Philadelphia has had a freeze as early as Oct. 5 (1961) and accumulating snow as soon as Oct. 10 (1979). In October 2021, however, the region is about to get a taste of mid-June. Forgive the leaves if they are showing a certain resistance to change.
Despite a persistent sun-obscuring gloominess, temperatures so far this month have averaged about 6 degrees above long-term normals, and that figure is going to grow the next two days, with temperatures perhaps in the mid-80s on Friday. But it’s the nighttime warmth that has been extraordinary.
“The [overnight] low temperatures are really driving the departures,” said Brian Haines, the science and operations officer at the National Weather Service in Mount Holly. Daytime highs the first 12 days of the month averaged 3.4 degrees above normals and the lows 8.2 degrees higher.
While nighttime warming has been evident as global temperatures have risen, what has happened this month at the official measuring station at Philadelphia International Airport qualifies as over the top. With an average temperature of 61.3 degrees, the first 12 nights of October were the seventh-warmest for any Oct. 1 to 12 period in records dating to the 1870s, the weather service said.
Blame, or credit, goes to the clouds, which have been trapping daytime heating. “It’s been so cloudy the radiation doesn’t get back into space,” said Paul Walker, senior meteorologist with AccuWeather Inc. Philadelphia hasn’t recorded a clear day since Oct. 1.
Haines said the cloudiness has been the result of an unusual pattern.
Ordinarily, the cloud-suppressing high pressure dominating the East would promote sunny days, and almost predictably, the weather in the eastern half of the country is the mirror opposite of what’s been going on out West. “It’s the old seesaw,” said Walker. Heavy snows in the Rockies have been followed by hard-freeze warnings.
The backlash was evident in Los Angeles during the playoff games between the Dodgers and San Francisco Giants, with players and spectators wearing sweaters and jackets against the biting winds generated by the strong storm to the east.
Philadelphia is sun-deprived because of the leftovers of the weak coastal storm that affected the region during the weekend, said Haines. While high pressure is dominating the mid- and upper-levels of the atmosphere, he said, low-level moisture from the “disintegrating” storm is being shunted westward.
Clouds will be hanging around Thursday and Friday, but he said the sun is going to make a comeback. Highs Thursday are forecast to approach 80 and reach the mid-80s Friday, although the record for the date, 87, set during a 1975 warm spell, is likely out of reach.
A cold front will follow, along with quite a decent October weekend. “I would say ‘gorgeous,’” said Haines, “but it really will be where it should be this time of year.”
Walker said the cool spell isn’t likely to last long, and warmth might be the rule for the rest of the month.
The ideal conditions for fall color are sunny days and cool nights, says Marc Abrams, a Penn State professor of forest ecology and physiology, who has been watching foliage seasons for 40 years.
For the foliage show, the outlook couldn’t be much worse.