After August-like heat, a first frost is possible this weekend in the Philly region
Winds could gust to 40 mph Friday. Then big changes are coming.
It wouldn’t be a bad idea to maintain a certain wardrobe flexibility for the next several days.
After a retro run of August-like heat on Wednesday, it is likely that frost will be covering some of the pumpkins and other surfaces in the region during the weekend away from the Philly urban-heat island.
Warm winds from the south were forecast to boost readings well into the 80s by day’s end Wednesday. That would be normal for August; however they were unlikely to challenge the record for an Oct. 14 in Philly, which is 88 degrees.
Clouds and light showers have “put a damper” on that potential, said Alex Staarmann, meteorologist in the National Weather Service’s Mount Holly office.
Then the atmosphere is expected to demonstrate its mercurial side, so typical of seasonal transition periods when the colder air masses form the north encounter the remnants of summer.
The forecast for the Philly region through the weekend
With a frontal passage, the sun is forecast to rule Thursday with highs in the upper 60s, just about normal for mid-October.
In advance of another of another front, on Friday temperatures may approach 80 degrees, with showers and winds that could gust to 40 mph as strong winds aloft could mix to the surface, the weather service is warning.
The cool-down will be significant. On Saturday, temperatures may get no higher than the mid-50s, and come early Sunday morning they may fall into the upper 30s in the city and mid-30s in the surrounding counties.
That is “likely” to generate the first frost of the season in some areas, said Staarmann — even if temperatures remain several degrees above freezing.
The difference between a ‘frost’ and a ‘freeze’
The ideal recipe for for frost would be clear skies and light winds that allow the sun’s heat to escape efficiently into space, and a dash of moisture in the air.
Cold air sinks, and surfaces exposed to the sky “are radiating heat away from the earth so that they are much cooler than the air 5 or 6 feet above it” — where the air temperatures are measured — said Staarmann.
The surfaces “can be 30 degrees while the air temperature is 35 or 36,” he added.
It is even possible to get a “light frost” with temperatures at 37 or even 38 degrees, he said.
For a freeze, the air temperatures, not just the stuff below the thermometer, has to reach 32 degrees.
On average, that doesn’t happen at Philadelphia International Airport until early November.
