Frost is possible outside Philly on Thursday and Friday, even though temperatures should stay above freezing
Temperatures won't go below freezing, but frost could form outside the city.
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With the forecast calling for the coolest overnights in the region in six months, the National Weather Service says scattered frost is possible both Thursday and Friday mornings away from the Philly heat island.
It likely will be close, said Ray Martin, a lead meteorologist at the National Weather Service in Mount Holly. In addition to the cooler temperatures, frost would require clear skies and nearly calm winds.
A frost advisory is up for counties just to the north and west of the region, and the weather service says areas of frost are possible Thursday morning outside the city in Bucks, Burlington, Chester, and Montgomery Counties and in the Pine Barrens.
Another round is in play for Friday morning in the outer reaches of those counties and the Pine Barrens.
Temperatures could drop below 40 degrees in Philly for the first time since April on Thursday morning, and as low as 34 in northern Montgomery County.
If temperatures stay above freezing in the Philly region, what explains the frost?
For frost to form, the temperatures don’t have to drop to freezing.
Frost formation is inextricably tied to microclimate and terrain features. Although no hard temperature criteria exist, generally frost can develop with a temperature around 36 degrees, said Martin’s colleague Lee Robertson.
Two important conditions are clear skies and little to no winds. Clear skies allow daytime heating to radiate efficiently into the atmosphere.
When winds don’t stir the atmosphere, thin layers of cool air atop metal surfaces and areas near the ground can make them several degrees cooler than the “ambient” air temperatures, measured several feet above the ground.
One factor that could work against the frost this week, said Martin, is the overall lack of moisture in the atmosphere and the soil.
In terms of rain, so far the region is 0 for October. For the last 30 days, according to the weather service’s Middle Atlantic Forecast Center, Philadelphia has had 8% of its normal rainfall, and Camden County, 7%.
And Martin said no rain is on the horizon.
What does the cool spell portend for the Philadelphia winter?
In all probability, nothing.
Long-range forecasters do look at trends around the globe in October, but not so much what happens in Philly.
Siberian snow cover in October may have effects on the North American winter, according to research by Judah Cohen, polar scientist with Atmospheric and Environmental Research in Massachusetts. But so far, he says, the snow hasn’t been anything special and hasn’t sent any clear signals.
The seasonal forecasters are taking long looks at the cooling of sea-surface temperatures in the tropical Pacific, which they expect to become a long-duration La Niña event that would have impacts across the United States throughout the winter.
The government’s Climate Prediction Center sees a 60% chance of La Niña developing, but it’s not a done deal, and precisely how it would affect Philly and the rest of the Northeast is unclear.
In any event, La Niña will be a factor in the winter outlooks, such as AccuWeather Inc.’s, which was released Monday. It called for another mild winter here, with below-normal snowfall.
The National Oceanic and Atmospheric Administration will announce its outlook Thursday.