Alternating rain and snow showers switched completely to snow by midday.

A blizzard-like blast from a storm that has been raging across the United States since before Thanksgiving slammed the Northeast on Monday, snarling travel ahead of the evening rush hour.

New York Governor Andrew Cuomo declared a state of emergency and deployed 300 members of the National Guard.

Airline problems had more than doubled by midday, with most of the more than 4,000 U.S. flight cancellations and delays at airports in Newark, San Francisco, New York, Boston, Philadelphia, Chicago and Washington.

Alternating rain and snow showers switched completely to snow by midday, and were forecast by the National Weather Service to pile up by the workday's end to 1 to 3 inches in New York and 4 to 6 inches in Boston.

"There is a projection now for certain parts of the city, particularly the more northern parts of the city of up to five to eight inches," New York Mayor Bill DeBlasio told reporters.

Heavier snow totals were expected in upstate New York, Pennsylvania, northwestern New Jersey, Connecticut, Massachusetts, southern Vermont, southern New Hampshire and Maine, with some areas already receiving a foot of snow, said meteorologist Bob Oravec of the National Weather Service's Weather Prediction Centre.

"When it's all said and done, some areas will have over two feet of snow from this storm, especially over parts of the Poconos and Catskills," Oravec said of the mountain regions.

New Jersey Governor Phil Murphy had said all non-essential workers should head home at noon due to weather conditions.

The storm that started on the West Coast ahead of Thanksgiving, the busiest U.S. travel holiday, slowly rolled across the entire country, drenching some areas with rain, blanketing others with snow and blasting still others with winds. Three tornadoes were reported northwest of Phoenix.

"It's uncommon to have a tornado in Phoenix, but it's not uncommon to have multiple types of weather with a big winter storm like that," Oravec said.

The storm was expected to linger in New York until just before sunrise on Tuesday, in Boston until early Tuesday afternoon and in Maine into Wednesday morning.

"There have been huge impacts from the storm since it occurred during the Thanksgiving week of travel and coming home from the holiday," Oravec said.

"It hit about possibly the worst time it could hit, and it went right across the entire country."