Ice skating at the Rothman Rink outside City Hall in Dilworth Park

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Expecting a new scarf or mittens under the tree? You might need them in Philadelphia this Christmas, which is forecast to be one of the chilliest in recent memory.

A cold front at the end of the week will blast the Philly region with arctic air, according to the National Weather Service.

“It’s not expected to get above freezing, from when we drop below sometime during the day on Friday … through the weekend,” meteorologist Sarah Johnson, who works at the local NWS bureau in Mount Holly, told Billy Penn.

The official forecast for the city says the mercury won’t top 22 degrees Fahrenheit on Saturday or 25°F on Sunday. That combo is set to break a decades-long record.

“The last time we had both Christmas Eve and Christmas Day highs in the 20s was 1989,” Johnson said. “So this will be the coldest combination of Christmas Eve and Christmas Day in over 30 years.”

Holiday air and highway travel is expected to snarl as a bomb cyclone hits the U.S. Midwest and sends wind and rain down to Philadelphia — which could then cause icy roadways.

There’s no guarantee the prediction will hold, but weekly forecasts have been getting more and more accurate. That’s because of increased computing power, according to Johnson.

Every forecast is based on complex calculations by a simulation that runs a model of how the weather will change over time. There are so many variables in play that the full physics is too much for even today’s computers to calculate, but the number of assumptions being made has gone down drastically. A 2015 paper in the journal Nature even called the progress a “quiet revolution of numerical weather prediction.”

The other big factor is the accuracy of the initial data fed into the prediction algorithms, Johnson explained — and that has also increased. Weather balloons are a big source of that data, and the continental U.S. well-covered, with about 100 of them. But of course, “weather doesn’t stop at a country’s border,” she said.

Last year, NOAA touted a big upgrade for its “flagship U.S. global weather model,” and part of that was supplementing those readings with data from orbiting satellites and observations made from aircraft.

It has all contributed to us being able to better guess what to pack on vacation or when to bring an umbrella to the office.

Thirty years ago, Johnson noted, NWS was only doing official forecasts for 3 to 4 days out. Now info is published for 7 days into the future — so she’s pretty confident about the forthcoming frosty holiday.

To clarify, there has been a Christmas Day with below-freezing temps since 1989. In 2000, the high was 27°F. But Christmas Eve that day was much warmer, so this will be the first year in decades that both dates are that cold.

The chill will upend the overall trend of relatively normal seasonal temps for meteorological winter, which starts at the beginning of December and runs through the end of February.

The normal average high in Philly is 40°F, per Johnson, and this year it has averaged 40.5°F. That’s actually counter to the longterm prediction: “Going into the winter we were thinking it had better chances for being above normal.”

Unfortunately, the forecast doesn’t include snow, although there could be some wintry mix on Friday after relatively warm rain on Thursday. Johnson advised holiday travelers to be careful.

“We encourage everybody to be weather-aware and know there might be some icy conditions on the roads,” Johnson said, “even though there probably won’t be a white Christmas.”