The season’s first freeze is possible in Philly on Halloween morning

In what is going to the coldest morning since the clocks moved forward an hour for daylight saving time on March 8, Philadelphia could have its first official freezing temperature of the year on Saturday.

And on what will be the last day of DST, Halloween will feel more like the first week of December with temperatures in the mid-40s around trick-or-treating time (such as it will be in this challenging year).

Wind chills at lunchtime Friday were in the mid-30s, and snowflake sightings were reported in upstate Pennsylvania, northern New Jersey, and as nearby as Northampton County.

In the early morning Saturday, readings could drop into the 20s in areas outside the city as a cold air mass spills across the region, and they have a very good shot at making it to 32 at Philadelphia International Airport, which would be a rarity these days.

The official temperature has reached freezing at the airport in October only once since 1992, and that was in 2013, on the day of a freak snow-and-ice storm.

“It will be close,” said Jonathan O’Brien, a National Weather Service meteorologist in the Mount Holly office. It’s never easy at the airport: In addition to the warming effects of the nearby buildings and runways, which balk at giving up daytime heat, the official thermometer isn’t all that far from the Delaware River and a swamp.

» READ MORE: Where is temperature measured in Philly, and why?

But if the skies are clear and winds die off in the early morning hours — the ideal conditions for allowing daytime heat to radiate into space toward that full, blue Halloween moon — the temperature could drop to 32 or even lower at the airport, O’Brien said.

If it does happen, it would be well ahead of schedule. On average in the 21st century, that first 32 reading at PHL has occurred on Nov. 14.

By contrast, in the 1960s, the airport was reading 32 by Oct. 25 on average. The freeze of Oct. 5, 1961, remains the earliest ever.

» READ MORE: Explaining this year's rare Halloween 'blue moon'

The later date likely is related to worldwide warming, with which Philadelphia’s temperatures have tracked well. However, the date has been wildly variable through the years. In the 1930s, the average first-freeze date was Nov. 18.

The weather itself is going to be jumpy in the next several days. Temperatures will make a run at 60 on Sunday, with showers possible, then dry and no higher than the 40s on Monday and back into the 50s Tuesday.

» READ MORE: Election Day weather: Near 100% chance it won’t affect voter turnout

In fact, Tuesday is shaping up to be quite a peaceful day “across the whole country;” at least from the atmosphere’s perspective.

Nationally, O’Brien said, the forecast is calling for “an anomalous quiet Election Day."