A late autumn storm is expected to dump as much as six inches of snow today across the northeastern United States, threatening to snarl road, rail and air traffic on one of the busiest travel days of the year.

"It's going to be a nasty day to travel," said Robert Sinclair Jr., a spokesman at the Automobile Club of New York, of the day before Thanksgiving. "You're going to have a double whammy, with folks on the roads and in the skies running into a lot of problems," he said.

Weather forecasters said they expect the system will bring from three to six inches of snow from Ohio to Massachusetts, with heavier amounts inland.

Light rain began to fall in the Washington region last night as the leading edge of a storm approached from the north and west. However, travel by road and air was proceeding without reported delays, authorities said. Conditions at Reagan National Airport were "pretty good," an official said.

Forecasters said that while the storm would not be a major one, it would still be significant because it will hit the region on a big travel day.

The Port Authority of New York and New Jersey said its staff, armed with heavy snow equipment and thousands of tons of salt, sand and de-icing chemicals, would work around the clock to avoid travel delays at regional airports.

Snow disrupted air traffic yesterday at Chicago's O'Hare International Airport. Arriving flights were being delayed by 3 hours, 18 minutes on average, and some were being pushed back by as much as 10 1/2 hours.