A major winter storm that could dump up to a foot of snow is poised to start pummeling the state this afternoon, the National Weather Service said.

A winter storm warning for most of the state is in effect until 7 a.m. Sunday. Meteorologists at the weather service’s Taunton office said they expect 4 to 8 inches of snow to fall on most of the state, while higher accumulations of 6 to 12 inches are now forecasted inland to the southwest of Boston in Norfolk, Plymouth, and Bristol counties.

“This is going to be a heavy, wet snow that packs down.... Travel will get tricky,” said weather service meteorologist Alan Dunham.

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Snow began falling in the Berkshires this morning and reached Boston by 3 p.m., Forecasters say bands of steady snow are still aimed at the city, which could end up with 4 to 6 inches of accumulation.

In a reversal of a previous announcement, Boston Mayor Thomas M. Menino’s office declared a snow emergency for 9 tonight, triggering parking bans across the city. Department of Conservation and Recreation officials announced a similar parking ban on roads under its control, also starting at 9 p.m. The department is responsible for roads around parks and waterways, like the Arborway.

While the storm is expected to bring significant accumulations, Dunham said the risk of coastal flooding is low, since winds are expected to be blowing out to sea. That should come as some relief to beleaguered residents of Plum Island, where a storm surge Thursday washed away a protective dune and damaged the foundations of four homes.

The storm will reach its peak between 5 p.m. today and 1 a.m. Sunday, Dunham said. A brief period of rain is possible in some areas in the early evening, Dunham said, but should quickly revert to snow.

The snow is unlikely to melt between now and New Year’s Eve, Dunham said, as high temperatures on Sunday and Monday will barely inch above the freezing point before plummeting into the teens at night.

New Year’s Eve will be chilly and overcast, according to Dunham. A flurry or two may come Monday night, but no significant weather is expected.

With thousands of visitors expected for First Night festivities this weekend, Boston city officials are asking residents to use public transportation and keep roads clear for plow and emergency vehicles.

In a press release, Menino said the city would be ready.

“Our team will be actively monitoring this weather pattern through the evening tonight, and are prepared to respond and get our roads and walkways clear in time for First Night celebrations on Monday,” he said in the release. “With so many people coming to visit Boston for First Night, safety is our number one concern.”

First Night spokeswoman Joyce Linehan said organizers don’t anticipate the storm will delay setup for the event, which is expected to draw 1 million participants over 12 hours of festivities.

“Rain is a problem; snow is not,” said Linehan. “The storm looks to be clearing with enough time so that we’ll be ready for Monday with no difficulty at all... There will be a nice little blanket of white around the sites.”

It was unclear if Menino, who was recently released from a months-long hospital stay, would participate in First Night activities.

“We would love to see him,” said Linehan.

State Department of Transportation crews and contractors are out in force today, pretreating roads and clearing snow that has already began to fall in the Berkshires, said Department of Transportation spokesman Michael Verseckes . About 4,000 plows, sanders, and de-icing trucks are available to keep roads open, he said.

“It looks like we’re in for a pretty significant storm,” Verseckes said. “Everybody’s on standby right now.”

Verseckes echoed official calls for motorists to keep off the roads.

“If you don’t have to leave home during the height of storm, that would be beneficial,” he said. “If you have to drive, do so carefully.”

Last year’s mild winter means the department has a stockpile of leftover sand and de-icing chemicals, Verseckes said; leftover funds were largely used to help the MBTA close a budget gap.

Verseckes said the Department of Transportation isn’t expecting a crush of highway traffic this weekend, since holiday travel around Christmas tends to be more staggered than on Thanksgiving weekend.

While officials don’t expect weather-related traffic snarls, the Sunday night closure of the Sumner Tunnel will require rerouting drivers through the I-90 Ted Williams Tunnel or over the Tobin Bridge. The tunnel is being closed while workers inspect its wall panels; a similar panel in the Callahan Tunnel crashed onto the road last week after its supports rusted through.

The encroaching storm has already caused at least once cancellation, as organizers of the “A Kwanzaa Song” music and dance performance at the Hibernian Hall ballroom in Roxbury called off today’s performance. The show will resume Sunday.