A reinforced shot of cool air will push southeast along the heels of a cool front crossing the region tomorrow afternoon. This front's passage through the region should largely be dry -- there might be a few sprinkles accompanying it to our north and northeast -- but there will be a noted wind shift to the northwest behind the front tomorrow afternoon and a sharp drop in dewpoint values.
If you've liked today's 50's for dewpoints, we'll see 30's and 40's in here overnight between Thursday into Friday. With northwest breezes tapering off overnight and clear skies, the potential to see temperatures taking a dive into the deep end of cool does exist as one travels north and west of the city.
Both the NAM (above) and GFS (below) show the potential for 40's in the suburbs on Friday morning. Some variants of the NAM on a higher resolution basis try to paint some 30's out by Pottstown and Coatesville -- not too unrealistic, but arguably a bit aggressive as lower to mid 40's seem much more likely. Around the city, lower 50's is not out of the realm of possibility but mid 50's probably is where we bottom out at. Assuming we get below 58 in the city, which seems likely, it'll be the coolest morning since May 27th's 49 degree start.
As we mentioned a few days back, frost is not out of the realm of possibility for the northern parts of Pennsylvania -- places such as Bradford could see their first frost of the incoming cool season with this incoming shot of cool. Doubt that risk of frost extends south of I-80, at least with this shot of cool.
Temperatures will recover Friday afternoon but our highs may not get to 75 by day's end...definitely a sign of what's forthcoming in increasing supply over the coming weeks and months.
Welcome to autumn!