Suffolk County officials issued a state of emergency for the county Saturday afternoon, and have called for a voluntary evacuation of Fire Island as Tropical Storm Hermine set its sights on the region.

But Suffolk police said a “code red” warning sent out by the Federal Emergency Management Agency at about 8:30 p.m. Saturday mistakenly called for the evacuation of the entire county, when it was intended to only announce voluntary evacuations for Fire Island. There are no additional evacuation orders for Long Island.

The alert was caught by Suffolk County Fire Emergency Services, which sent out a correct alert.

While there is no state of emergency in Nassau, officials in that county warned that those living in flood-prone areas should be prepared to leave their homes if necessary.

“The biggest concern we have is Fire Island,” County Executive Steve Bellone said at a news conference at the Fire Island ferry terminal in Bay Shore Saturday afternoon.

The voluntary evacuation should begin immediately, Bellone said, and should be completed by 1 p.m. Sunday — while ferries still are running and before Hermine is slated to begin hitting the region the hardest.

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“This storm comes at the worst possible time,” Bellone said. “Labor Day weekend, probably one of the most important weekends in Fire Island, where it is right now packed with people.”

Bellone said, based on predicted conditions, ferries may not be able to run as Hermine pounds the area, meaning people who have stayed on Fire Island may have to shelter in place for two to three days.

“This is a dangerous storm and we’re treating it as such,” Bellone said.

Fire Island ferries were canceling a series of outbound and inbound trips scheduled after 9 p.m. Saturday night, the ferry service’s website said. Service is set to resume Sunday morning, but might be suspended by the afternoon, the website warned.

Justin Meyers, a Suffolk County police spokesman, said it is imperative for those with a medical condition, pregnant women and seniors to evacuate Fire Island, because once the ferries shut down, it “could take a significant amount of time to reach a doctor or a hospital.”

The state of emergency began in Suffolk at 3 p.m. The county’s emergency operations center will open at 5 a.m. Sunday, and 14 shelters countywide that will be operated in conjunction with the American Red Cross are on standby, according to county officials.

The New York State Office of Parks, Recreation & Historic Preservation is closing all ocean beaches on Long Island to swimmers beginning Sunday, according to Gov. Andrew M. Cuomo’s office. Suffolk County beaches will be closed to swimming beginning Sunday “for an indefinite period of time,” the county said.

Cpl. Andrew Becher of the Ocean Beach Village police on Saturday also was encouraging vacationers to head home to the mainland.

“It’s really not the kind of conditions people would normally want to vacation in,” he said. “Right now, they are forecasting the walkways to be flooded, wind, erosion down on the ocean front, and possible power outages,” he said.

Nassau County Executive Edward Mangano urged those residents in areas prone to flooding to be prepared to leave their homes.

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“We are concerned here in the county about that storm surge warning,” Mangano said at an afternoon news conference in Bethpage.

Reflecting on Tropical Storm Irene, which battered the area in 2011, Mangano said: “Irene presented significant flooding, downed trees and power outages ... We want you to be prepared, not scared.”

He said Nassau County was preparing a shelter at the Nassau Community College campus in case it was needed during the storm.

Cuomo’s office also said Nassau has requested the use of a sandbagger and one pallet of sandbags to help prevent flooding.

Though Hermine, a Category 1 hurricane when it bashed Florida, later powered down to a tropical storm with 70 mph winds, the National Weather Service on Saturday morning warned Long Island and New York City to expect “tropical storm conditions” within 36 hours.

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A storm surge of 2 to 4 feet is anticipated, which brings “the possibility of life-threatening inundation during the next 48 hours at many coastal locations between Sandy Hook, New Jersey, and Bridgeport, Connecticut,” the National Hurricane Center said.

Localized storm surges could begin Sunday morning, according to the National Weather Service, creating an “elevated” threat to life and property. Low-lying spots or in areas farther inland near where higher surge waters move also could be inundated, it said.

Aaron Donovan, spokesman for the Long Island Rail Road, said the agency was “carefully monitoring the storm and prepositioning equipment and personnel to be able to deal with any effects of downed trees or coastal flooding.”

Workers for PSEG-Long Island were trimming trees in advance of the storm, and shoring up low-lying substations to keep water out, according to spokesman Jeffrey Weir. More than 700 workers from Canada and across the United States were poised to come to the area in the case of widespread damage and power outages, he said.

The Port Authority of New York and New Jersey issued a statement saying it planned to open its emergency operations center in Jersey City Saturday night in advance of the storm. The agency has flood barriers, generators and pumps ready, and is prepared to accommodate airline passengers who may end up stranded at the airports.

It also urged travelers to check with their airlines before departing for the airports, to make sure the flights have not been cancelled due to the storm.

Cuomo’s office announced the New York State Emergency Operations Center would be activated at noon on Sunday, and supplies and equipment — including meals and high-axle vehicles — would be stockpiled in Brentwood and at Kennedy Airport.

New York City beaches will be closed to swimming, because of potentially deadline rip currents, beginning Sunday and potentially Monday and Tuesday, said Mayor Bill de Blasio’s spokeswoman Natalie Grybauskas.

“We are prepared for this storm,” she said.

The mayor’s office was cautioning of anticipated strong rip currents, coastal flooding, rain and high surf.

More than 40 city agencies, including police, fire, sanitation and emergency management, are mobilized and coordinating out of the city’s “situation room” in Brooklyn.

Like other storms, Hermine is expected to lose speed and hover offshore, south of New York, but will remain powerful enough to inflict damage.

“There seems to be a pretty good consensus” about the storm’s path, said Carlie Buccola, an NWS meteorologist based in Upton.

“It has moved off the North Carolina coast, and it will very slowly move just southeast of Long Island,” she said. “It’s going to stall off the Delaware-Maryland-Virginia coast — at least until midweek.”

But New York and Long Island could still get pummeled over the weekend, though the storm surge is only expected to be at about half of superstorm Sandy’s surge.

“Basically, you don’t have to be in the center of the storm to be feeling significant impacts from it,” Buccola said.

Mid-Atlantic state governors announced storm preparations, with New Jersey Gov. Chris Christie saying he was declaring a state of emergency for Ocean County, Atlantic County and Cape May County “as a result of severe weather conditions expected later today and throughout the Labor Day weekend.”

Hermine already has cost two people their lives, damaged homes and businesses and caused hundreds of thousands of people to lose power from Florida to Virginia.