Meteorologists, trends — and history — argue for a sixth straight cold shoulder from winter in Philly
Warm winters in Philly are nothing new; they’re just happening more frequently.
For five consecutive winters, temperatures have averaged well above normal in Philadelphia, and last winter, nature treated snow as if it were a controlled substance, dispensing a mere 0.3 inches.
So will the payback be a word unsuitable for a family audience? Don’t count on it.
Aside from the unmistakable global warming trend, mild winters aren’t new. (How do you think ski resorts got so good at making snow?)
The variability of winters in the East was evident to colonists in the 17th century, and for reasons that researchers still are trying to untangle, colder and warmer winters historically have occurred in clusters.
What’s been different in the last three decades is that those mild winters are occurring more often around here, even as snow totals have reached historical highs — last winter being quite an emphatic exception.
What’s not different is that any cold-air mass that tries to settle over the region confronts an obstacle that well predates the Industrial Revolution, says Valerie Meola, a meteorologist at the National Weather Service Office in Mount Holly. That would be the Atlantic Ocean.
“We take for granted how big a role the ocean plays,” she said.
Sea surface temperatures off the Mid-Atlantic coast on average are in the 40s in December and stay well above freezing throughout the winter. “It’s really hard the first half of the winter to get any cold weather to stick around,” said Tom Kines, a meteorologist with AccuWeather Inc.
When areas of high pressure, which favor fair weather, are centered off the coast, they generously deliver warming winds off the ocean.
Winter storms, those bringers of snow, can also be bringers of warmth. Their onshore winds can snuff out cold spells by dragging ocean air landward. This happened famously in February 1961 when a powerful storm ended a record 15-day stretch in which the temperature failed to get above freezing.
If they hug the coast, snow can turn to rain along the I-95 corridor; if they track farther out to sea, snow can turn to partly cloudy.
“The storm tracks for the big snows are fairly narrow,” said Mike Halpert, acting director of the Climate Prediction Center.
What the numbers show
The Atlantic effect notwithstanding, in the last 30 years the lack of sustained cold has been strikingly apparent.
Defining a “cold spell” as four consecutive days in which the temperature fails to get above freezing, an Inquirer analysis found 12 winters since 1990 in which that standard wasn’t met even once.
That was about double the rate of the previous 128 winters in the period of record. On Jan. 1, 1998, the temperature didn’t get past freezing, and that was the only such day that entire winter.
Temperatures for the December-through-February meteorological winter have averaged above the long-term mean of 34.6 for five consecutive years. Overall, for winters it was an incredibly warm decade — but it wasn’t the warmest.
In a close race, that distinction belongs to the period that ended with 1932-33, in which not a single cold spell occurred in seven consecutive winters and all 10 had average temperatures above that mean.
That streak ended dramatically in February 1934 in a season that featured 33 days in which the high didn’t make it past freezing and an all-time low of 11 below zero, still a record in Philly.
Riding shotgun with the greenhouse signal is a maddening planetary variability.
Weather patterns tend to dominate in decadal and multidecadal cycles, here and all over the world, and in all seasons, said Halpert, all living happily on different time scales.
For example, hurricane active and lull periods have alternated in 25- to 40-year cycles, evidently tied to slow changes in the Atlantic.
Recent winters generally have been warm in the United States, and that appears to be related to a pressure pattern known as the Arctic Oscillation, which has been locked in a “positive,” or warm, phase.
The once-obscure El Niño/Southern Oscillation, or ENSO, now is an international celebrity. This year it is in its La Niña, or cool, phase, and that’s one reason the winter outlooks have such a warmish look. That’s not to be confused with the Pacific Decadal Oscillation.
Solving the riddles of the onsets and durations of the various cycles and how they might actually affect local weather is going to take a while: Life might even return to normal by then.
Halpert said one huge obstacle is the paucity of data, given that some of these cycles take decades to unfold.
The only given, he said, is that a warming planet runs through it all, and that increases the odds of more mild winters in the future.
“The climate-change signal is not going way,” he said.