Store the mowers, take in the plants. The lowest temperatures since March are about to chill the growing season.

After stinging Arctic winds and temperatures in the 30s generate wind-chills in the 20s, the official overnight low in Philadelphia could threaten the daily record of 21 degrees, set on Nov. 11, 1961.

The National Weather Service forecast low, 20, not only would be a daily record, it would match the lowest official temperature observed in Philadelphia in the first two weeks of any November.

"Crops and other sensitive outdoor plants will likely be killed," the National Weather Service said in issuing a freeze warning that goes into effect Friday night.

"The source region of this air mass is up near the pole," said Dave Dombek, a senior meteorologist at AccuWeather. "This would be no big deal if this were a month from now." But this comes after a near-record run of warmth

Saturday's predicted high, 42, would yield an average temperature of 31; that's about 20 degrees below normal. The record low average temperature for that date, 33.5, happened in 1987 during a surprise snowfall that buried the Washington area and dusted Philadelphia.

This would be the first official freeze of the season at Philadelphia International Airport, and the first region-wide one.

It should drop below 32 late Friday, and based on trends, that first 32-degree reading would be arriving close to schedule.

In the 21st century, on average, the first official freeze of the season has been recorded on Nov. 14.

The long-term average is Nov. 8, but the date has shown substantial decadal variation. In the 1960s it was arriving by Oct. 25; in the 1930s and in the first decade of the 20th century, Nov. 16.

Temperatures will moderate to near-seasonal levels early next week, but another cold shot is possible the following weekend.

It was just a week ago that the temperature hit 75. Those days are over for a while.

Said Dombek: "Maybe next spring."