Snowplows will be out for a storm expected to reach the Washington region by Saturday evening. (Michael Robinson Chavez/The Washington Post)

This story has been updated.

The first significant snowfall of the winter swept into the Washington region Saturday after forecasters had upped their predictions of depth and duration, increasing the likelihood of closures and delays on Monday.

Even before the first flakes fell midafternoon, forecasters had increased the possible snow totals for the immediate area to four to 10 inches, which would make it the biggest January snowstorm since 2016.

In Virginia, Gov Ralph Northam (D) declared a state of emergency, citing the probability of downed trees, power outages and transportation impacts.

For much of the day, attention had focused on forecasts and preparations. But by 7:30 p.m. the time for measurement had begun over most of the metropolitan area and beyond.

In many places, depths, although modest, were enough to prompt the thrusting of rulers into the accumulated flakes.

A photograph from Fairfax County showed a ruler-measured depth of 2.5 inches.

Forecasts suggested that a good deal more could come as the snow continued overnight and well into Sunday.

But the seemingly meager snowfalls of Saturday night proved adequate, even in the District, to cover both grassy areas and untreated sidewalks.

Light snow, fog and mist were jointly reported at the official measuring station at Reagan National Airport. Snow had been falling there since at least as early as 3 p.m.

The official snowfall reading for Washington for Saturday was 1.9 inches At Dulles International Airport, the official reading for Saturday amounted to 2.9 inches.

In addition, at National, the temperature had fallen to 2 degrees below freezing at 8 p.m., suggesting a good chance that any snow would stick. At Dulles, the midnight temperature was 28 degrees.

Well before midnight Saturday, snow began to accumulate on roads in western Fairfax County and in the Chevy Chase area of Montgomery County, except in the glistening tracks left by traffic. A witness described roads as “messy” in Takoma Park. Another witness said roads were “getting suspect” in the District’s 14th Street corridor.

The witnesses posted photos online that seemed to substantiate their assessments.

Not everybody was impressed. “This snow is pathetic,” said Emily McFied.

Indeed, even in an area where small amounts of snow can have major effects, Saturday night did not seem particularly fearsome.

A least in the earlier hours of Saturday night, the snowfall rate still seemed “pretty slow,” Maryland’s State Highway Administration tweeted around 8 p.m., saying it would give added time for road clearance.

At 8:30 p.m. the agency said it had more than 2,500 pieces of equipment out throughout the state.

“There are a few crashes,” the agency tweeted, “but no major incidents.” Many collisions were reported by the fire department in Montgomery County.

It appeared around midnight Saturday that the true impact of the snow would depend on what happened on Sunday, in the early morning and during the day. Forecasts suggested that a lot of snow was yet to come. .

The Weather Service predicted six to eight inches for the District and five to 10 inches for surrounding suburbs. The Washington Post’s Capital Weather Gang forecast four to eight for the immediate metro area, with more to the south, less to the north.

Areas west and south of the city — parts of Prince William County, southern Fairfax County and especially Fredericksburg and southern Maryland — could see five to 10 inches, said Jason Samenow, the Capital Weather Gang’s chief meteorologist.

“It’s a substantial snowstorm for the area,” he said. “It’s now looking to be a long-duration event.”

He and others had initially predicted an end to snow by early Sunday afternoon, but he said it is now likely to stretch into evening. That would give plows less time to reach secondary and neighborhood roads before Monday morning, making delays or cancellations more likely.

The National Weather Service extended its winter storm warning to 6 p.m. Sunday and warned that traveling would become “very hazardous or impossible.”

“We thought if it ended [earlier] Sunday, things would be back to normal by Monday,” Samenow said, “but probably not now.”

In Washington, the partial shutdown of the federal government will not leave federally controlled roads unplowed.

Although furloughed, National Park Service employees will continue to clear snow from parkways under its jurisdiction, spokesman Mike Litterst said. The roads include the George Washington Memorial Parkway, Rock Creek Parkway and the Baltimore-Washington Parkway, authorities said.

After crews sprayed chemicals to curb icing and enhance plowing effectiveness, plowing would start after one to two inches have fallen, said Jenni McCord of the Virginia Department of Transportation. She said crews would need to make several passes anywhere that gets the forecast maximum of 10 inches. Interstates and roads serving hospitals, fire stations and other emergency facilities will be prioritized, she said.

With pavement temperatures expected to dip below freezing overnight, McCord said, even pre-treated roads could become slick quickly.

Ashley Ross-Scott, a spokeswoman for the Maryland State Highway Administration, said motorists who can’t stay off the roads should at least stick to plowed roads.

Some residents prepared by snapping up sleds and stocking refrigerators. Cars headed for a Whole Foods in Bethesda backed up onto River Road. Shoppers at grocery stores in Arlington and Northwest Washington said carts were scarce and lines long.

At Annie’s Ace Hardware Store in Petworth, three sleds remained early Saturday afternoon.

At Strosniders Hardware Store in Bethesda, clerks unloaded cases of ice melt as a light snow fell outside.

“The panic has begun,” a clerk declared.

Morgan Smith, Dana Hedgpeth and Lori Aratani contributed to this report.