STATE COLLEGE, PA. ? Sliding cars and jackknifed trucks snarled highway traffic, and flights were grounded Tuesday as a storm blew out of the Midwest with a threat of up to 2 feet of blowing, drifting snow.

The National Weather Service posted winter storm watches and warnings from Iowa and Missouri across the Ohio Valley into parts of New England.

As the storm plowed eastward, more than 2 inches of snow fell by midday at Pittsburgh, while parts of Indiana measured more than 11 inches with drifts up to 6 feet, the weather service said.

Up to 18 inches of snow was forecast for northern Pennsylvania, with a chance of 2 feet at higher elevations. Twenty-inch accumulations were possible in parts of New York state, where communities on the eastern end of Lake Ontario have endured a week of lake-effect snow that totaled more than 11 feet.

Schools were canceled or delayed in Pennsylvania and elsewhere. The University of Illinois in Champaign canceled classes for the first time since 1979, and today’s Indiana-Purdue men’s basketball game was postponed until Thursday.

The federal government in Washington dismissed most employees at 2 p.m., forcing the area’s transit system to adjust its schedule and begin its rush hour service. The commuter rail line between Washington and Baltimore tried to adjust to the early flood of passengers.

At least four traffic deaths were blamed on the snow and ice – three in Nebraska and one in Missouri. A 9-year-old girl was killed when an ice-covered tree branch fell on her in Ohio.

“I want to get out of here and away from this weather,” trucker Terry Kettering of Youngstown, Ohio, said at the Flying J Travel Plaza on the south side of Indianapolis.

Illinois State Police reported whiteout conditions and roads blocked in some places by waist-high drifts.

“It’s very dangerous; it’s real bad,” said Gustavo Chavez, who was at a gas station in Sherman, Ill., north of Springfield, after paying $200 to have his car pulled from a ditch. “It doesn’t stop.”

Pennsylvania’s two largest airports, at Philadelphia and Pittsburgh, reported no major problems by afternoon.

Elsewhere, however, Chicago’s O’Hare International Airport canceled more than 900 flights, airport and airline officials said. Midway Airport canceled about 300 flights. About 20 percent of the flights out of Cincinnati’s main airport were canceled because of poor conditions elsewhere, spokesmen said.

Ohio state officials said southbound lanes of Interstate 75 in the Dayton area reopened after being closed by jackknifed tractor-trailer rigs.

“People are sliding off everywhere,” said Joe Whittaker, emergency management director in west-central Indiana’s Fountain County.

Along the southern edge of the snow belt, freezing rain coated roads, tree limbs and power lines with as much as three-quarters of an inch of ice. About 10,500 Duke Energy customers lost power in Indiana by late afternoon, said spokeswoman Angeline Protogere.

Ameren Corp. said about 7,600 customers were without power, mostly in Illinois.