Most of the United States is being assailed with extreme winter weather this week as Arctic air blasts south from Canada, snow tracks up the Northeast coast and a potentially crippling winter storm takes aim at the South.

Snow was falling in the Mid-Atlantic and New York City on Sunday. The coldest air in several years will descend across much of the country starting Monday, forecasters said.

Houston mayor John Whitmire said it's "all hands on deck" as the city expects a winter storm that promises heavy snow, sleet and freezing rain. "I've been convinced that we are about to experience a very serious and dangerous whether episode,

"The level and extensiveness of the frigid air may be tough to match the rest of the winter," forecasters warned.

The heaviest snow was expected to fall in New Jersey between 4 and 9 p.m. Sunday, with totals of up to 8 inches.

While it's not clear what impact a mix of rain, freezing rain, sleet and snow may have on the capital city, officials say now is the time to begin preparations.

The National Weather Service issued an updated winter storm warning at 1:53 p.m. on Sunday in effect until Monday at 7 a.m. for Berkshire County.

The NYC area is forecast to see significant snow on Sunday as New Jersey declares a state of emergency. Here's how much to expect, when the storm will stop and end, plus how to track live coverage:

Storm Warning has been posted for central and southern New Hampshire from Sunday afternoon through early Monday morning.

"I've been convinced that we are about to experience a very serious and dangerous weather episode," Whitmire said during Sunday's press conference.

Escambia County Alabama closed Tuesday and Wednesday.