If you thought the weekend’s highs in the mid-30s were cold, better rethink the meaning of cold.

The end of the workweek on Long Island is forecast to see temperatures head down to the 20s — that’s for the highs — with overnight lows in the low to midteens, according to the National Weather Service’s Upton office.

The surge of arctic air is forecast to bring Wednesday’s overnight lows down to around 20 degrees, the weather service says.

Thursday sees highs in the low 20s, but conditions overnight into early Friday morning hit the low teens, the weather service said.

Adding to that, breezy conditions mean the wind chill early morning Friday will make it feel more like below zero, said Carlie Buccola, weather service meteorologist. That’s according to the weather service’s late Monday afternoon forecast, which is subject to some fine-tuning.

There’s also a 30 percent chance of flurries or snow showers Thursday night, she said.

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“As we have to readjust to winter temperatures,” the season’s first really cold blast can be brutal, said Jessica Spaccio, climatologist with the Northeast Regional Climate Center, based at Cornell University. That’s especially “coming off a warm autumn,” which has been the case this year.

What’s more, on average for the month of December, Long Island MacArthur Airport sees two days with highs below 30 degrees, she said.

Still, the cold doesn’t last through the weekend, the weather service said, as a turnaround brings daily highs up again to around the low 40s for Saturday — with Sunday possibly closing in on 50 degrees.