Temperatures dropped below freezing early Sunday in parts of the region for the second time in three days, and widespread frost threatened to ice some plantings .

The National Weather Service has a freeze warning posted for outer Bucks and Montgomery Counties, and a frost advisory up for everywhere but Philadelphia and Delaware Counties.

By 1 a.m. readings were in the upper 20s in Pottstown and Millville, in below freezing in Mount Holly and Blue Bell, Montgomery County.

Light-blue areas are under a frost advisory; dark blue denotes a freeze warning.
National Weather Service
Light-blue areas are under a frost advisory; dark blue denotes a freeze warning.

Philadelphia once again was in somewhat of a bagel hole under the protection of the urban heat-island effect — dramatically evident early Friday — although it falling well into the 30s there also. And parts of Delaware County also likely be frost-vulnerable.

Whatever happened to April?

“I think everyone had high hopes of a warm spring with how warm the winter was,” said John Feerick, a meteorologist with AccuWeather Inc.

Actually, April did get off to a relatively warm start, with the high hitting 75 back on April 8, but daytime highs the last four days have been closer to the normal for Dec. 1.

For snow-lovers, the sleigh horse has long left the barn, but for the last several days the region has been under a pattern typical of January and February cold spells.

An upper-level “trough” of lower pressure, which favors the chill, has settled over the East. For most of the winter, its mirror opposite, a “ridge” of higher pressure, which favors warmth, was in control.

The winter was governed by a pattern that kept the polar vortex confined to the high latitudes for the most part, along with its chill, said Feerick, sparing our region from prolonged cold spells.

One consequence of the pattern was the fact that the Arctic snow-and-ice cover reached its highest winter extent since 2013, and Philadelphia’s snow — all 0.3 inches of it — reached its second-lowest extent in records dating to 1884.

The run of April chill is due to continue around here during the workweek with high temperatures in the 50s, several degrees below normal, although the winds that drove wind chills into the 30s Saturday were due to back off at night.

That’s one reason for the frost and freeze alerts: Light winds and clear skies are ideal for letting heat escape into space.

Buildings and paved surfaces in cities are prone to inhibit that radiational cooling. On Friday morning, readings got down to 24 in Pottstown and Millville, but it was 10 degrees warmer at Philadelphia International Airport.

Once the sun comes up Sunday, temperatures are due to reach the mid-60s, which is where they should be on April 19. For a day at least, temperatures will approach “normal,” even if nothing else does.