A massive nor’easter bearing down on New England is expected to dump 12 to 20 inches of snow over much of Maine by the time it ends Wednesday morning.

The potentially crippling storm stretching from Washington to Maine will affect tens of millions of people and pack high winds that could produce blizzard conditions in many areas Tuesday.

On Monday, the National Weather Service placed all of coastal New Hampshire, York County and all of Cumberland County under a blizzard warning for Tuesday, meaning severe winter weather is expected and could create whiteout conditions that make travel extremely dangerous.

“It’s going to be a strong winter storm that brings blizzard conditions with it,” said weather service meteorologist Stacie Hanes. “This is going to bring visibility down quite a bit, with the heaviest snow falling in Portland in the afternoon and evening. We don’t use the term ‘blizzard’ lightly.”

Snow will be falling at the rate of 2-3 inches an hour during the height of the storm, she said.

The weather service is predicting that most of Maine will get between 12 and 20 inches, with some isolated areas in York and Cumberland counties looking at the possibility of being buried under 18 to 24 inches.

The blizzard warning will go into effect at 7 a.m. Tuesday and remain in effect through 5 a.m. Wednesday. The weather service defines a blizzard as a severe winter storm that limits visibility to less than a quarter mile, produces falling or blowing snow, generates sustained wind speeds or gusts of 35 mph or higher, and lasts for at least three consecutive hours.

The last storms to hit Portland that were officially classified as blizzards occurred Jan. 27, 2015, and Feb. 8, 2013.

School districts across southern Maine did not waste any time in making the decision to cancel classes ahead of Tuesday’s storm. Portland, South Portland, Scarborough, Cape Elizabeth, Biddeford, Kittery, Saco, Wells and Ogunquit, Westbrook, and York all announced Monday that there would be no classes Tuesday.

Some school superintendents were being more cautious. Freeport, Yarmouth and Brunswick school officials advised that there likely would be early dismissals Tuesday, although they left open the possibility they could change their minds Tuesday morning.

The scope of Tuesday’s storm was enormous. Meteorologists were calling for snowfall totals as high as 20 inches in New York City, and Boston could get 12 to 18 inches.

THOUSANDS OF FLIGHTS CANCELED

Air travel already was thrown into turmoil, with more than 5,000 flights in the Northeast region canceled as of Monday evening.

By Monday afternoon, about 90 percent of the flights scheduled to arrive at the Portland International Jetport had been canceled, Director Paul Bradbury said. He said many of the flights that were scheduled to land Monday night also were canceled because the airlines did not want their airplanes and crews stranded in Portland.

If the storm winds down as expected overnight Tuesday into Wednesday, Bradbury said the jetport should be able to resume operations Wednesday morning.

The Amtrak Downeaster announced on its website that it plans to operate all trains on Tuesday, except for trains 688 and 689.

Concord Coach Lines canceled nearly all of its scheduled Tuesday afternoon bus service between Maine and Boston. Concord’s website indicated that most of its morning routes would operate as scheduled.

Monday’s blizzard warning and snowfall totals for Portland – 17 inches are predicted to fall on the city – represent a slight upgrade from Sunday when the weather service wasn’t sure that blizzard conditions would occur and that snowfall totals for Portland would be closer to 12 inches.

But as of Monday afternoon, the weather service said the chance of Portland receiving a foot or more of snow was 84 percent.

Portland typically waits until the day of a storm before announcing a citywide parking ban, but wasn’t taking any chances with this nor’easter. City Communications Director Jessica Grondin issued a statement Monday afternoon alerting car owners that a citywide parking ban would be enforced from 10 p.m. Tuesday through 6 a.m. Wednesday.

Old Orchard Beach, Freeport, Falmouth, Kennebunkport, Windham, Yarmouth, Kittery and Gorham also announced on-street parking bans, with Gorham saying its ban would go into effect Monday at midnight and remain in effect through Thursday at 6 a.m.

Temperatures are predicted to be in the 20s, keeping the snow relatively light and making it less likely to cling to branches and power lines.

However, Central Maine Power Co. said deep snow could still create dangerous travel conditions that could lead to vehicles damaging utility poles.

“Every hour of every day, we are prepared to respond to power interruptions, but in a case like this all employees throughout the company are on heightened alert,” Sara Burns, CMP’s president and CEO, said in a written statement.

FEAR OF WIND-CAUSED OUTAGES

Emera Maine, which serves customers in northern and eastern Maine, is hoping the storm will produce, light fluffy snow.

“Our biggest concern at this point is the anticipated wind gusts Down East, which could bring trees and limbs down onto power lines, so we’ll be watching that closely,” Kevin Peterson, an Emera Maine spokesman, said in a written statement.

The Maine Emergency Management Agency said Monday that it is closely monitoring the storm’s approach. It urged motorists to prepare for the worst.

“Fortunately, we’ve been through plenty of storms by this time of year, so we’re used to it,” said Bruce Fitzgerald, director of MEMA. “But it’s always good to remind people to play it safe and not become complacent.”

MEMA said travel should be avoided, but if people need to go outside in the storm, drivers should update their emergency kits and pack a shovel, flashlight, battery-powered radio, blankets, booster cables and emergency flares.

The University of New England posted a message on its website letting students know that classes have been canceled at its Biddeford and Portland campuses. The University of Southern Maine canceled Tuesday classes on its Portland and Gorham campuses.

Winter storms have taken their toll on students across the state. Portland and South Portland have used up their five allotted snow days, which means Tuesday’s snow day will push the end of the school year even farther into summer. The last day of school for both districts was June 21.

Dennis Hoey can be contacted at 791-6365 or at:

[email protected]