Temperatures are expected to fall into the 40s overnight in parts of the region, and Philadelphia should see its lowest readings since May 12.
The National Weather Service advises that under clear skies and generally light winds tonight, excellent for radiational cooling, patchy frost could whiten parts of the Pine Barrens and Poconos.
Because the area immediately near the ground tends to be colder than the air where the temperature is measured, it is possible to get frost even when the readings are above freezing.
In Philadelphia, the forecast low for tomorrow morning is 51; it hit 50 back on May 12, and hasn't dropped into the 40s since April 28.
We have to believe this run of chilly nights is good for the foliage show, and the first flecks are showing up around here.
As noted earlier, Marc Abrams, foliage specialist at Penn State, points out that cool nights in late September and early October are an important part of the recipe for virbant colors.
With the exception of the Thursday overnight period, temperatures are forecast to into the 50s the rest of the week, about where they should be.
The Climate Prediction Center's two-week outlook favors near-normal temperatures.
We'll have more to say tomorrow about the progress of the foliage season.