The National Weather Service has issued a “blizzard watch” for Long Island and the metropolitan area, beginning Monday night to Tuesday evening.

Long Islanders should brace for 12 to 18 inches of snow that could start arriving late Monday, the Upton-based service said.

Tuesday’s nor’easter could reduce visibility to less than a quarter of a mile at times, the NWS said on its website.

“This could lead to whiteout conditions and make travel very dangerous,” the NWS said.

Northeast winds of 20 mph to 30 mph can be expected, with gusts of 40 mph to 50 mph, the service added.

“It looks like this could have the most snow we’ve had all season,” said Carlie Buccola, an NWS meteorologist in Upton.

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The closest contender was the 14.3 inches of snow that fell in Islip on Feb. 9, she said. That location recorded 4 inches of snow Friday.

The blizzard forecast comes on the heels of Saturday’s frigid cold and windy start to the weekend, courtesy of a high pressure system from Canada that dropped temperatures 20 degrees below normal, the NWS said.

Saturday afternoon’s temperature rose to the upper 20s, a far cry from the normal high of 46 degrees at Long Island MacArthur Airport, according to Faye Barthold, an NWS meteorologist in Upton.

Saturday night, temperatures will fall into the mid-teens again, according to Barthold, adding that the normal low at the Ronkonkoma airport for this time of year is 30 degrees.

Overnight into Sunday, the skies will be partly cloudy to clear, said Matt Hammer, a News 12 meteorologist.

The record low overnight Saturday is 14 set in 1984. “I wouldn’t be shocked if we tied or broke that low tonight,” Hammer said Saturday.

Daylight saving time will begin at 2 a.m., when we will “spring forward” to 3 a.m., losing an hour of sleep but gaining an hour of sunlight. Sunrises will now be around 7 a.m. and sunsets at 7 p.m. starting Sunday, Hammer said.

Sunday will still be cold, but it will be a dry day and winds are expected to die down, Barthold said. Temperatures will peak in the upper 20s but “probably not quite getting to 30,” she said.

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This weather system will last “through the weekend and into early next week, at least,” she said.

Hammer said the chances of Tuesday’s storm heading out to sea are slim, and other factors could mean “major snow accumulation” if the storm leans more toward wintry weather than wet.

“This could easily be a mainly snow event, but we’re fine tuning the details throughout the weekend,” Hammer said.

High winds and coastal flooding “could be a pretty big deal Tuesday if everything sets up correctly,” Hammer said.

Tuesday’s snowfall might stick around for much of the week, as the thermometer is unlikely to rise above the 30s, Buccola said.

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“It looks like it’ll be a pretty chilly week,” Buccola said. The weekend could change that. She added: “We’ll get into the low 40s by next Saturday.”