Turkey and all the drippings: rain, snow may snarl U.S. holiday travel NEW YORK (Reuters) - Treacherous weather predicted for the day before Thanksgiving prompted some of the 46.3 million Americans expected to travel this holiday weekend, the year's busiest, to bump their flight and driving plans up to Tuesday. Up to 10 inches of snow was forecast overnight for mountainous parts of the Northeast and heavy rain and snow was predicted for New York, Boston, Philadelphia and Washington, said National Weather Service meteorologist Michael Musher. "Even if it's just rain, there are going to be delays which could cause havoc across the country," Musher said. "We're talking about a lot of moisture moving in overnight into tomorrow morning," he said. On the roads, there is the potential for standing water, which could cause hydroplaning. Rain was expected to turn to snow during the day on Wednesday, with accumulations of up to 5 inches in New York City, up to 4 inches in Philadelphia, up to 2 inches in Boston and less than an inch in Washington, according to the National Weather Service. Snow totals were expected to be higher outside the metropolitan areas. Altogether, 46.3 million Americans are expected to travel over the holiday weekend, the highest volume since 2007, according to the AAA. With nasty weather predicted for Wednesday, American, United, Delta and Southwest airlines offered a free one-time change for fliers who want to move up or delay their flights over the holiday weekend. Some 3.5 million were expected to be flying over the holiday weekend, according to AAA. "We're doing everything we can to get our customers to their destinations," said United spokeswoman Karen May. U.S. road travelers drive an average 549-miles round-trip at Thanksgiving to feast on turkey and all the fixings with their loved ones, said AAA spokeswoman Julie Hall. "We're telling drivers to consider altering their travel plans to give themselves extra time," Hall said. If they can get ahead of the storm or if they can delay to avoid the storm, they should do that. Reporting by Barbara Goldberg; Editing by Jill Serjeant and Mohammad Zargham