The past few days in Central Pennsylvania have been very warm and humid. Some cities and towns reach the lower 90s Wednesday afternoon. Thursday, was a bit cooler but it was still warm and humid for September. The high temperatures reached the mid to upper 80s Thursday. High pressure is about to break the summer time pattern.
Friday: High pressure will move over New England. The winds around high pressure always move clockwise. The clockwise winds will usher in cool and moist air from the Atlantic Ocean into Central Pennsylvania. The cool moist air will settle into the valleys east of I-99. The cool air has a very hard time getting over the mountains allowing the western counties to be a bit warmer. The cool moist air in the valleys lead to a cloudy sky and drizzle. This weather phenomenon is call cold air damming. The high temperatures will be in the mid to upper 60s for our eastern counties but the upper 60s to the lower 70s for the western counties.