Heavy rain and howling winds served up a double dose of nasty weather for Long Island Monday as a nor’easter took dead aim on the region, overflowing streets and leaving thousands in the dark by nightfall, with the potential for the worst flooding Tuesday morning.

Weather watchers and state parks officials warned that potentially damaging effects from the storm — even as it diminished overnight — were possible, leading to a risk of significant coastal flooding Tuesday.

Widespread reports of power outages continued well into Tuesday morning after PSEG officials late Monday night said they had restored power to 17,000 customers since the storm began. At about 3:30 a.m. Tuesday, 2,439 PSEG customers were without power, according to the utility’s website.

A wind advisory warning of northeast gusts remained in effect until 6 a.m. Tuesday, according to the National Weather Service, which at about 10:30 p.m. Monday canceled an earlier high-wind warning forecasting gusts of up to 70 mph.

A coastal flood warning remained in effect from 4 to 11 a.m. Tuesday for South Shore beaches and eastern Suffolk bays, with the potential for major overflow in isolated spots at high tide, officials said.

A flood watch for Nassau County that began at 1 p.m. Monday was set to lift at 6 a.m. Tuesday.

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Heavy rain of between a quarter- and a half-inch was expected by 4 p.m. Tuesday with steady northwest winds of 17 to 23 mph before the storm moves on by the evening.

Residents in flood-prone areas were advised to “be alert for rising water . . . and take appropriate action to protect life and property,” according to a statement from the National Weather Service.

Late Monday night, state parks officials said half of Gilgo Beach was submerged in seawater pushed ashore by the rising tide and offshore gusts of up to 60 mph. Elsewhere on the South Shore, as well as the Long Island Sound, state and local officials reported minor beach erosion and flooding but remained concerned about significant coastal damage at high tide Tuesday morning.

And while beaches flooded at high tide Monday afternoon, most dunes remained in place, an early hopeful sign that the nor’easter might not do the sort of long-term shore damage wreaked by past storms, said George Gorman, New York State Parks deputy regional director for Long Island.

“It was not eroding the dunes, so that’s great,” he said. “I was pretty happy — I hate to say that because of the flooding, but nothing of concern like in some of the major storms that we’ve seen.”

As the storm gathered strength Monday afternoon and into the night, towns across Long Island reported street flooding and trees and power lines toppled by gusts of up to 45 mph in some locations.

Outages were spread somewhat evenly across the island, said PSEG spokesman Jeff Weir.

PSEG brought in 190 outside line workers to help in the restorations, Weir said, along with 100 outside tree trimmers.

He said crews were expected on the streets overnight with replacements “set to take over in the morning.”

“There’s nothing we won’t be able to handle,” Weir said.

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In fact, Long Islanders — veterans of damaging storms of the past like Sandy — were taking the storm in stride and shoring up areas prone to flooding ahead of Tuesday morning’s high tide.

In Lindenhurst Village, residents below Montauk Highway began to see street flooding by midafternoon, or “the usual” as they call it. Mayor Tom Brennan said the village had not taken extra measures, but he anticipated more flooding after evening high tide and had public works and fire department crews on standby.

“Hopefully it doesn’t get that bad, but if it does, we’re ready,” he said.

Areas nearby in the Babylon Town section of Lindenhurst, such as American Venice, Copiague, and Amity Harbor, continued under close watch, according to town spokesman Kevin Bonner. In addition, he said, the town’s departments of public works and environmental conservation had monitored the South Shore and barrier beaches throughout the day. He said the town has been working the past few months to clear storm drains in anticipation of large storms.

Just before 11 p.m. Monday in storm-wary Long Beach, Park Avenue in a West End neighborhood near the Reynolds Channel was flooded, as was Broadway near the boardwalk. Pavement was still visible in both areas.

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Piles of sand on the beach at New York Avenue and National Boulevard at the boardwalk — both street entrances to the shore — were holding back the tide from entering Broadway.

The West End’s streets, many lined with homes still being repaired from the damage of superstorm Sandy in 2012, were largely deserted.

Forecasts Sunday tracked the storm on a northern course after it carved a soaking and, in some locations, lethal path through parts of the southeastern United States.

Riverhead and other East End communities were expected to get hit hardest by the powerful winds. By late Monday night, the storm’s strength was hard to miss in Riverhead.

Nearly the entire stretch of Heidi Behr Way on the town’s Peconic River waterfront was under nearly a foot of water just before high tide at about 9 p.m. Police barricades closed off most of the roadway as high winds and swelling waters from the river cascaded over the road and formed a small lake in an adjacent parking lot.

The floodwaters had so far spared Riverhead’s Main Street Monday night. Officials closed the street earlier in the evening after winds damaged part of a utility pole, leaving dozens of homes and buildings temporarily in the dark. Power appeared to be restored by 8 p.m.

In Freeport, Mayor Robert Kennedy took to his car to check for flooding and other damage Monday night. Along Hudson Avenue, one of Freeport’s most flood-prone streets because of canals on both sides, water had topped bulkheads and the curb-line by 6:30 p.m.

Water at Hudson Avenue and Martha Street, Kennedy estimated, had risen between 12 and 15 inches.

“Once the water goes over the bulkheads, you can’t do anything about it,” he said.

A PSEG worker at the scene of downed wires caused by strong winds on Grove Avenue in Patchogue on Monday, Jan. 23, 2017. A nor'easter is expected to hit the area with strong winds and heavy rain. Photo Credit: James Carbone