National Weather Service (NWS) meteorologists are urging some New Yorkers to postpone their trips this weekend as heavy snow moves into the area.

The NWS has been monitoring the possibility of lake-effect snow in parts of New York this weekend, and its Buffalo office issued a winter storm warning earlier this week. On Wednesday afternoon, meteorologists upgraded the clock to a lake effect snow warning for Wyoming, Chautauqua, Cattaraugus and southern Erie counties and the eastern Lake Ontario region for this weekend.

“Heavy snow with lake effects is expected. Several feet of snow will likely fall in the most persistent snow on the lakes through the weekend,” the warning said. “The highest accumulations will occur near the Tug Hill plateau. Snowfall is likely to exceed 5 cm per hour at times.”

Once snowfall exceeds two inches per hour, snow plows have difficulty clearing roads, NWS meteorologist David Thomas said. Newsweekmeaning roads will quickly become covered if the snow falls that quickly or faster. This could cause extremely dangerous driving conditions, or temporarily close roads in some places, although it is too early to tell which roads may be affected.

“Travel will be very difficult and sometimes almost impossible. The hazardous conditions will certainly impact Friday evening commutes,” the warning said. “During lake effect snow, weather can vary from locally heavy snow with greatly reduced visibility to dry conditions just a few miles away. Be prepared for rapid changes in weather, visibility and road conditions.”

The safest course of action is to postpone the trip until after the storm, or leave before it arrives.

“If you must travel, drive with extreme caution. Leave plenty of space between you and the motorist in front of you and allow extra time to reach your destination,” the warning said. “Avoid sudden braking or acceleration, and be especially careful on hills or when making turns.”

The lake effect snow warning goes into effect Friday at 7 a.m. local time and lasts until 7 a.m. Monday. Poor driving conditions could impact Thanksgiving travelers on their way home or Black Friday shoppers.

The warning comes as nearly 80 million Americans are expected to travel across the country for the Thanksgiving holiday.

Elsewhere in New York, a winter storm watch issued by the Albany office warned of heavy snow of 7 inches or more that began falling Wednesday evening and continued through Thursday night. The snow may hamper roads for last-minute travelers.

“Roads, especially bridges and overpasses, are likely to become slippery and dangerous. Be aware of slippery roads,” the guard said. “The hazardous conditions could impact travel around the Thanksgiving holiday.”