Heavy rainfall from a powerful storm careening up the East Coast is expected to batter the New York region starting Sunday night as officials warned commuters to brace for disrupted travel conditions, strong winds, and major flooding.

The National Weather Service said that a low-pressure system in the Gulf of Mexico could bring as much as four inches of rainfall to some areas across New York City and northeastern New Jersey after it passed over South Carolina and North Carolina earlier on Sunday.

Downpours were also forecast for areas east of Interstate 95 in Washington and Baltimore. Flood watches were issued for those areas. The Allegheny Mountains could get as much as 10 inches of snow, forecasters said.

“We’ve seen the rain moving up the eastern coast of the U.S. right now,” said Alan Reppert, a senior meteorologist at AccuWeather, a weather forecasting service. The storm, by midafternoon on Sunday, had drenched parts of South Carolina with more than nine inches of rain.

By Sunday evening, the heaviest rain was falling over Delaware, where parts of the state, including New Castle, are under a flood watch, Mr. Reppert said. Rain will move into New York, New Jersey, and Connecticut in the coming hours, he said.

“We’re looking at the heaviest rainfall in the New York City area closer to midnight,” he said, adding that it was expected to be a “much stronger” storm than recent ones that have hit the region.

The National Weather Service in New York issued a coastal flood warning from 10 p.m. Sunday to 6 p.m. Monday for southern Westchester County and several communities in southern Connecticut, including New Haven.

Along the New York coast, flooding with up to two and a half feet of inundation and sustained winds of 25 to 40 miles per hour, with gusts of 55 to 60 m.p.h., could damage power lines and topple trees, the Weather Service said.

Officials bracing for the storm implored those who live in basements to move to higher ground and advised all residents to download the city’s emergency alert application.

They also cautioned residents to brace for powerful gusts. Winds in the city were expected to be the strongest early to mid-morning on Monday, officials said, with gusts up to 60 m.p.h. sweeping through Brooklyn and Queens. Waves 12 to 16 feet high could pummel the coastline, the Weather Service said.

Widespread rain of two to four inches is also expected to fall over the Philadelphia region from Sunday into Monday morning, elevating concerns about potential urban and coastal flooding.

“River levels may continue to rise after the rain ends on Monday, with some rivers forecast to crest as late as Monday night through Tuesday,” the National Weather Service for Philadelphia and Mount Holly said in a Sunday evening briefing.

The service warned that strong winds there could also bring down tree limbs and cause power outages.

A storm watch was also issued for stretches of the Massachusetts and Rhode Island coasts.

Charleston, S.C., recorded its fourth-highest tide on record, and the highest tide without a hurricane, said Steven Taylor, lead meteorologist with the National Weather Service office in Charleston. The high tide on Sunday reached 9.86 feet. The record high was 12.52 feet, recorded during Hurricane Hugo in 1989, he said.

Ben Almquist, the emergency manager for Charleston, said a few roads had to be closed because of flooding.

“We have several buildings that had water damage reported and 50 to 80 people had to be pulled out of their vehicles,” he said. No serious injuries were reported.

The storm is the second in recent days to rough up the East Coast. A week ago, many of the same communities were under similar watches and warnings.

“We will even see some power outages, beach erosion, and coastal inundation from the waves and the wind pushing the water,” Mr. Reppert said.

Forecasters said the storm was expected to move away from the northeastern coast of the United States on Monday evening, but they cautioned residents to prepare.

“Even though the big storm will begin to depart the Northeast Monday evening, the huge circulation of the storm will overspread the entire eastern U.S. with very blustery conditions,” the Weather Service said.

Rebecca Carballo and Johnny Diaz contributed reporting.