherbell
After 16 days of relentless Pacific storms, including a bomb cyclone, and a constant stream of atmospheric rivers, most of the Bay Area is slated to see a comfortable break in the wet pattern Thursday.
This is thanks to a shift in the flow of air thousands of feet above the ground — the jet stream — its winds gradually shifting north in the coming days, meaning the storm door will finally begin to close.
But before it does, two more storms will roll into far Northern California this weekend and into early next week. Some of the rain and snow will spill into the rest of the Golden State, raising the risk for light to moderate bands of rain in areas that are filled to the brim from all the recent storms.
Two monster storms about to bomb already drenched California this weekend. Map via Windy.com
A storm door that just won’t shut
Thursday’s shift in the jet stream has some far-reaching implications to the overall weather pattern in the Bay Area and much of California. Its tilt farther north will send atmospheric rivers into Eureka and the Oregon coast, rather than the Bay Area and Southern California. The last time the core of atmospheric rivers made landfall that far north was in mid-December, when most of the Bay Area was just starting to see brief rounds of moderate rain.
This shift in the jet stream’s flow is causing a weather pattern that resembles mid-December’s flow of moisture.
The European weather model’s 500mb heights (contours) with lower-than-average anomalies shaded in blue hues and higher-than-average anomalies in yellow and orange hues, valid for Thursday afternoon and evening. The Bay Area will be right along the boundary between the two anomalies as the low-pressure system off the coast glides north toward the Eureka coast and Oregon. Diaz, Gerry / Pivotal Weather
This means that as we head into Friday night and Saturday, when the next Pacific storm is set to roll into California, the system will have less moisture to work with than previous ones. The rest of the ingredients for a rainy weekend — convective energy, lift from the mountains and weather fronts and instability from the storm — will be there. But moisture levels will be lacking.
That’s not to say that there won’t be a lot of rainfall. Totals for the storm on Friday night and Saturday will run at around 1 to 2 inches for the Bay Area and 2½ to 3 inches for higher elevations. By Sunday night into Tuesday, the next storm will bring similar totals. There will be around 2 to 5 inches of rainfall accumulated across the Bay Area by Tuesday.
That’s a far cry from the rainfall the bomb cyclone and the New Year’s Eve storm brought to the region. And while these numbers are still impressive for this time of the year, the fact that it’s going to take two storms to reach values that a single storm was able to generate just a couple days ago is a sign that the shift to a drier pattern is right around the corner.
The European weather model’s forecast rainfall totals (shaded) through Tuesday for Northern California, with totals for the next few days tallying up to 2 to 3 inches for most of the Bay Area and closer to 5 inches on the Sonoma coast, North Bay mountains, Marin Headlands and Santa Cruz Mountains. Diaz, Gerry / Pivotal Weather
Floodwaters will take some time to recede
Looking at the accumulated rainfall that fell during the last three weeks and forecast rainfall expected in the next week as a bell curve, rainfall is currently trending downward. As the jet stream continues to tilt farther north, the storm door will continue to close until it finally shuts sometime during the third and fourth week of January.
But until then, weak storms will bring light to moderate rains that make it difficult for water levels in rivers and streams to recede. This includes the Russian River, San Lorenzo and several other waterways in the Bay Area and the rest of the state that will need a couple weeks to fall back down to normal levels. And until the storm door completely shuts, it’s going to take some time to air out all the excess water from this incredible display of atmospheric rivers and storms that California endured at the start of 2023.
San Francisco: For the first time in several days, mostly cloudy weather will give way to a fair share of sunshine across the city today. The air will remain warm and humid this afternoon as temperatures rise to the lower 60s in the Sunset and Richmond districts, the Presidio, Golden Gate Park and Lake Merced. Muni commuters from West Portal to downtown will notice a bit more of a warm-up as temperatures east of Sutro Tower are slated to reach the mid-60s. This mild, tropical-like air will stem from a distant storm off the coast. This comfortable weather will be a nice reprieve from the platitudinous dispensations of rainfall over the past couple of weeks. Unfortunately, the storm door isn’t completely shut just yet, as drizzles return to Ocean Beach, the Embarcadero and Hunters Point tonight.
Pacific Coast and Peninsula: Moisture-laden air will filter into all corners of the Peninsula this afternoon, raising temperatures along Highway 1 between Pacifica and Half Moon Bay to the lower 60s. Mostly cloudy skies, brief rounds of sunshine and humidity levels near 90% will make it feel a bit tropical along the coast this afternoon. This warm, moist air will stream east of Highway 92 into South San Francisco, Daly City, Millbrae and San Francisco International Airport by the afternoon hours, raising daytime highs to the mid-60s by the bay. The rest of the Peninsula, including Redwood City, Foster City and Atherton, can expect warm air and rounds of sunshine as well this afternoon as the region enjoys a nice break from the recent rains.
North Bay: The tail end of an atmospheric river off the coast will seep into the northernmost stretches of Marin, Sonoma, Napa and Solano counties today. Look for warm, tropical-like air, high humidity levels and rounds of sunshine from mostly cloudy skies. Daytime highs in Santa Rosa, Napa and Petaluma will reach the mid-60s this afternoon, while a light breeze off San Pablo Bay and the delta will keep most of Marin and Solano counties in the lower 60s. Residents should watch for any ponding and isolated flooding on Highways 29 and 101 since rivers are already swollen from this spate of storms. The Russian River and several other streams in the area are still running high near towns like Guerneville and Healdsburg, so plan for any alternate route suggestions from local authorities if floodwaters begin to rise again.
There is a slight chance for intermittent rain showers in the Mayacamas Mountains and just north of Santa Rosa today, but most of these will be brief.
East Bay: Warm, moist air will hover over Contra Costa and Alameda counties today, raising mostly cloudy skies that allow some sunshine to filter in this afternoon. Look for mostly quiet weather for residents east of I-680, including Walnut Creek, Antioch and Brentwood. The San Ramon and Livermore valleys will likely see their fair share of sunshine today while a light delta breeze keeps daytime highs in the lower 60s.
Warm air will seep into San Francisco Bay during the day. This will help raise temperatures in Alameda, downtown Oakland and most of the I-80/I-880 corridor to the mid-60s. Residents in the Oakland and Berkeley foothills, including Piedmont, Albany and Castro Valley, will see some of their standing floodwater recede today, but be sure to watch for any pockets of ponding and closed-off roads thanks to recent landslides.
South Bay and Santa Cruz: River levels continue to slowly recede on the San Lorenzo River, streams and creeks near Highway 17 today. Humid air and mostly cloudy skies will make for a more comfortable afternoon with daytime highs in the lower 60s. The intermittent sunshine will help speed up the evaporation of floodwaters and dry out some of the soils along the CZU burn scar.
Rounds of sunshine will raise temperatures in the Santa Clara Valley to the lower to mid-60s this afternoon, while floodwaters on stretches of 280 and the 101 corridor between Morgan Hill and Gilroy will continue receding.