An Arctic air mass is driving freezing weather in the Bay Area. Here’s where it will get the coldest

A cold, low-pressure system is bringing in some of the coolest air the Bay Area and Northern California have seen in weeks. This cold snap started over the weekend as the low moved over San Francisco Bay, and it will continue through the first half of this week. Bitter northwest winds will keep daytime highs in the 50s across most of the Bay, while nighttime temperatures will drop quickly – especially in the inland valleys of the East Bay and South Bay.

So how long will this blast of Arctic air hover over Northern California?

The relentless cold

For many residents in the Bay Area, this recent drop in temperatures was sudden, with overnight lows dropping to levels not seen in the region since the first week of December. Where did it come from? This cold snap stems from a mass of Arctic air that quickly engulfed most of the Western United States.

The weak bubble of warm air (yellow) at the heart of a ridge of high pressure hundreds of miles offshore is surrounded by cold, Arctic air that has infiltrated most of the western United States.

San Francisco Chronicle

The overwhelming size of the Arctic air mass – which covers most of North America – is largely to blame for how cold it has been in the Bay Area and along most of the California coastline. But this past weekend’s cold front from a low pressure system helped leave a trail of northerly winds that will continue to bring below-freezing temperatures to the North Bay Highlands, East Bay Hills, Diablo Range and Santa Cruz Mountains over the next couple of nights.

The long, cold nights ahead

The North American weather model predicts another night of freezing temperatures in interior parts of the North Bay, South Bay and East Bay. This includes the risk of frost and temperatures that could drop into the low 20s overnight. Some of this frigid air will pool in the Santa Clara Valley, Peninsula and parts of Oakland and Berkeley tonight and into Wednesday morning. That means it’s not out of the question for residents near San Francisco Bay to wake up to morning lows in the mid-30s again.

The risk for mid-30s will also be possible in the hill microclimates of San Francisco, including Twin Peaks, West Portal, Noe Valley and Bernal Heights, to name a few. Widespread upper 30s to lower 40s will paint most of the coastline and bayfront overnight, leading to an increased risk of freezing for homeless residents in San Francisco, Oakland and the wider Bay Area.

Temperatures will quickly warm into the upper 40s across the Bay Area after sunrise Wednesday morning. But until this cold air mass lifts out of California, expect this cold break to keep overnight temperatures below 40 degrees across much of the region tonight and tomorrow night.

Fortunately, warm air over the Pacific will come ashore by Thursday morning, ushering in a warming region throughout that will keep temperatures from dipping below 40 degrees across most of the Bay Area by the end of the work week.

Tuesday collapse

• San Francisco: The center of a low pressure system will turn toward the coast this morning, bringing light to moderate northwest winds gusting to 15 mph throughout the day. Brief gusts of 20 mph will reach the Presidio, where some of the coolest air in the city will cling to the beaches and piers this afternoon. Look for daytime highs there and in Pacific Heights to reach only the lower 50s, while spots of mid-50s are more likely in the outer Sunset and Richmond districts.

For residents visiting the Golden Gate Bridge tonight, you may catch a glimpse of some low clouds approaching the coast. Be sure to bundle up as temperatures will drop into the 40s once the sun goes down. Winds will continue to pick up along the bay this evening, with neighborhoods like North Beach, Nob Hill and Dogpatch seeing gusts up to 20 mph. Lighter winds will prevail tonight between Castro Street and 101 as temperatures in downtown, SoMa, the Castro and Mission drop into the lower 40s before midnight. Sutro Tower, Bernal Heights, Mount Davidson and a few other hills at West Portal have a good chance to drop into the upper 30s tonight, raising concerns that frost will develop overnight. These dangerous nighttime temperatures will also pose a risk to unsheltered populations in the city, especially those in wind-exposed areas along the Embarcadero and Hunter’s Point.

• Pacific Coast and Peninsula: A bitterly cold morning is on tap for the Peninsula’s foothills and peaks this morning. Commuters on Highway 92 between Half Moon Bay and San Mateo will see lows in the mid-30s in the morning. Fortunately, warmer Pacific air will keep the coastline from Pacifica to Half Moon Bay a few degrees warmer, with morning lows there in the lower 40s. Cold air will linger throughout the day, but sunshine throughout the day will help push temperatures into the upper 50s this afternoon on the coast and in Daly City, South San Francisco and Millbrae. Because of the colder air in the foothills, most of the 101 corridor from Foster City to Redwood City and Menlo Park won’t get into the lower 50s today. There’s a small chance patchy fog will develop along the San Bruno Gap after sunset, though most of it should remain concentrated on the peaks along Highway 92. Look for overnight lows in the 40s along the coast, while cities along the 101 corridor south of Millbrae will likely drop into the mid-30s tonight. This cold blast will pose a risk to pets, people and pipes across this stretch of the Peninsula, so be sure to plan for a freezing night.

• North Bay: The whiplash of 30-degree temperatures returns today for residents in the Petaluma, Santa Rosa and Napa valleys, along with some peaks in the Marin Headlands. Commuters traveling along highways 101, 29 and 17 this morning will experience frost and morning temperatures right around freezing. Widespread 30s will also increase the chance for patches of freezing fog in northern parts of Napa and Sonoma counties, including Healdsburg and Guerneville.

Sunshine will then quickly raise temperatures to the 60-degree mark along the Sonoma County coastline and the banana belt along 101 from Novato to San Rafael. Widespread upper 50s to lower 60s will then spread eastward across San Pablo Bay and the delta to include cities along highways 37, 80 and 780 in Solano County. This warmth will prevail through sunset. But make sure you have a coat handy if you’re out and about tonight. Cold, northwesterly winds gusting to 20 mph will spill into the plains and valleys of Napa, Sonoma, Marin and Solano counties, dropping temperatures to the lower 40s by 8 p.m. and upper 30s before midnight. This cold air has the potential to bring patches of freezing fog to parts of the Napa, Sonoma and Petaluma valleys, so residents working late should plan for a slight chance of ice on roads and windshields.

• East Bay: Aside from Alameda Island, most East Bayers along San Francisco Bay will wake up to temperatures in the upper 30s this morning. Those closer to the water will receive a little extra protection from the cold air thanks to the warmer water off the bay, keeping their morning temperatures above 40 degrees. Frost spots are likely until 9 a.m. in the Oakland and Berkeley hills, while residents farther inland and east of the I-680 corridor, places like Concord, Walnut Creek and the San Ramon and Livermore valleys, more widespread frost and will see temperatures below. Freeze. Fortunately, temperatures will recover quickly after 9am as sunshine thaws out the East Bay. Look for daytime highs in the upper 50s across Alameda and Contra Costa counties before cold, northwest winds spread from the delta to the rest of the region after sunset. Bitter, cold winds will roll in from the delta and San Francisco Bay, reaching 20 mph gusts in the Oakland and Berkeley hills. These winds will sweep into the inland valleys before subsiding after midnight. But their cold air will pool in most of Contra Costa and Alameda counties, so expect another night of chilly temperatures in the 30s, along with a chance for freezing temperatures in the San Ramon and Livermore valleys.

• South Bay and Santa Cruz: Another morning of bitterly cold temperatures is on tap for the Santa Clara Valley and the Santa Cruz Mountains. Bayside residents in San Jose, Milpitas and Sunnyvale, and coastal residents in Santa Cruz, Aptos and Capitola will all be spared the coldest air the next few mornings thanks to the warm water along the coast and bays. Look for morning temperatures in the lower 40s.

As for residents across the rest of the region, expect bitterly cold temperatures this morning, with lows in the 30s around sunrise. Some of the coldest air will drift over East San Jose and the foothills of the Santa Clara Valley, where lower 30s will lead to frost spots until 9 a.m. This cold air will also occur along Highway 17. But be ready for a big warm-up during the day — especially for residents along the 101 and 17 freeways — as daytime highs clock in the lower 60s. Some of the hottest spots will be San Jose, Los Gatos and Morgan Hill, where mid-60s are possible as long as skies remain clear throughout the afternoon.

By tonight, the South Bay and Santa Cruz Mountains will experience a rapid drop in temperatures as cold air filters into the Santa Clara Valley, with the Cupertino Hills and East San Jose the most likely overnight lows around freezing and the return of patchy frost will see.

Gerry Díaz (he/they) is a San Francisco Chronicle newsroom meteorologist. Email: [email protected] Twitter @geravitywave