As of 5 p.m. Monday the temperature in Hellertown was just 21 degrees according to the thermometer outside National Penn Bank on Main Street.

After an exceptionally mild December, the coldest air of the winter season so far is settling in across the Saucon Valley.

Monday night’s low temperature is forecast to be just 6 degrees, and the National Weather Service warned in a Hazardous Weather Outlook that “wind chill values below zero” are expected.

The frigid air isn’t record-setting, but it is coming as a shock to area residents, many of whom enjoyed high temperatures that were 20 to 30 degrees above normal during the holidays.

It was the first Keystone State Christmas in memory when shorts, sandals, flowers and green grass were a common sight.

Tuesday’s high temperature will be around 31 degrees, but by Wednesday daytime highs will rebound to near 40 and rain is likely Friday night, according to the latest NWS forecast for Hellertown.

There is no snow in the near-term forecast.