People walk past Robin Riddick, 43, as she creates a makeshift space for sleeping along First Street NE near Union Station in Washington. (Nikki Kahn/The Washington Post)

With blasts of arctic air expected to reach the Mid-Atlantic this week, plunging daytime temperatures below freezing, local officials are gearing up to move hundreds of homeless people off the streets and into shelter.

The ramping-up of outreach and services for the homeless could include the opening of churches, libraries and recreation centers as havens for those unable to squeeze into year-round shelters in the District, Maryland and Northern Virginia.

Planning is particularly intense in the District, which at last count had more than 8,300 homeless people, according to a U.S. Department of Housing and Urban Development report last month.

A new report by the U.S. Conference of Mayors found that the nation’s capital had the highest homelessness rate among 32 U.S. cities.

Although only one in 25 of Washington’s homeless is thought to lack temporary shelter — an estimated 318 people, overall — frigid days and nights can quickly turn deadly for those on the street.

Last winter, two homeless people died of hypothermia, according to the District’s Interagency Council on Homelessness.

During the previous five winters, hypothermia was the primary or contributing cause of death for 47 people, although officials had not determined how many were homeless.

Laura Zeilinger, director of the District’s Department of Human Services, said the chronically homeless — who may already be weakened by other medical and psychiatric issues — are particularly vulnerable during cold snaps. A typical street dweller in his 50s can have the health problems of somebody 20 years older, Zeilinger said.

The good news, she said, is that many can be persuaded to come in from the cold.

“Most people will come into our cold-alert or hypothermia sites, even if they won’t come into our other shelters,” Zeilinger said. “We absolutely will do everything possible so that folks avail themselves of a place to come inside.”

In a conference call Wednesday, Zeilinger and other D.C. government officials decided to activate a citywide emergency alert because of the impending cold, which some forecasts say could dip to single digits with wind chill by early Friday.

The decision means that officials may open government buildings — such as libraries and recreation centers — as overnight warming sites. They will also offer the homeless free, on-call transportation to shelters.

D.C. officials plan to open additional shelters if temperatures reach freezing, or 40 degrees Fahrenheit if it is also raining. Even more sites will be opened if the temperature drops to 15 degrees, or 20 degrees with snow or high wind, Zeilinger said. Forecasters said there is a chance of snow and ice Friday night into Saturday morning.

The public can call 311 or (202) 399-7093 to request outreach workers to check up on someone on the street or to arrange transportation to a shelter.

In suburban Maryland and Virginia, officials were also bracing for greater demand for shelter space among the homeless.

Fairfax County will keep its six emergency shelters open to all comers during the freezing weather, county spokeswoman Lisa Connors said.

Last month, the county of 1.1 million residents also launched its annual hypothermia prevention program, a network of 45 religious organizations that convert to shelters and food pantries through March.

Arlington County’s 24-hour homeless-services center has expanded its capacity to handle 25 additional people, for a total of 80 beds. The center, at 2020 14th St. N. in the Courthouse neighborhood, opened just over a year ago.

If demand goes higher, the sheriff’s department will open space in its lobby, where homeless people can spend the night, said Kathleen Sibert of Arlington Street People’s Assistance Network, which operates the center for the county.

Those who need to warm up but don’t need overnight accommodations will be directed to libraries, community shelters and shopping malls, said Kurt Larrick, spokesman for the county’s Human Services Department.

In Montgomery County — which has nearly 1,000 homeless people, a tenth of them without any shelter — about a dozen shelters will be open during the day, said Mary Anderson, a spokeswoman for the county’s Health and Human Services division. If those shelters fill up, the county has designated overflow space. County outreach workers will encourage people living on the street to seek shelter.

The effectiveness of such programs can mean the difference between life and death. Jesse Rabinowitz of Miriam’s Kitchen, a homeless-services organization in Foggy Bottom, said that after the historic snowstorm that hit the D.C. area last year, the group’s staff discovered a man under a nearby bridge whose prosthetic legs had frozen to the ground.

He was rescued by emergency medical workers and survived, Rabinowitz said, but “that story shows to me how exposed and dangerous it is to live on the streets.”

For some, the risk from the extreme weather that is forecast has not sunk in — or is just the latest of the many dangers and indignities that accompany homelessness.

Robin Riddick, 43, huddled on a blue tarp spread out beneath an overpass near Union Station one night this week. Riddick said she once worked as an IT consultant to small businesses in New Jersey until health problems forced her to stop working.

“Not having the amenities you usually have or want to have, it’s kind of depressing,” she said, her feet poking out in socks from beneath a worn comforter. “But I’ve been through some hard times.”

Riddick said she had arrived in the District only several days before from South Carolina, where she had most recently been staying with a relative. She said she didn’t fear extreme weather. “I like the cold. I’m a cold freak,” she said, stretching out her arms. “I’m a camper.”

But there are limits. If the temperature drops below freezing as expected, Riddick said, she plans to call the District’s emergency hotline for help.

Patricia Sullivan, Arelis R. Hernández and Antonio Olivo contributed to this report.