Freezing northwest winds will quickly sweep across the Bay Area today, lowering daily highs by 7 to 10 degrees in parts of the region. This rush of bitter, cold air will be amplified by today’s passage of a cold front that will leave a trail of frost in its wake. Today’s cold snap will bring in some of the coldest air of the winter season, with temperatures in inland towns like Walnut Creek and Santa Rosa falling into the low 30s by tonight.
The coldest day (and night) of the season
Coastal residents will wake up to temperatures in the low 40s for the first time in weeks, with some patches of shoreline potentially dropping into the high 30s before sunrise. This unusual coastal weather pattern is being driven by an intensified cold front.
Due to the influence of warmer sea surface temperatures, these fronts often struggle to lower coastal and bay temperatures in the region. But this week’s setup will be different, and that’s because of how strong this upcoming cold front will develop.
Frost watches are out Wednesday from 2 a.m. to 9 a.m. for the entire Bay Area as cold air settles in the region. The coldest temperatures will be found inland, but coastal residents can also expect some impact from this Arctic eruption.
Chronicle of San Francisco
Weather models are signaling a cold front for Tuesday and Wednesday, stretching from Vancouver to San Diego, with cold, arctic air sweeping across most of California. That means warm air over the waters of San Francisco Bay and the Pacific Coast will be largely wiped out by Tuesday night. It’s the atmospheric equivalent of trying to heat an apartment with open windows with a space heater.
There just won’t be enough warm air to keep temperatures from plummeting into the 30s tonight and into Wednesday morning. The National Weather Service issued frost clocks Wednesday from 2 a.m. to 9 a.m. for the entire Bay Area — including San Francisco and the Peninsula.
Temperatures forecast for Tuesday across San Francisco, with the coldest pockets likely to form around Twin Peaks, Noe Valley and Glen Park.
Chronicle of San Francisco
The cold air poses a risk to unaccommodated residents and those with limited heating options. Homeowners further inland should check their plumbing, as this type of cold air can often cause older plumbing to burst.
Luckily, this cold front appears to be moving fast. Both the European and American weather models are consistent with the system leaving California on Wednesday afternoon. This allows temperatures to quickly recover to the upper 50s on Wednesday, while overnight temperatures will see the biggest recoveries – only falling into the low 40s.
Breakdown on Tuesday
san francisco: Cold north-west winds will hit the coast this morning creating light to moderate north-west gusts. Brief gusts of 20mph will occur along the Presidio where morning lows are likely to fall in the upper 30s. In most parts of the city, watch for daily highs that only hit the low 50s, while spots in the mid-50s are more likely along the Pacific Coast along outer Sunset and Richmond counties, and on the Bayfront — including Mission Bay and Dogpatch.
All local residents heading to the Golden Gate Bridge tonight should definitely pack their bags. Temperatures are expected to drop into the 40s just after sunset. Overnight lows are likely to fall to the 40 degree mark by the bay, with neighborhoods like North Beach, Nob Hill and Dogpatch possibly measuring some upper 30s after midnight. This cold air will rush through Castro Street and 101 tonight as temperatures in downtown, SoMa, Castro and Mission drop into the high 30s.
Sutro Tower, Bernal Heights, Mount Davidson and some other slopes at West Portal also have a good chance of falling into the upper 30s tonight, raising concern that frost could develop on the slopes overnight. These dangerous nighttime temperatures also pose a risk to vulnerable populations in the city, particularly those in Mission and Glen Park, where some of the city’s coldest airs will settle.
Pacific Coast and Peninsula: A bitterly cold morning awaits the foothills and peaks of the peninsula. Commuters on Highway 92 between Half Moon Bay and San Mateo should watch out for frost as morning lows drop to the mid-30s.
The daytime sun will help push temperatures into the low 50s this afternoon along the coast and in Daly City, South San Francisco and Millbrae. Due to colder air in the foothills, there is a chance that daily highs will remain just below 50 degrees along most of the 101 corridor from Foster City to Redwood City and Menlo Park. Warmer air along the coast from Pacifica to Half Moon Bay will keep daily highs a few degrees warmer than the rest of the peninsula, but weather models still expect temperatures to plateau around the low 50s by midday.
Thanks to the cold air, there’s a slight chance of fog patches forming along the San Bruno Gap tonight, although most of it should be concentrated on the peaks along Highway 92. Keep an eye out for overnight lows in the 40s along the coast, while towns along Corridor 101 south of Millbrae are likely to fall in the low 30s tonight. This cold snap poses a risk to pets, people, and pipes on this section of the peninsula, so be sure to plan for a chilly night.
North Bay: A cold draft blows over the valleys of Petaluma, Santa Rosa and Napa as well as over some of the peaks of the Marin headland. Commuters traveling along Highways 101, 29 and 17 this morning will face frost patches and freezing temperatures. Widespread 30s also increase the likelihood of patches of freezing fog in the northern portions of Napa and Sonoma counties, including Healdsburg and Guerneville.
Sunshine will then do its best to contain the cold snap, pushing daytime temperatures into the mid-50s along the Sonoma County coast and banana belt along the 101 from Novato to San Rafael. Locate mid-50s daily highs in San Pablo Bay and the Delta, including cities along Highways 37, 80, and 780 in Solano County.
This atmospheric attempt at a warm afternoon quickly fades after sunset. Bitter, cold air will pour into the plains and valleys of Napa, Sonoma, Marin and Solano counties tonight, pushing temperatures down to below 40’s by 8 p.m. and mid-30’s before midnight. This cold air has the potential to reintroduce freezing nebulae along parts of the Napa, Sonoma and Petaluma Valleys, so residents who work late should expect a small chance of ice forming on roads and windshields.
east bay: Residents along the San Francisco Bay Area will wake up to bone-chilling temperatures in the upper 30s this morning. Those closer to Alameda and the Bayfront get an extra layer of protection from the cold air thanks to the warmer waters off the bay, as morning temperatures are closer to 40 degrees.
Frost patches are likely on the hills of Oakland and Berkeley through 9 a.m., while residents farther inland and east of the I-680 corridor, places like Concord, Walnut Creek, and the San Ramon and Livermore valleys, are seeing widespread freezing and morning temperatures will be below freezing.
Fortunately, the cold winds will ease in the afternoon and the sun will help temperatures recover. Look for daily highs in the upper 50’s in Alameda and Contra Costa counties. After sunset, cool air will rush down from the hills of Oakland and Berkeley into the inland valleys. This pool of cold night air will envelop most of Contra Costa and Alameda counties. So expect another night of cold temperatures in the 30s along with a chance of patches of freezing frost in the San Ramon and Livermore valleys.
South Bay and Santa Cruz: Freezing temperatures are forecast for this morning in the Santa Clara Valley and Santa Cruz Mountains. Residents along San Francisco Bay and coastal residents in Santa Cruz, Aptos and Capitola will all be spared the coldest air thanks to the warm offshore waters and bays. Watch out for morning temperatures in the low 40’s on the coast.
For residents of the rest of the region, expect bitterly cold temperatures this morning, with lows in the 30s just after sunrise. Some of the coldest air will reach East San Jose and the foothills of the Santa Clara Valley, where the low 30s will favor a round of frosts through 9am. This cold air will also prevail along Highway 17.
Don’t expect too much of a warmup today. Weather models are predicting a cold, overcast afternoon — particularly for those living along the corridors of the 101 and 17 freeways — as daily highs will struggle to reach the mid-50s. This is thanks to the cold front approaching California during the day. Some of the warmest spots will be San Jose, Los Gatos and Morgan Hill where some temperatures above 50 are possible as long as skies aren’t fully cloudy. By tonight, temperatures in the South Bay foothills and Santa Cruz Mountains will drop as the low 30s filter into the Santa Clara Valley. The Cupertino Hills and eastern San Jose are most likely to experience overnight lows below freezing after midnight.
Reach Gerry Díaz: gerry.diaz@sfchronicle.com; Twitter: @geravitywave
