Dec. 18 (Bloomberg) -- A blast of wintry weather across the U.S. that left a record-breaking snowfall in Las Vegas threatens to drop as much as 5 inches (12.7 centimeters) of sleet and snow on New York City tomorrow and disrupt travel across the country.

A winter storm watch issued by the National Weather Service for the region that includes New York has been changed to an advisory. Snow accumulation for the region tomorrow is now estimated at 3 to 5 inches in Manhattan, down from 6 to 10, according to a statement on the service’s Web site.

There is a chance snow may start falling as early as 4 a.m. before switching to a mix of snow, sleet and rain after midday, with precipitation tapering off about midnight.

“Snow is expected to overspread the region Friday morning and could become heavy at times in the afternoon,” according to a forecast issued at 4:12 p.m. New York time. “Higher snowfall amounts are expected as you move inland.”

Boston Mayor Tom Menino declared a snow emergency and canceled school in the city for tomorrow. Last year, a mid- December snowstorm halted traffic for as long as eight hours for drivers trying to leave Boston.

The storm is moving across the Ohio Valley today, bringing heavy snow to the Midwest and Canada before reaching the East Coast. The snow may continue in New York through the weekend, the service said.

‘On the Fence’

Tom Kines, a meteorologist with AccuWeather.com in State College, Pennsylvania, said New York City “is kind of on the fence,” in the middle of the changeover line between snow and rain.

To the south, the storm will probably be mostly rain, Kines said by telephone. Northeast and northwest of the city, the storm may dump 6 inches of snow, he said.

“It is going to have a major impact on the major cities from Chicago to the East Coast and that is going to cause a lot of travel headaches both in the air and on the ground,” Kines said.

Winter storm warnings and watches now stretch from Chicago to Portland, Maine, including the northern suburbs of New York City. The storm is also expected to blanket New England, where thousands have been without power for a week.

A winter storm warning is issued when 7 or more inches of snow and sleet are expected in the following 24 hours, while an advisory covers a potential accumulation of 4 to 6 inches, according to the National Weather Service. A warning also is used when there is a potential “threat to life and property.”

Storm Watch

A winter storm watch is issued when a storm may affect an area “but its occurrence, location and timing are still uncertain,” according to the weather service.

The storm began in the Southwest and moved across the Midwest today. The desert playground of Las Vegas received 3.6 inches of snow yesterday, breaking the record for the biggest snowfall for December since official records began in 1937, according to the National Weather Service.

“Measurable snow has only fallen on five instances since 1937” in December, including yesterday, the weather service said. Yesterday’s storm is the eighth-largest in the Nevada city’s history, the service said.

New York-area transit agencies prepared for the precipitation today.

Salt and Sand

The Port Authority of New York and New Jersey said it has thousands of pounds of salt, sand and de-icing chemicals and hundreds of pieces of equipment, including blowers, heaters and plows, ready to clear snow at airports, bridges, seaports, tunnels and train stations.

Metro-North Railroad, the second-busiest U.S. commuter railroad, has delivered salt to all of its 120 stations in New York and Connecticut and fueled up snow blowers, spokeswoman Marjorie Anders said.

Boston may get a foot of snow. Parts of southeastern Massachusetts, Connecticut and Rhode Island may receive as much as 14 inches, according to Alan Dunham, an NWS meteorologist in Taunton, Massachusetts.

Snow from the storm may fall at a rate of 1 to 2 inches per hour at times, Dunham said.

“It is going to mean a lot of snow,” Dunham said by telephone.

The storm comes as crews in New York, Massachusetts, Vermont and New Hampshire struggle to restore power to homes hit by a Dec. 11-12 ice storm.

In New Hampshire, 53,000 customers were still without power as of this morning, according to Colin Manning, a spokesman for Governor John Lynch. Six state roads and 83 local roads were still blocked by fallen trees and debris, and 315 people were in 34 shelters around the state, he said.

Snow is likely during the day in Boston on Dec. 20, and there is a 90 percent chance of rain and snow the following day, according to the National Weather Service.

“Today is the quiet before a fairly extended period of fairly stormy weather,” Dunham said.

To contact the reporter on this story: Brian K. Sullivan in Boston at bsullivan10@bloomberg.net; Chris Dolmetsch in New York at cdolmetsch@bloomberg.net.

Last Updated: December 18, 2008 17:32 EST