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About two weeks ago, we were standing in line to get tables at restaurants and bars to enjoy 70-degree weather. And now we’re hunkering down for a forecasted 12 inches of snow thanks to ‘Winter Storm Stella‘ when it’s actually supposed to be warm.

What gives? This wasn’t even an El Niño winter. Why has weather gone from unseasonably warm in most of January and about all of February to bitter cold in March?

The answer, said National Weather Service Mt. Holly meteorologist Mitchell Gaines, has to do with a weak jetstream turned strong, and winds from the north and west finally overpowering those from the south and east. And, oh, El Niño’s sister pattern, La Niña, is part of it.

In January and February, a couple of factors led to the stronger push from the south. One reason was La Niña was essentially a no-show, so water temperatures in the Pacific didn’t drop as much as anticipated. At the same time, sea temperatures in the Arctic were warmer than normal.

That meant a pleasant January. On all but seven days, the high temperature exceeded the historical average high for each given day. People were wearing shorts during the Mummer’s Parade. The lone major snowstorm happened early in the month.

February was even better, producing record warmth for Philly, according to the NWS. Temperatures crept past 60 five times and past 70 another five times.

Credit: Accuweather

Sometime in mid-February, sea temperatures began returning to normal in the Pacific and the Arctic. With the temperatures no longer above average, the jetstream has picked up, too, and that’s why Philadelphia is freezing cold. The southern part of the United States is warmer than average right now and for the last few weeks we’d been getting most of their weather patterns. Not anymore thanks to that amplified jetstream.

So Philly’s March has been frigid. After above average warmth for a few days, we’re now experiencing highs about 20 degrees lower than average.

Was this inconsistency predicted? Not especially. Most forecasters predicted a fairly average, if slightly cold winter for Philadelphia. The Farmer’s Almanac, the unscientific predictors, were actually the closest, calling for a milder winter.

But Gaines said what we’re actually seeing this week shouldn’t be considered that weird.

“Having snow in the month of March is not all that unusual,” he said, “especially in the first half of the month.”

Credit: NWS Mt. Holly

The National Weather Service is calling 12 inches the most likely possibility for Philadelphia; expect at least seven inches, though. The service is saying there’s a potential for as much as 21 inches.

This particular snowstorm, Gaines said, is happening because a storm front is entering this battle between fronts from the north and west and south and east. With the system from the north and west winning thanks to that jetstream and now-average ocean temperatures, we’re going to be snowbound in March.

Mark Dent is a reporter/curator at BillyPenn. He previously worked for the Pittsburgh Post-Gazette, where he covered the Jerry Sandusky scandal, Penn State football and the Penn State administration. His...