Snippet from Elliott's discussion today - this seems a good bit earlier than most years if it materializes:
Thursday will be the coolest day since late May despite abundant sunshine. It won't be as windy as tomorrow, but a north-northeasterly breeze of 8-16 mph will make the outdoor air feel even chillier. I then expect the first widespread frost of the season across northern MD and the Lower Susquehanna Valley Thursday night. A Canadian high pressure system will settle into southern New England and northern PA late this week and promote efficient, radiational cooling after sunset. Winds will be very light or calm Thursday night, and dewpoints should be in the upper 20s to low 30s under clear skies. Temperatures often bottom out near the dewpoint on clear, crisp fall nights, and this case should be no exception. In fact, temperatures may bottom out between 30-32°F in the deepest valleys and most outlying areas early Friday morning. They'll be significantly higher (38-40°F) in urban centers such as York, Lancaster and Harrisburg. In the majority of locations, lows will range from 32-36°F (see below). Regardless, the frost will likely put an end to the growing season in most spots, so bring tender plants/vegetation inside or apply a protective covering over them Thursday evening.