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An icy medley of snow, sleet and freezing rain continued to coat parts of the Mid-Atlantic and the Northeast on Wednesday morning, as nearly a foot of snow and heavy winds are expected to hit the East Coast as part of the fourth nor'easter in less than three weeks.

About 75 million people are under winter weather advisories from Indiana to Maine.

A snow emergency was already declared in Philadelphia, which is expecting 5 to 9 inches, as areas along the Interstate 95 corridor were bracing for moderate to heavy snow from northern Maryland to southern New Jersey by mid-afternoon. Washington could see from 3 to 6 inches of snow, New York from 6 to 10 inches and Boston from 4 to 7 inches, forecasters said.

With wind gusts in New York City expected to be as strong as 55 mph, the Department of Buildings advised "all builders, contractors, crane operators, and property owners to secure their construction sites, buildings, and equipment."

Snowfall rates could reach 1 to 2 inches per hour, with the evening commute being particularly treacherous, forecasters added.

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More than 3,800 flights were already canceled Wednesday, mostly at airports in New York, Philadelphia, Boston and Washington, according to FlightAware.com. American Airlines said it was scrapping all flights at LaGuardia Airport in New York after 11 a.m. ET. United said Wednesday that it had canceled more than 300 flights out of Newark Liberty International Airport in New Jersey.

Amtrak also canceled Wednesday's Acela express service from Washington through New York to Boston and limited other service sharply throughout the region. New York and Philadelphia city schools will be closed Wednesday.

Regional utilities reported only a few thousand customers without power scattered across the sprawling northeastern quadrant of the country Tuesday night. But that number was expected to skyrocket Wednesday as the icy mess moves up the Northeastern and New England coasts before ending as all snow in New England on Thursday, the National Weather Service said. Some of the strongest wind gusts could be on eastern Long Island, New York, and southeastern Massachusetts, with up to 45 mph.

"Travel will be very difficult to impossible, especially during the evening commute" on Wednesday in cities like New York, Philadelphia and New Haven, where snow accumulations of more than a foot were predicted, the National Weather Service said.

The new system extends a streak of late-season nor'easters that have bashed the East Coast this month.

The first nor'easter of the month, on March 2, halted flights and caused chaos for commuters. A second nor'easter, on March 7, left thousands of customers without power and up to 2 feet of snow on the ground. The third, on March 12, hammered parts of the Northeast and New England.

But Ari Sarsalari, a meteorologist for The Weather Channel, said this week's system could top them all.

"For New York City, this is probably going to end up as the biggest of the four nor'easters, at least as far as snow totals go," he said. "This is going to be a straight-up, all-day snow event."

Jeff Kneuppel, general manager of the Southeastern Pennsylvania Transportation Authority, or SEPTA, urged people to get early trains out of Philadelphia on Tuesday, telling NBC Philadelphia that Wednesday would be a very difficult travel day.

The coincidence of spring's arriving in tandem with another nor'easter made for an odd display of merchandise at a Busy Beaver building supply store in Washington, Pennsylvania, about 25 miles southwest of Pittsburgh.

"We've had a rather unique month with March," Mike Monath, the store's general manager, told NBC affiliate WPXI of Pittsburgh on Tuesday. "We've tried to transition into spring, [but] the weather is not cooperating, so we have a mixture of Easter flowers, gas grills and also rock salt and ice-melter, which is still popular this morning."