The National Weather Service warned of a ‘slight risk’ of flash flooding and severe thunderstorms in the Lower Hudson Valley.,

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New York region braces for possible flooding from another storm.

Flooding on the Franklin D. Roosevelt Drive in Manhattan last week left cars stranded as the remnants of Hurricane Ida hit the New York region.Credit…Stephanie Keith for The New York Times

Sept. 8, 2021Updated 11:25 p.m. ET

Thunderstorms began increasing across south-central Pennsylvania on Wednesday night, part of a storm system that could bring heavy rain overnight and early Thursday to parts of the New York region still reeling from the remnants of Hurricane Ida.

The National Weather Service said there was a “slight risk” of flash flooding and severe thunderstorms mainly in northeast New Jersey and the Lower Hudson Valley. The Weather Service also warned of the potential for damaging winds and hail and said that an “isolated tornado cannot be ruled out.”

By 11 p.m., bands of rain moved west into Warren County, where a flash flood warning was issued by the weather service. It also warned of potential flash flooding on stretches of Interstate 78 and 80.

The storms were moving into the region as many communities were still recovering from the damage caused by the remnants of Ida, which killed at least 46 people across four states one week ago.

Overall, the storms were expected to bring 0.5 to 1.5 inches of rain to the New York region, although 2 to 2.25 inches were possible in a worst-case scenario, the Weather Service said. It noted that flooding was possible in urban areas and in communities where small rivers, streams and creeks were already swollen.

New York City was expecting 0.75 inches to 1 inch of rain, with some areas possibly receiving 1.25 inches.

“These systems are not forecast to be severe at this time, but please be prepared as conditions may change,” the New York City Emergency Management Department said.

Mayor Bill de Blasio of New York said at a news conference on Wednesday morning that the heaviest downpours were possible after midnight, bringing about half an inch of rain.

“That, normally, would not be an issue at all,” he said. “Obviously, we’re concerned about folks who were hit that any new water could be a problem. And we’re also concerned that we could see more than is projected.”

Mr. de Blasio urged homeowners and business owners to take precautions and said that city workers would be “out in force,” cleaning catch basins, particularly in areas hard hit by Ida.

Gov. Philip D. Murphy of New Jersey also asked residents to be aware of possible flooding.

“Please take every precaution given that the ground in many places remains saturated,” Mr. Murphy said on Twitter. “The potential for flash flooding always exists with a quick-moving thunderstorm. Do not attempt to drive into standing water.”

The possibility of additional rain came as the authorities were still tallying the damage from Ida’s floodwaters, which killed at least 25 people in New Jersey, 16 in New York, four in Pennsylvania and one in Connecticut.