(CNN) — A monster snowstorm is expected to slam into the Northeast on Monday and Tuesday, dumping what could be up to 3 feet of snow in Boston and New York, along with freezing rain and strong gusts possibly reaching hurricane level. The storm is expected to hit northern New Jersey through southern Connecticut and freeze Rhode Island and parts of Massachusetts.
By 5 p.m. ET Sunday, storm warnings had stretched to the Canadian border.
“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.”
Earlier Sunday, the National Weather Services upgraded its blizzard watch to a blizzard warning for the area from northern New Jersey through southern Connecticut, including New York City. Twenty to 30 inches of snow is possible with winds gusting 55 to 65 mph.
The Weather Service says whiteout conditions will make travel extremely dangerous in the entire affected area. 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 at Charles Street Supply in Boston, said customers have been coming into the hardware store since it opened Sunday 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 told CNN 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.
CNN’s Sara Ganim contributed to this report.