It was cars -- as many as 15 at a time -- and not snow that piled up here last night as the Washington area was coated by freezing rain rather than the flakes weather forecasters had been predicting for days.

And after its unfulfilled warning of about six inches of snow yesterday, the National Weather Service was hedging its bets on this morning's rush hour. The official prediction: occasional freezing rain and drizzle, ending sometime in the early morning, with temperatures around 30 degrees.

"It will be dangerous in the morning," weather service forecaster Walter Green said last night, after yet another revised prediction eliminated the warming temperatures that earlier had been expected overnight.

The most serious accident believed to have been caused by icy roads yesterday occurred shortly before 5 p.m. near Croom in southern Prince George's County, fatally injuring an Edgewater, Md., woman.

Maryland state police said Ida Marie Mitchell was driving north on Rte. 301 near Marlton Avenue when she lost control of her car, hit a curb, crossed two lanes of traffic and collided with a flatbed truck.

Mitchell died about 7:20 p.m. at Prince George's General Hospital, police said. The driver of the truck was not injured and no charges were filed, police said.

The snow the area was supposed to get ended up in Pennsylvania and northward, and Green said the cause was an unexpected blast of warm air that moved into the area in the upper atmosphere.

But a pocket of cold air was trapped near the ground and that caused the rain to freeze, spreading a glaze of ice that made many roads and bridges impassable and spawned hundreds of minor auto accidents.

Police throughout the area said the spate of accidents started about nightfall and continued unabated throughout the evening hours.

"The ice is deceiving," explained a Fairfax City police sergeant. "You can't see it until you are right on top of it."

One radio traffic announcer advised motorists not to drive "any faster than you'd like to be going when you run into something."

"The Beltway is just a sheet of ice," said a Maryland state police dispatcher in College Park.

"Utter chaos," is how a Maryland state police dispatcher in Forestville described the road conditions that caused a six-car chain-reaction collision on the inner loop of the Beltway near Georgia Avenue.

"The bulk of the accidents have been people trying to stop and sliding into other cars," said Lt. George Joca of the Fairfax police. "I guess it's just fate and good luck that no one here has been seriously hurt."

No major power outages were reported as a result of the storm, but scores of evening meetings and classes were canceled and Dulles International Airport closed at 7:14 p.m. because of ice on the runways and taxiways. National and Baltimore-Washington International airports were able to maintain normal schedules, officials said.

Generally, road conditions seemed worse north and west of the city.

Near Fairfax Circle, a Metrobus slid out of control and ended up tilted in a ditch near Jermantown Road and Gray Street. The accident occurred about 9:30, officials said. There were no passengers aboard and the driver was not injured.

The largest pileup of the night, 15 cars, was reported at Arlington Boulevard and Covington Street just east of Fairfax Circle about 6:40 p.m. and the next largest, 13 cars, occurred about the same time at Gallows Road and Cottage Street, just north of I-66.

The District was not spared the icy conditions, and neither were District police and fire vehicles. Five police scout cars had spun out on the slick streets by 10:30 p.m. -- including one that slid into a tow truck that was helping a disabled car on the 11th Street bridge.

And a fire department ladder truck returning from a call fishtailed as it rounded a corner in the 3000 block of Gainesville Street SE and landed on top of a parked car.