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A first frost is possible early Sunday in the Philly region, the weather service says

Frost can form at temperatures above 32. If it happens, it would be well earlier than average.

A pond freezing over on an early morning in the Pine Barrens. Temperatures won't reach freezing, but frost could form in some areas Sunday morning.
A pond freezing over on an early morning in the Pine Barrens. Temperatures won't reach freezing, but frost could form in some areas Sunday morning.Read moreTOM GRALISH / Staff Photographer

Although temperatures are forecast to stay safely above freezing, the National Weather Service has posted an advisory for a first frost in outer Bucks, Chester, and Montgomery Counties, with “patchy frost” possible elsewhere.

The exceptions would be Philadelphia and Delaware County, thanks to the protective custody of urbanized heating, said Patrick O’Hara, meteorologist with the weather service in Mount Holly.

Temperatures in the advisory area could drop as low as 33, and frost can form with temperatures as high as 35 or 36, said O’Hara. Air temperatures are taken several feet above the surface ground, but surfaces closer to the ground can be colder.

“Frost could kill sensitive outdoor vegetation if left uncovered,” the weather service said.

» READ MORE: Snow may be scarce this year, say the early outlooks

Overnight cooling conditions should be ideal with clear and brilliantly moonlit skies and light winds that will allow the daytime warmth to radiate into space.

Also, those conditions in all likelihoods will create quite a range of temperatures throughout the region.

Readings shouldn’t drop below the lower 40s in Philly, he said, with its sun-absorbing buildings and paved surfaces that retard overnight cooling.

By contrast, certain far less urbanized areas of South Jersey likely will be considerably cooler, he said, notably Millville, where overnight readings often are several degrees below Philadelphia’s.

» READ MORE: Millville is one cool town

If a first frost does make an appearance, it would be on the early side. On average, frost first appears on Oct. 14 in Coatesville and on Oct. 13 in Millville, says Alex Staarmann, meteorologist at the National Weather Service in Mount Holly. In Philly, it doesn’t occur until Halloween on average.

It so happens that the earliest measurable snowfall in Philadelphia in the period of record occurred on Oct. 10, 1979, all of 2.1 inches. That record won’t be threatened this year.

Temperatures will rebound Sunday, with highs the next three days in the 60s, with cool and clear nights and 100% chance of moon shadows.

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