Travelers returning home after Thanksgiving once again faced severe weather around the country, as some roads in the Midwest were impassable and a plane in Buffalo, New York, slid off the runway.

NBC News forecasters estimate more than 56 million Americans could be affected by the pre-winter weather.

There were delays across the country, and 423 flights were canceled Sunday, according to flight tracking website Flight Aware.

Delta Airlines said in a statement Saturday that 22 of its Northeast destinations and 16 of its upper-Midwest stops were likely to be affected by weather delays, and it offered flight changes with no extra charges for customers scheduled to use New York's LaGuardia Airport on Sunday and Monday.

In Buffalo, a flight arriving from LaGuardia slid off the runway Sunday morning when taxiing, and got stuck in grass, the airline said in a statement. Passengers exited the plane from a staircase and were bused to the terminal. Upstate New York is forecast to get eight to 16 inches of snow, the national weather service said.

American Airlines issued a travel alert for more than 40 North American airports from the Dakotas to Michigan and throughout the Northeast, including its hubs at New York's John F. Kennedy International Airport and Philadelphia International Airport.

The carrier said the alert warning of possible delays could impact 2.7 million of its customers through Tuesday. It also offered no-fee flight changes for weather-delayed travelers using select hubs.

The National Weather Service's forecast for Sunday called for heavy snow for the upper Mississippi Valley, the upper Great Lakes and in parts of the Northeast, freezing rain in parts of the Mid-Atlantic and southern New England, showers and thunderstorms in the Tennessee and Mississippi valleys and rain in some parts of Southern California to the Pacific Northwest.

Ice storm warnings are in effect from western Maryland to central Pennsylvania, where freezing rain will cause "severe travel disruptions," the weather service said.

Conditions are "extremely treacherous" in northern Minnesota after an evening of very heavy snowfall, the National Weather Service said. Roads near Duluth are fully covered in snow, and "most are impassable," according to weather officials, which said numerous vehicles got stuck in snow drifts and ditches Sunday morning. From the eastern Dakotas to Northern Michigan, the potential for a foot of Sunday snow is also causing travel woes.

In Massachusetts, officials are warning of rough afternoon conditions for Sunday on the state's highways, where snow expected to start at noon and last through the evening will create "treacherous travel," the local National Weather Service said.

In California, where more snow and high winds are expected in northern parts of the state on Sunday, a Thanksgiving Day storm was blamed for suspension of rail service Saturday between the state's two largest cities, Los Angeles and San Diego. Workers were shoring up an eroding seaside bluff that comes perilously close to tracks in Del Mar.

The tracks used by Amtrak and a regional commuter line were expected to be back online Sunday, said Kimy Wall, spokeswoman for San Diego's North County Transit District.

In Arizona, the bodies of two children were found Saturday in Tonto Basin after a vehicle they were in was flooded during a rainstorm. A third child remained missing.

The severe Thanksgiving week weather has been blamed for vehicle pileups in the Southwest, three tornadoes in Maricopa County, Arizona, and blizzard conditions in Rapid City, South Dakota.