Bay Area residents, lulled by short-sleeve days over the holiday weekend, can expect to be smacked with a reminder that it's December come Wednesday, with temperatures predicted to dramatically drop to below freezing as the season's first cold snap hits the region.
"Historically, when you look at these cold systems, they generally begin around mid-December and run through early January. So we're ahead of that by about a week," said Duane Dykema, forecaster for the National Weather Service. "Occasionally, you get a respite in between systems. We'll have to see what happens this time."
The icy cold should arrive with the most force Wednesday night and into Thursday morning, Dykema said, dropping 15 degrees in the central and eastern parts of Contra Costa County, as well as the easternmost area of Alameda County. Temperatures in cities such as Concord, Walnut Creek, Antioch and Livermore are expected to dip to as low as 27 degrees.
The South Bay and the Peninsula should bottom out at about 31 degrees early Thursday, and forecasters said there is a freeze watch for the Bay Area from late Tuesday night until Thursday.
And, if that isn't enough, the intense cold could linger for a week or longer.
So what's bringing on this sudden cold spell? A dry, cold mass that originated in Canada is pushing out a warmer bubble that fostered balmy temperatures around 70 degrees -- about 10 degrees warmer than normal -- in most Bay Area cities on Sunday.
Despite the cold front, Dykema said most cities will avoid setting records for low temperatures. Most low-degree milestones in the Bay Area are for temperatures in the low 30s and high 20s, though forecast Wednesday lows of 34 and 35 degrees, respectively, would set records in Oakland and Richmond, he said.
Temperatures in San Francisco also are expected to fall into the mid-30s on Wednesday, and Santa Cruz could see temperatures as low as 30 degrees, according to forecasters.
Throughout the Bay Area, homeless shelters were bracing Monday for the cold spell. In San Jose, EHC LifeBuilders launched its Cold Weather Shelter Program on Monday, creating 125 emergency beds at the former Sunnyvale National Guard Armory, 100 at the Gilroy National Guard Armory and 50 at the Boccardo Reception Center.
"With the cold weather coming in, we expect that we're going to fill up sooner," said Jenny Niklaus, CEO of LifeBuilders. "Usually it takes a little while to ramp up to capacity, but with the cold, we could be full on Wednesday."
Niklaus said that while people freezing to death is "pretty rare" in the South Bay, cold weather often exacerbates other medical conditions such as diabetes or heart disease.
"It doesn't have to get that cold for someone to experience hypothermia," she added. "The fact is weather is dangerous."
Monica Marshall, 50, was one of about a dozen homeless people lined up for a bed in Sunnyvale on Monday evening. Some said they would otherwise ride an all-night transit line or seek out shrubs and other foliage that would provide a modicum of insulation.
"You can stay warm anywhere if you are not afraid, if you get out there and be an explorer," said Marshall. "But if you've been indoors all your life, you're not going to make it."
In Contra Costa County, homeless shelters such as the Bay Area Rescue Mission in Richmond expect to house more people than normally would be allowable.
"We're going to be able to take our dining area and put sleeping bags in there," said Gary Kingsbury, the vice president of operations for the shelter. "It helps temporarily."
In Oakland, the Henry Robinson Multi-Services Center has replaced the now-closed Oakland Army Center as a haven for those who receive a referral from agencies that work with the homeless, said city of Oakland spokeswoman Dana Perez-St. Denis. The Salvation Army, CityTeam and Crossroads at 7515 International also will offer housing during the cold.
Those who may have shelter but lack heat are advised by public safety authorities to dress in layers, use thick blankets and to keep their feet covered with socks. They also are advised to move into a shelter with heat, if possible.
Outdoor plants and pipes also are at risk during the cold weather, authorities said. Plant-lovers are encouraged to get plants under shelter or inside a house because buildings give off radiant heat. Using newspaper, cardboard, tarps and sheets to create a tent over a plant also helps, authorities said. The Contra Costa Water District said wrapping pipes in Bubble Wrap or newspapers can keep pipes from freezing.
Rain is unlikely to fall anywhere during the early portion of the cold spell, but a front with more moisture may replace the cold one, and several cities could see rain by late Friday night or into Saturday morning, he said.
The cold system also could trap pollution in the air by the end of the week, according to Bay Area Air Quality Management District spokesman Tom Flannigan, though no Spare the Air alerts have been forecast.
Contact Rick Hurd at 925-945-4789 and follow him at Twitter.com/3rdERH.
Here's how cold it's expected to get in selected Bay Area cities on Wednesday, with the lowest temperatures expected an hour before the sun rises:
CONTRA COSTA COUNTY
Martinez: 26 degrees
Concord and Walnut Creek: 27 degrees
Brentwood and Antioch: 28 degrees
Livermore, Pleasanton: 28 degrees
Fremont, Hayward and Oakland: 31 degrees
SANTA CLARA COUNTY
Los Gatos, Saratoga, Cupertino and Palo Alto: 29 degrees
San Jose, Santa Clara, Mountain View, and Sunnyvale: 32 degrees
SAN MATEO COUNTY
Atherton, 30 degrees
San Mateo and Hillsborough: 32 degrees