By Tuesday, parts of the Northeast will see up to a foot of snow, the service said.
Central Maine Power Co. said Saturday that it was closely monitoring weather forecasts and preparing to respond to potential power outages that could occur as a result of the snowstorm.
"You could face messy conditions on Sunday, so exercise caution and give yourself lots of extra travel time", NYC Emergency Management Commissioner Deanne Criswell said on Saturday.
A new system was forecast to develop in the mid-Atlantic on Sunday, moving as a nor'easter into Monday.
There were 100 flights canceled Saturday and another 182 delayed at Denver International Airport because of winds.
New York City's best chances for accumulating snow will probably come Monday, when 1 to 3 inches could fall before the storm finally ends Tuesday.
The National Service officially released a Winter Storm Watch for Orange, Putnam, Rockland and Westchester counties with 8 to 12 inches of snow predicted for those areas.
Just outside Duluth, the Hermantown Police Department reported late Saturday that "multiple roads contain multiple stuck cars making some areas impassable and bottlenecking all traffic". But residents in inland locations should brace for snow and a wintry mix.
All roads in and out of Casper were closed Saturday morning, including the entire 300-mile stretch of Interstate 25 in Wyoming.
Travel was also hard in Colorado on Saturday as winds blew around snow that had fallen in previous days.
In northern Montana, more than a foot of fresh snow and strong wind gusts are expected to combine to create ground blizzard conditions along the Rocky Mountain front.
Roads were closed Saturday across eastern and southern Wyoming due to whiteout conditions.