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Snow? Again? Yep, and in time for the evening commute

Yet again, snow blanketed the region: 2.2 inches fell on Friday, making it the third time in a week we had accumulating snow. That qualifies as a meteorological rarity.

A pedestrian braves the snow with an umbrella near 23rd and Spring Garden streets as snow falls on Friday, Dec. 15, 2017. TIM TAI / Staff Photographer
A pedestrian braves the snow with an umbrella near 23rd and Spring Garden streets as snow falls on Friday, Dec. 15, 2017. TIM TAI / Staff PhotographerRead moreTimothy Tai

One remarkable aspect of this winter season is that as far as the solar system is concerned, it's not even winter yet.

That technicality aside, for the third time in a week accumulating snow fell on the region in what qualifies as a meteorological rarity.

From two to three inches of snow was reported Friday in the immediate Philadelphia area, but while the amounts were prosaic, the snow was particularly disruptive because it arrived in time to put the entire homeward bound commute on ice.

Snow began falling about 1 p.m., and with temperatures in the mid-20s, whatever fell quickly bonded to surfaces. "Numerous" accidents were reported on major roads, a PennDot official said, and by 3 p.m., westbound traffic on the Schuylkill Expressway was at a standstill. Even the streets of Center City were snow- and ice-covered.

The problems downtown were further compounded when a power outage knocked out traffic lights at Logan Square and the busy confluence of Arch Street and John F. Kennedy Boulevard. SEPTA reported delays of up to 20 minutes on most Regional Rail lines, 35 minutes on the Wilmington line.

Actually, the totals for the snowfall last Saturday, the first of the season and the first in the sequence, were considerably higher. Officially, 4.1 inches was measured at Philadelphia International Airport then. An additional 1.4 inches fell on Wednesday, and Friday's 2.2 inches at the airport made it three official snows in six days. That's something that has happened only two other times by mid-December in records dating to 1884. The others were in 1917 and 2005.

Historically, White Christmas mythology aside, December is not a particularly snowy month around here, for a variety of reasons.

For one thing, the aforementioned solstice comes late in the month, and with the lag in solar effects on the Earth's surface, the coldest period usually doesn't come for three or four weeks, just as the peak summer heat doesn't hit until July.

Another big factor is the Atlantic Ocean, which cools way more slowly than land masses. Our snow often comes from coastal storms that generate onshore winds that bring in warmer air from the ocean, where water temperatures are still in the mid-40s.

"All these systems are kind of fast-moving," said Al Cope, the senior science officer at the National Weather Service office in Mount Holly.

Thus they have been unable to lure much in the way of warmth from the ocean, and on the other hand, they haven't had time to pick up much moisture.

So why the storm frequency?

"One of the big reasons," said Dan Kottlowski, a meteorologist with AccuWeather Inc.,  is that "we got hit by a big surge of Arctic air.

"Not only have we been hit with it, it's been persistent," he said.  Upper-level winds — 80 to 90 mph at 18,000 feet — have been whisking so-called Alberta clipper storms across the country, he added. They haven't been particularly juicy but still have managed to wring out nuisance snows.

The big storms that tend to brew in the Gulf of Mexico haven't had a chance to come north because it's been so cold in the South, he added.

Friday's storm was a bit different from the others because it redeveloped off the Delaware coast, he said, and did have access to some local Atlantic moisture.

After this round, things should settle down, although some mixed precipitation is possible in the early hours of Monday. On Tuesday, however, temperatures are expected to reach 50.

And in case you're wondering, the astronomical winter arrives officially on Thursday.