MINEOLA, N.Y. (AP) — A sloppy mix of rain and snow rolled into the Northeast on Wednesday just as millions of Americans began the big Thanksgiving getaway, grounding hundreds of flights and turning highways hazardous along the congested Washington-to-Boston corridor.

By early afternoon, more than 600 flights had been canceled, the bulk of them in the Northeast, during what is typicallyone of the busiest travel periods of the year. Thousands of flight delays were also expected as the snow from the nor’easter piled up.

Some travelers tried to change their plans and catch earlier flights to beat the storm, and major airlines waived their re-booking fees. But most planes were already filled.

Numerous auto accidents were reported across the Northeast, where by midafternoon the line between rain and snow ran roughly along Interstate 95, the chief route between Washington and Boston.

Pat Green and her husband drove from Saugerties, New York, to the Albany airport for the first leg of their trip to San Francisco. She said the drive on the New York State Thruway was “a little hairy,” but they made it, and their flight was on schedule so far.

“It was snowing so hard you couldn’t see the car ahead of you,” she said. “We slowed down so we were fine. We also give ourselves a lot of extra time.”

Major Northeast cities were likely to see moderate to heavy rain most of the day, though New York could see 1 to 4 inches of snow, and its northern suburbs 6 to 8 inches, the National Weather Service said. Higher elevations west of the I-95 corridor could see as much as 6 to 12 inches.

New York Gov. Andrew M. Cuomo activated the state’s Emergency Operations Center and had 1,800 plow operators standing by to clear snow between New York City and Albany.

Dan Albert hoped to beat the snow as he, his wife and 15-year-old daughter refueled their SUV Wednesday morning along I-81 in Hagerstown, Maryland, about halfway between their Greensboro, North Carolina, home and their Thanksgiving destination in Mahwah, New Jersey.

“Traffic was fine last night, no problems at all, but today’s going to be a real booger,” Albert said. But he added: “Got to see the folks. We only get to see them once a year. Got to muscle through it, right?”

At a rest stop on the New Jersey Turnpike, tired families grabbed large cups of coffee and breakfasts of pizza and fried chicken before heading back to the slushy road.

Abdullah Masud, a lawyer who lives in Kuwait, was heading from Boston to Washington with a cousin.

“We were originally planning on leaving Wednesday morning, but when we heard about the snowstorm we changed our mind and left Tuesday night. But I don’t think it made that much of a difference,” he said, noting the heavy traffic.

By early afternoon, airlines had canceled more than 10 percent of their flights at Philadelphia, Newark Liberty and LaGuardia airports.

The wintry weather was not confined to the Northeast. An Alberta clipper left many highways in North and South Dakota slick, and a winter storm warning was issued for parts of Minnesota.

The Wednesday before Thanksgiving is often called the nation’s busiest travel day of the year, but that’s probably not true. A U.S. Transportation Department study based on 2001 figures found that when car trips are taken into account, Thanksgiving Day itself is a heavier travel day.

As for air travel, the Wednesday before Thanksgiving didn’t even rank among the Top 10 busiest days of 2013, the Transportation Security Administration found.

The top airline travel day last year was the Sunday after Thanksgiving, with 2.2 million passengers. The Friday before Memorial Day was second at 2.1 million.