Temperatures plunged as a mass of frigid air poured over the northern states from the Rockies to the Northeast yesterday on the heels of a storm that iced over highways, snarling traffic and killing at least six people.

Afternoon temperatures remained below zero from North Dakota to upper Michigan, and below freezing in much of the region between the Appalachians to the Rocky Mountains.

Blustery winds made it feel even colder. It was 9 degrees below zero in Grand Forks, N.D., where the wind chill reading was 35 below.

Wind gusting to near 35 mph at Mason City, Iowa, produced a wind chill of minus 51.

Snow advisories were posted in Ohio, Pennsylvania and New York near Lakes Erie and Ontario.

Freezing rain glazed roads in New York, New Jersey and Pennsylvania yesterday morning, causing dozens of minor accidents.

Freezing rain closed Westchester County Airport, and Newark International Airport in New Jersey and John F. Kennedy International Airport in New York closed temporarily so runways could be sanded.

In northern Iowa, hundreds of students spent the night away from home as school officials decided roads were too unsafe for buses, and people with homes near the schools opened their doors to the children.

"Visibility was whiteout to 10 feet at times," said Lt. John Dempsey of Mason City.

The storm dumped up to 14 inches of snow on Grand Rapids, Minn., and about one foot at the airport in Duluth, Wis.

At least four people were killed on treacherous roads in New Jersey, yesterday morning, in accidents that included a pileup involving a commuter bus, a tractor-trailer and 12 other vehicles.

Two traffic deaths in Iowa were blamed on the storm.