A powerful storm system began to move into the Northeast on Sunday, prompting flood and wind alerts as well as travel advisories as some areas braced for power outages and up to six inches of rain, forecasters said.

The National Weather Service in New York issued a flood watch for portions of southern Connecticut, northeast New Jersey and most of New York City and Long Island and parts of the Hudson Valley. The flood watch is in effect from Sunday afternoon through Monday afternoon.

“It is a pretty large storm system covering the Northeast,” Joe Dellicarpini, a meteorologist with the Weather Service office in Boston, said on Sunday.

“We are looking at a potential for flooding, not just the roadways, but some rivers and small streams as well, and that is primarily across Connecticut and Massachusetts,” he said.

Parts of central Connecticut, roughly as far north as Hartford, up to Lowell, Mass., and into northwestern Rhode Island were upgraded to a moderate risk of excessive rainfall on Sunday. Prolonged and heavy rainfall could bring up to seven inches of rain and flash flooding.

Light to moderate rain was already falling in New York City early Sunday afternoon. The heaviest rain was expected in New York later and will spread into the Boston area by the evening.

Forecasters said three to five inches of rain could fall across parts of Long Island and southern Connecticut, with six inches possible in some areas. Forecasters said most of the rain would fall in a 12-hour period.

Winds of 25 to 35 miles per hour and gusts of 45 to 55 m.p.h. are expected along the coast on Sunday evening. A high wind warning was posted for Suffolk County and a wind advisory was in effect for Nassau County, Queens and Brooklyn. Damaging wind gusts of up to 60 m.p.h. are likely in Suffolk County Sunday night, forecasters said.

“Expect poorly secured outdoor objects to be damaged or blown away,” the National Weather Service said.

The combination of wind, rain and in some places snow in New York “could cause some dangerous issues for New Yorkers, including flooding, dangerous travel and power outages,” Gov. Kathy Hochul said on Friday.

The National Weather Service office in Albany posted winter storm warnings for parts of the North Country in New York and said accumulating snow was likely for Sunday night into Monday, making for difficult travel for the morning commute.

A Storm Prediction Center map showed winter storm warnings posted throughout Northern New York and into Vermont and storm warnings along the coast from Long Island north to Maine.

Emergency management officials in New York City issued a travel advisory for Sunday and Monday.

Flooding could lead to traffic disruptions and delays. Officials asked New Yorkers to exercise caution when traveling, allow for extra travel time and consider using public transportation.

“Everyone should take the necessary precautions to protect themselves and their belongings in the event of potential flooding conditions in low-lying areas,” Mayor Eric Adams said in a statement.

The storm comes weeks after record-breaking rains in September overwhelmed New York City’s subway system, grounded flights and flooded streets.

There were no storm-related fatalities then, but emergency teams made 28 rescues from “raging waters” in the Hudson Valley and Long Island, according to Ms. Hochul.

Elsewhere in the Northeast, the Philadelphia/Mount Holly Weather Forecast Office posted a flood watch from 1 p.m. Sunday through 4 p.m. Monday in New Jersey, Delaware and parts of Pennsylvania.

About an inch of rain has already fallen in parts of southern Delaware and New Jersey, the office said.

“While there will be a lull in the rain through late afternoon, a cold front will enhance the rain this evening and tonight,” the office said, noting that up to two more inches of rain were expected.

The severe weather could cause power outages, PSE&G, the gas and electric service provider in New Jersey, warned. “Our crews are prepared and ready to respond safely and as quickly as possible if outages occur,” the company said.

There is also potential for damaging winds in Cape Cod, Martha’s Vineyard and Nantucket, where gusts could reach up to 60 m.p.h. late Sunday into Monday, Mr. Dellicarpini said.

The Northeast was not the only part of the United States to experience severe weather this weekend. On Saturday, officials said six people were killed and more than 20 hurt after powerful storms blew through parts of Tennessee. Tornado watches and warnings as well as severe thunderstorm warnings were posted throughout the South.

Johnny Diaz contributed reporting.