Philly winters aren't what you think

The weather varies a lot every winter and from year to year. We looked at 130 years of data to show you.

• Aseem Shukla, Anthony R. Wood
• Published 

  Dec 7, 2021

If the word winter evokes icicles, snowy landscapes, and rock-hard waterways suitable for ice-skating, it may be time to check your assumptions.

Snow does happen in Philadelphia, but so does cold rain and prolonged warm spells. In fact, of all the seasons, winter has the most-dramatic temperature swings.

Amid all the noise, underlying warming is evident: Climate is always changing, and these days the temperatures have been trending in one direction. But within any given winter, temperatures still vary wildly.

The Inquirer analyzed 130 years of official records for the December-through-February meteorological winters, along with the region’s very elastic snow seasons.

What emerges is a nonlinear, often messy and chaotic profile of a local climate changing in sync with the planet’s.

In day-to-day weather, especially in winter, “normal” is hardly the norm.

Temperature: 90 degrees of separation

Philadelphia’s winter temperatures have gone as high as 79 degrees (Feb. 25, 1930) and as low as 11 below zero (Feb. 9, 1934).

Philadelphia and other areas in the middle latitudes are in the battleground zones of restless tropical and Arctic air masses. In winter, said polar scientist Judah Cohen, of the climate analytics company Atmospheric and Environmental Research, the difference is greatest between the solar radiation reaching the tropics vs. the poles. That makes winter’s temperature contrasts the sharpest of the seasons.

The result of this variability means there’s far from a “typical” winter day. In the meteorological community, 30 years is typically used to track climate trends. Between 1991 and 2020, on average:

❆ Fully a third of winter days didn’t drop below freezing at all — even at night.

❆ Days with highs below 38 and above 49 occurred equally as often.

❆ Only 13 days a year stayed entirely below freezing. By contrast, 13 days exceeded 56 degrees in winter.

❆ Only 13 days per year dropped into the teens.

• Here’s a dot representing a 41-degree day, the average high temperature in January in Philadelphia over the last 30 years.

• And here’s a dot showing an 88-degree day, the average high in July.

• But averages conceal a lot. These dots represent the high temperature for every January day. Although the most frequent highs were in the 40s, the lowest was 6 degrees and the highest was 73, a 67-degree difference.

• By comparison, July high temperatures in that 30-year period varied only from 65 to 103, a 38-degree difference.

• The winter months are the most volatile, ranging from bitterly cold to pretty warm, while the summer months stay mostly warm or hot.

And while some recent winters have been quite cold and snowy, others dominate the ranks of all-time warmest. No winters in the 21st century have rivaled the 1960s and ’70s, the coldest period on record.

• From 1891-1920, high temperatures in winter fluctuated widely around an average of about 40.5 degrees.

• The range of high temperatures a hundred years later, from 1991-2020, looks similar at first. But the average increased nearly 3.5 degrees, to just under 44.

• So while highs in the 30s and 40s happened about as frequently during the two periods, look at the two extremes to see the shift.

• Days in which the temperature fails to get above freezing are occurring less frequently. From 1891-1920, about one in five winter days stayed entirely below 32 degrees. One hundred years later, that number shrank to one in eight.

• And warm winter days have become more common. Days with temperatures reaching above 50 degrees went from making up one in five of all winter days in the older period to one in three in the last 30 years.

The snow lottery

Frances Moore, a professor of environmental science and policy at the University of California, Davis, recently studied how people’s perceptions of weather are shaped.

“People seem to be basing [their] expectation of normal weather on the last two to eight year average,” she said.

And in Philadelphia, snowfall is so volatile that “normal” just isn't all that helpful. For the 1991-to-2020 period:

❆ On average, the city received about 23 inches of snow a season.

❆ Totals varied from 0.3 inches in the winter of 2019-2020 to a whopping 78 inches — six-and-a-half feet — in the winter of 2009-2010.

❆ Measurable snow fell on average just 12 days a year — but that number’s been as low as 2 days and as high as 22.

❆ Snow accounts for just under a third of winter days with precipitation. The rest of the time, it’s rain, sleet, or “wintry mix.”

Just what effects a warming world has on snowfall remains a source of intense analysis.

Cohen and researchers such as the National Oceanic and Atmospheric Administration’s Amy Butler have theorized that rapid Arctic warming might be contributing to outbreaks of heavy snow and severe cold at the midlatitudes. Even as winters warm, changes in the Arctic might disrupt the upper-air flow in such a way as to allow unusually powerful pulses of polar air to spill southward. That happened last February when near-record chill iced Texas and up to 4 feet of snow accumulated over multiple storms in parts of the Philly region.

“It’s going to take a long time for a statistically significant signal to emerge from a very noisy snow signal,” said David Robinson, the New Jersey state climatologist who runs the Rutgers University Snow Lab.

Whatever the cause, the five biggest snowfalls in Philadelphia have occurred in the last 25 years, starting with the 30.7 inches of Jan. 7-8, 1996.

But the 21st century also has seen some of the most snow-poor years on record.

As for a white Christmas, the government’s climate data say the probability is just 8% in Philly in any given year. We’re too close to the Atlantic, which takes its good old time cooling off and is a source of warm air.

There are also wide fluctuations in when snow actually falls in Philly.

Measurable snow has on average first appeared in the second half of December and made its last appearance in March. But the Philadelphia snow season has begun as early as Oct. 10 and ended as late as April 27.

And an early start doesn’t mean an early finish or vice versa: In some years, as many as five months elapse between the first and last snow, and in others it’s a matter of weeks or even just days.

The winter of 2021-2022

The atmosphere has been stingy with signals, but meteorologists’ consensus forecast favors near “normal” snowfall and temperatures embedded in another wildly variable winter.

Of course, long-range forecasting is not for the fearful. CBS3’s Kate Bilo has described predicting snow as “a fool’s errand.” NBC10’s Glenn Schwartz has stopped issuing his winter outlooks because of climate change.

The Climate Prediction Center’s outlook says chances are good that most of the country will be mild this winter. Given the trend, going warmer in a winter forecast should be a safe bet. But, “some years you’re going to be woefully wrong,” said Mike Halpert, the center’s longtime winter forecaster.