More wintry weather could complicate travel leading up to the Thanksgiving holiday, according to forecasts across the United States, while a federal agency has issued a warning over air traffic controller shortages.

The National Weather Service (NWS) office in Sacramento, California, issued a winter storm warning for the Sierra Nevada through Tuesday, with heavy snow expected at higher elevations and wind gusts potentially reaching 55 mph. Total snowfall of roughly four feet was forecast, with the heaviest accumulations expected on Monday and Tuesday.

Winter storm warnings and advisories were also in effect in parts of Idaho, Montana, Colorado, Utah, Nevada, upstate New York, Vermont, New Hampshire, Maine, Alaska, and New Mexico, according to an NWS map issued on Monday.

The Midwest and Great Lakes regions will see rain and snow on Monday, and the East Coast will be most impacted on Thanksgiving and Black Friday, forecasters said.

“The system doesn’t look like a powerhouse right now,” Hayden Frank, a meteorologist with the weather service in Massachusetts, said on Sunday. “Basically, this is going to bring rain to the I-95 corridor, so travelers should prepare for wet weather. Unless the system trends a lot colder, it looks like rain.”

Frank said he isn’t seeing any major storm systems arriving for the weekend anywhere in the country, so travelers heading home Sunday can expect good driving conditions. Temperatures, however, will get colder in the East while warming up out West.

But parts of the Midwest and East Coast can expect to see heavy rain, and there’s potential for snow in Northeastern states.

A storm last week brought rain to New York and New Jersey, where wildfires have raged in recent weeks, and heavy snow to northeastern Pennsylvania. The precipitation was expected to help ease drought conditions after an exceptionally dry fall.

Private weather forecaster AccuWeather warned that a storm that could produce snow may impact the eastern portion of the United States on Thanksgiving Day, including in areas such as the Northeast and Ohio Valley.

The head of the Federal Aviation Administration (FAA) said the agency may need to slow down air traffic during the Thanksgiving travel week due to air traffic controller shortages, namely in the northeastern United States.

“During the Thanksgiving travel period, we will use traffic flow management initiatives to deal with any staffing shortages on that particular day in this airspace, and we expect to have some of those shortages. So, we’re working diligently to make sure that that can operate as efficiently as possible,” FAA Administrator Mike Whitaker told reporters during a news conference on Nov. 21.

His agency is anticipating that Tuesday will be the busiest day for air travel and traffic, he said. Slowdowns will be implemented for safety, if need be, he added.

“These initiatives keep the system safe, and people should know that safety is never at risk. If we are short on staff, we will slow traffic as needed to keep the system safe,” Whitaker said.

Trade group Airlines for America said in a report issued earlier this month that it expects record numbers of travelers to fly between Nov. 22 and Dec. 2.

More than 31 million people are expected to fly on U.S. carriers during that time period, it forecast. That’s up from 29 million travelers during the same time frame last year.

The busiest days are expected to be on Wednesday, Nov. 27, and Sunday, Dec. 1, according to the group.

The Associated Press contributed to this report.