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, the Weather Service said, and 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.