A historic winter storm has begun lashing the Northeast US, placing 59 million people under weather warnings, causing power outages, and prompting a travel ban in New York City.

States of emergency have been declared in multiple US states, including Massachusetts, Connecticut, Delaware, New Jersey and Rhode Island. Several have instituted travel restrictions or bans due to the blizzard, and thousands of flights have been cancelled.

Forecasters say much of the US northeast and the Canadian Maritimes will be impacted from Sunday evening to Monday.

It's expected to be the most powerful nor'easter storm in nearly a decade for much of the region, bringing snow, fierce winds and coastal flooding.

About 40 million people are affected by the blizzard warning, and another 19 million are under winter storm warnings that cover the Central Appalachians to coastal Maine.

"While we do get plenty of these nor'easters that produce heavy snow and strong impacts, it's been several years since we saw one of this magnitude across this large of a region in this very populated part of the country," Cody Snell, a meteorologist at the National Weather Service's Weather Prediction Center, told the BBC's US partner CBS News.

Already, power outages are mounting with more than 20,000 people without electricity in New Jersey. Thousands more are without power in Virginia, Delaware and Maryland, according to the tracking website PowerOutage.us.

About 3,900 US flights have been cancelled on Sunday, and hundreds of others have been delayed, according to flight-tracking website FlightAware.

The National Weather Service has warned that the storm will bring an estimated 2-3 inches (.05-.07m) per hour, and snowfall can reach 1-2 feet (0.3-0.6m), resulting in "nearly impossible" travel conditions.

The blizzard conditions will also make travel "extremely treacherous" in parts of the northeast US, the service said.

Snow started to fall Sunday evening in parts of the northeast, including Boston, Philadelphia, and New York, where some of the harshest conditions are forecast.

It's the first time in nine years that New York City - the most populous US city - has been under a blizzard warning. The city is expected to be hit particularly hard, with 18-24 inches (0.5-0.6m) of snow forecast for the metro area and temperatures dropping to about 20F (-6 °C).

A full travel ban is in place in New York City from 2100 local time (0200 GMT) on Sunday to 1200 local time (1700 GMT) on Monday, with Mayor Zohran Mamdani saying schools would be cancelled and all streets, highways and bridges would be shut to traffic, except for emergencies.

During a news conference, Mamdani said the heaviest snowfall is expected overnight, with up to 28 inches (0.7m) in some areas. He declared a state of emergency in the city. This is the second major snowstorm of the first-time mayor's administration, coming after 19 people died during a three-week cold snap in January.

Across the state, the highest wind gusts are expected along the coast, up to 65 to 70 mph (104 to 112 kph), with officials warning of downed tree limbs and power outages too.

New York Governor Kathy Hochul declared a state of emergency for the whole state ahead of the nor'easter storm and activated about 100 members of the National Guard.

"This will be something the likes of which we've not seen in years," she said. "People will be in the dark. Long Island and New York City and lower Hudson are literally in the direct eye of the storm."

Due to the storm, all Broadway shows on Sunday evening have been cancelled and the New York Racing Association has cancelled Sunday's eight horse races at the Aqueduct Racetrack.

Conditions are expected to be harshest along the coast, with New Jersey, New York, Massachusetts and Rhode Island forecasted to be hit the worst.

In Rhode Island, Governor Dan McKee issued a travel ban across the state and enabled the activation of the National Guard as needed. He also closed state government offices on Monday.

"We're asking Rhode Islanders to take this storm seriously and stay home," McKee said. "This is an all-hands-on-deck response. Our state agencies have the resources and manpower they need to respond and keep Rhode Islanders safe."

New Jersey Governor Mikie Sherrill also declared a state of emergency and halted service on the state's transit network from Sunday evening for all buses, light rail, and Access Link.

She said this could be "the worst storm we have seen since 1996" and encouraged residents to stay home and off the roads.

In Connecticut, which, along with New York and New Jersey, comprises the metropolitan Tri-state area, Governor Ned Lamont signed an emergency order halting all commercial vehicles from travelling on highways across the state.

The order takes effect at 1700 local time (2200 GMT) and remains in effect until further notice, Lamont said.

Massachusetts, Delaware and Rhode Island have also declared a state of emergency ahead of the storm and the city of Boston, Massachusetts, has also closed schools due to the storm.

In Pennsylvania, Governor Josh Shapiro has signed a disaster declaration ahead of the storm to ensure state agencies "have every resource they need to prepare and keep people safe," and has told residents to avoid unnecessary travel and to follow instructions from local officials and agencies.

Although the nation's capital is not expected to be as severely impacted, school districts near Washington, DC, have announced closures. Federal government offices have delayed the start of the workday and said workers could take unscheduled leave or work remotely due to the weather.

In Canada, Nova Scotia's east coast is expected to be the worst-hit area, with many parts currently under an orange weather warning from Monday morning until early Tuesday.