Arguably the greatest 24-hour rainfall on record outside of the tropics occurred around Smethport, PA on July 17, 1942. A post-event survey by the Weather Bureau provided a widely accepted maximum storm rainfall of 34.50 inches within a 12 hour period, of which an estimated 30.60 inches fell in just 6 hours.
However, the official 24-hour rainfall record for Pennsylvania occurred in conjunction with Hurricane Agnes in June 1972. One of the pump stations located southwest of the city of York, PA (York County) recorded a total of 13.50 inches on June 22nd.
Source: Weather Prediction Center | NOAA