The National Weather Service is using terms such as "life-threatening" and "historic" to describe the system ready to spread across the Northeast.
The storm could dump up to 3 feet of snow in Boston and New York before it ends Tuesday, along with freezing rain and strong wind gusts possibly reaching hurricane level.
Blizzard and winter storm warnings are up from New Jersey through Maine and into Canada.
Airlines began canceling flights by the hundreds. FlightAware, a travel website, put the total at more than 1,800 early Monday
"We plan to cancel all flights Tuesday at Newark, LaGuardia and JFK, as well as Boston and Philadelphia," said United Airlines Spokeswoman Mary Ryan.
Other airlines were implementing similar measures.
Check first if you're flying into or out of the region.
"I want everyone to understand that we are facing -- most likely -- one of the largest snowstorms in the history of this city," said New York City Mayor Bill de Blasio.
He repeated throughout a Sunday afternoon news conference that people should expect delays, very slow or stalled commutes Monday evening. Whatever people do to be cautious, he said, do more.
"My message to all New Yorkers is prepare for something worse than we have seen before. Prepare to be safe. Take every precaution. Now is the time to get ready for this extreme weather," he said. "Don't underestimate this storm."
The Weather Service says whiteout conditions will make travel extremely dangerous in the entire affected area. Wind gusts in the New York City area could hit 65 mph.
The blizzard warning is in effect from 1 p.m. Monday through Tuesday, with the worst conditions expected from late Monday evening through midday Tuesday.
In Massachusetts, emergency management officials warned that the winter storm will be potentially "historic and destructive."
Snow in the southeastern part of Massachusetts could morph into a period of freezing rain. And a major nor'easter is likely to develop on Monday and move up the northeast coast, forecasters said.
Travel across Massachusetts during the storm could be impossible and life-threatening, according to the Massachusetts Emergency Management Agency. It's unclear just how much snow might be on the way, but the National Weather Service is saying 1 to 2 feet is possible.
The Rhode Island Emergency Management Agency tweeted Sunday that the state would be under a blizzard watch Monday.
That area's storm conditions aren't predicted to let up until Wednesday morning.
Christine Carew, a sales associate in Boston, said customers have been coming into the hardware store to grab last-minute snow supplies.
Parents and their children have popped in to buy sleds and others are picking up shovels, ice melt and snow brushes.
"This is kind of typical," she said about Boston getting a lot of snow. "We're more prepared for it. We know it's going to happen."
There's little chance the store will be closed Monday, no matter how hard it snows. The manager, she said, lives above the store.
Eight counties in Massachusetts are already under a winter storm watch: Berkshire, Franklin, Hampshire, Hampden, Worcester, Barnstable, Nantucket and Dukes.
A blizzard watch is in effect for Essex, Middlesex, Suffolk, Norfolk, Bristol and Plymouth counties. Barnstable, Nantucket and Dukes counties also face a high wind watch.
Derek and Jim Missert have lived in the Boston area for years. On Sunday they prepared to ride out the storm by being as prepared as they usually are, with plenty of food and water on hand. Jim Missert lives outside the city, so he expects to lose power and has a generator. Derek Missert expects that he'll not lose electricity as he works from home in the city.
There could be coastal flooding in the state starting early Tuesday with pockets of major flooding on east-facing coastlines, the state emergency agency said.
New York Gov. Andrew Cuomo urged New Yorkers to prepare immediately for the storm, saying they should plan for "major disruptions" during Monday and Tuesday morning commutes. Roads could be closed and power could go out, he said in a news release Sunday, and New Yorkers need to look out for downed power lines and tree limbs.
Cuomo directed all state agencies to prepare. New York has at least 1,806 plows and more than 126,000 tons of salt to spray on roads across the region. The National Guard will also have six dozen personnel and 20 vehicles stationed throughout the state starting on Monday morning.
The mayor of Danbury, Connecticut, Mark Boughton, recently made the news
for creating the popular Twitter hashtag "#Icezilla
" when a major storm hit his area.