Why do Philly’s western suburbs always get more snow?

Because the city is hot, crowded and low.

Danya Henninger / Billy Penn

Philadelphia is a hot potato, and the suburbs are pieces of broccoli.

This is the first principal takeaway in the crash course “Rocks for Jocks” lesson from The Weather Guy, a local blogger and storm aficionado whose site swells with thousands of daily hits when severe weather looms.

The dinner foods are useful analogies, he said, to show why snow is usually much more prominent in the ‘burbs than in the city proper. In the case of this week’s storm, for example, predictions called for close to 4 to 6 inches of snow in Center City, but double-digit accumulation around the outskirts.

Anyone familiar with local forecasts recognizes the pattern — but why does it happen?

Per The Weather Guy, aka Dr. Glenn F. Schreiber, a lot of it is because of something called the “urban heat island” effect.

“All of the people, the concrete, the buildings,” Schreiber explained, “all of that makes it slightly warmer than, say, the suburbs.”

On the allegorical dinner plate that is Philadelphia, he said, the broccoli pieces include places like Hatboro, Narberth, Lansdowne, and King of Prussia. Snow that falls there will have a greater chance of sticking, while flakes that land on Broad Street sidewalks — which are warmed by pedestrians from above and whirring infrastructure below — are much more likely to melt, so they don’t build up as quickly.

There’s even a difference between neighborhoods within the city, noted Schreiber, who is a root canal specialist by day and meteorology hobbyist by night. (Hey, every superhero needs a day job.) Compared to Center City, snow is more likely to build up in areas like West Philly or Chestnut Hill, which have more greenery and less foot traffic.

Yep, a couple of miles can make or break the epicness of your neighborhood’s snow day.

There’s another big factor contributing to regional snow total discrepancy: elevation.

Per Don Barber, geology professor at Bryn Mawr College, when air hits the areas northwest of Center City that are higher above sea level, it quickly cools and is forced to release moisture.

“As one goes higher in the atmosphere,” Barber explained, “temperatures are cooler. Just as warm air can hold MORE moisture, cool air holds LESS moisture.” As wind comes in from the northeast, it rises as it crosses City Line Avenue into the relatively hilly ‘burbs, then cools and drops water in the form of snow.

Philadelphia proper’s lack of snowy precipitation and temperate weather probably contributed to it being chosen as a center for British colonizers, and later as a temporary capital of the U.S., Schreiber suggested.

However, the climate is not the same as it was back in the Revolutionary Era. Warming has led to more available moisture, which has in turn led to heavier and more frequent storms here — of both the rainfall and snowfall variety.

And also more gnarly, semi-exhilarating thundersnow.

“Because heavy precipitation is often associated with a convective uplift in the atmosphere,” Barber said, “more intense rain — and sometimes thundersnow — reflects the greater amount of heat energy present in the atmosphere due to global warming.”

So potentially, whether you reside on a broccoli or a hot potato, bombastic blizzards with fanfares of lightning are likely to become more commonplace in the nor’easters of the future.

Maybe it’s time to upgrade those makeshift sleds to cross-country skis, Philly.