Temperatures plummeted into the teens, snowfall appeared in the forecast and local governments and homeless shelters intensified their efforts yesterday to help those at most risk cope with an early and potentially dangerous cold snap.

Hypothermia centers that provide emergency beds for those seeking shelter from the cold have been at capacity for several days, area officials reported yesterday. Demand is expected to increase if forecasts hold and temperatures remain below freezing throughout the week.

Last night, as temperatures in the District dipped well below the freezing mark, city-contracted "hypothermia vans" and volunteers roamed city streets in search of homeless people willing to go to shelters.

At a bus stop on Constitution Avenue and 15th Street NW, a slight man stuffed into a tight, red jacket hugged Jessica Arwine, an outreach specialist for First Helping, an assistance and feeding program for the homeless.

"Amigos," Arwine said, pointing to her chest and then his. Then she plucked a leaf stuck in his "Bush-Cheney Inauguration" knit cap.

"I found him here without a blanket or anything," Arwine said. She found him with the help of James, a homeless man who was riding shotgun in Arwine's sport-utility vehicle, guiding her to the people he knew would be in trouble on a cold night.

The man didn't speak English, and Arwine's schoolgirl Spanish came to her in fits and starts. But within minutes, she had gotten him an outfit from the Salvation Army and called the hypothermia van to take him to a shelter.

"Telefono me," she said to the man, stuffing one of her business cards into his backpack. He hugged her again, moving stiffly in his new layers of winter clothes, then boarded the van.

"Where next, James?" Arwine asked.

The hypothermia centers, which operate regularly each night from Nov. 1 through March 31 in the District, remain open 24 hours on days when the temperature dips below 32 degrees. Already this fall they have been open round-the-clock on 10 days, while plans to expand several D.C. shelters lag behind schedule.

"This year we had our first [24-hour] day on Nov. 1, which hadn't happened in many, many years," said Cornell Chappelle, chief of program operations for the Community Partnership for the Prevention of Homelessness, which implements and monitors homeless programs for the District.

If this late autumn feels completely different from the last one, that's because it is -- temperatures during the first week of December last year flirted with the 80-degree mark. Forecasters said no one should expect this fall's weather to mimic last year's any time soon.

Temperatures in the 20s and low 30s are expected for most of the week, said Jim Travers, a meteorologist with the National Weather Service's Baltimore-Washington headquarters in Sterling. In addition to the low temperatures, a weather system arriving tonight is expected to bring precipitation and stay through tomorrow: The Weather Service issued a winter storm watch, which means there is a potential for more than four inches of snow.

"A lot of times we wonder what form the precipitation will take, but there's not much question this time," Travers said. "This should definitely be snow. I'd say people should dust off their shovels, make sure the snowblowers are working and be prepared."

Area shelters for the homeless were trying to prepare yesterday by stocking up on blankets and coats, and they were trying to find room for an influx of residents.

"We just have so many people coming to sleep that we've recently begun running out of cots," said Grace Stapleton, development coordinator for Carpenter's Shelter in Alexandria. "We can just offer people a blanket to sleep on the floor."

Some shelters are running low on such supplies as blankets and sleeping bags.

"We usually get boatloads of that stuff at Christmas," said Pamuel Michelle, executive director of Mondloch House, a shelter in Fairfax County. But, she added, "it is cold now."

D.C. government officials have been trying to add bed space while advertising a 24-hour hotline number -- 800-535-7252 -- that people can call if they spot someone in need of shelter.

"It's just a full-court press on letting the entire city know we need their help in identifying families and individuals," said Stacy Rodgers, deputy director of programs for the District's Department of Human Services.

In late October, city officials said they expected that emergency capacity at hypothermia centers would grow from 446 beds to more than 700 by Dec. 1. And, although new beds have been added, most weren't ready by the target date. Officials said funding problems caused delays.

"Right now, we have 472 hypothermia beds," Chappelle said. "We should have 806 by the end of the season. That's far more than we've had in past years."

The city's largest hypothermia center, at Gales School near Union Station, is to expand soon. It has 108 beds, but a new room accommodating 25 more is set to open tonight, officials said. By the weekend, another new room should increase the shelter's total capacity to 160.

For now, though, "we're pretty much at capacity," said Chapman Todd, regional director for homeless services at Catholic Charities, which operates the shelter for the city. "I don't think there have been any open beds for several days now, at least."

Mary Ann Luby, outreach coordinator for the Washington Legal Clinic for the Homeless, said the city's efforts to expand hypothermia programs, while better than in years past, should have started earlier, before cold temperatures gripped the region.

"We're far beyond where we were a few years ago, but it's not perfect," Luby said. "And when we're dealing with life and death, I think we have to be perfect, or as close to it as we can get."

Hypothermia can occur when body temperature dips below 95 degrees. Nine homeless people died of hypothermia in the District during the past two winters. This year, no official hypothermia deaths have been reported. An unidentified man was found dead at 14th and K streets on Thanksgiving, Chappelle said, but the cause of death has not been determined.

Staff writers Sewell Chan and David Cho contributed to this report.

Rafe Marlow, right, of the United Planning Organization, hands a blanket to Reno Nelson, 72, near the McPherson Square Metro station.Nelson tries to warm up inside the station after receiving his blanket. City-contracted "hypothermia vans" also were out looking for people wanting to go to shelters.A pedestrian keeps moving in yesterday's cold. Temperatures in the 20s and low 30s are expected for most of the week, the Weather Service says. Last year at this time, temperatures were flirting with the 80-degree mark.