There is a growing risk for flash flooding for millions in the Northeast Friday, including the New York City area, as moisture from the remains of long-dead Tropical Storm Ophelia combines with a new coastal storm to deliver heavy rainfall.

"The remnant of Ophelia will merge with a new low pressure developing along the Carolina coast today before strengthening as it heads northeastward tonight and Friday, spreading rain back across the region, especially coastal areas," the National Weather Service office in Philadelphia said Thursday morning.

The rain will begin for many starting Thursday, peaking on Friday and Friday night from Maryland to Massachusetts.

Rainfall totals are expected to range between 1 and 3 inches, but some areas could receive more than 4 inches. Up to 5 inches is possible in the New York City area through Saturday.

Uncertainty remains for exactly where the worst rain will fall since weather models are in disagreement with how close the low pressure system will be to the coast.

A track closer to the coast would increase rainfall totals. A more offshore track would keep the heaviest rain and storms out to sea.

Flooding could happen even in areas that only receive one inch of rain, since the ground is already saturated across many areas of the Northeast from recent heavy rainfall. At least two inches of rain has fallen over the last seven days from Virginia through Massachusetts, with some areas picking up over four inches.

The system will be very slow to exit the region this weekend, so more rain is expected from Boston to Virginia Beach on Saturday. The sun finally returns in full force on Sunday as rainfall-limiting high pressure returns to the Northeast and mid-Atlantic.

A supermoon coastal flood threat

Another factor may exacerbate flooding across the Northeast and mid-Atlantic -- a supermoon.

The fourth and final supermoon of 2023 will occur Friday morning, creating more extreme tidal cycles that could increase the flood potential in communities along the East Coast.

As a result, coastal flood advisories extend more than 300 miles from Bridgeport, Connecticut, through Virginia Beach.

Up to one foot of inundation above ground level is expected in low-lying areas near shorelines and tidal waterways. The weather service warns some partial or full road closures are possible.

Rip currents and very high surf are another concern. At least 76 people have died in rip currents so far this year, NOAA data shows.