The Philadelphia region should expect warmer temperatures and stormier conditions with around 16 to 20 inches of snow this winter. Those are the main takeaways from the NBC10 First Alert Weather team’s winter weather forecast.
“El Niño leads to a winter with more storminess and more precipitation,” NBC10 First Alert Weather meteorologist Justin Godynick said. “Also, you’ve got to expect more warmth. Temperatures certainly will run above average throughout this winter.”
The First Alert Weather team also predicts about 18 inches of snow for Philadelphia during the winter.
“It’s a little bit below normal but in an El Niño winter, most of that snow could fall in one storm,” Justin said.
Here are all the factors contributing to the winter forecast:
Higher pressure can redirect storms and slow their trip along the eastern seaboard. That’s one phase of the North Atlantic Oscillation (NAO), which is an atmospheric climate pattern defined by the difference in sea-level air pressure between a subpolar low-pressure system and a subtropical high-pressure system. The NAO controls the intensity and direction of westerly winds as well as the paths of storms in the North Atlantic.
The NAO block occurs when a persistent high-pressure ridge develops over Greenland. If the block is in the right position, cold air can linger while a coastal storm takes longer to move away, meaning more hours of precipitation and onshore wind. If the block shifts, however, a storm misses the Philadelphia region to the south.
“El Niño does lead to more blocking however, it doesn’t mean that the block is going to be in the right position for us to get a winter storm every single time,” Justin said.
In the Philadelphia area, there’s a 48% chance for a wetter than normal winter this season. That means more chances for coastal storms, such as nor’easters.
El Niño – a climate pattern caused by variation in winds and sea surface temperatures –tends to strengthen the Pacific jet stream and extend it across the southern United States, leading to a more active route for storms. Whenever a system turns up the east coast, it can pull Atlantic moisture inland which sets up a winter pattern with more precipitation. This means more opportunities for rain or snow when cold air is available. More moisture doesn’t automatically mean more snow, however, as it’s dependent on a storm tracking inland, along the coast or far offshore.
Cold air increases the chances of more snow. An El Niño winter can tilt the odds towards warmer temperatures. The Climate Prediction Center shows a 40% to 50% chance of above average temperatures this winter across the Philadelphia region for December, January and February.
Even though the pattern shifts the polar jet stream a bit toward the north during strong El Niño years, the jet stream is constantly rippling, allowing arctic air to dip to the south, right into the Philadelphia region. This means we could have more mild stretches of weather interrupted by cold snaps rather than constant warmth during the winter.
If arctic air, moisture, and the right storm track all come together, we’ll have more snow.
Sand dunes act as a protective buffer during storms. During a nor’easter, onshore winds raise coastal water levels, letting waves reach higher and cut into dunes. Wherever sand has been lost and not recovered, a lower, narrower barrier gives the next storm less resistance. Waves can then wash over it more easily.
In the Philadelphia region, flooding has a different pathway. Persistent winds can pile water into the Delaware Bay and raise the tidal river. A combination of a high tide and low-lying waterfront lead to coastal flooding. Rain also adds runoff and higher levels can slow drainage.
A combination of moisture, cold air and the right storm track are what lead to snow. While El Niño can help supply moisture, it doesn’t guarantee the cold and instead often favors milder conditions. If a storm moves inland, winds can pull warmer ocean air across Philadelphia, changing snow to rain. Farther offshore, colder air may stay in place, but if the storms track too far out, the heaviest precipitation will miss the Philadelphia region while areas north and west can hold onto the cold air longer than the city and the shore.
Even in a mild winter, one storm with the right setup can make a big difference in seasonal snowfall.
El Niño can influence the polar vortex throughout the winter. At first this favors a continued mild trend across the Philadelphia region. However, as winter drags on to late January and beyond, the chance for arctic air increases. In an El Niño winter, the polar vortex can collapse, bringing down brutal shots of arctic air, despite an overall forecast for a mild winter.
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