Rivnuts from Homestead,PA on 4/30/2021 8:34:50 PM:
To provide a quantitative answer to your question see the link below that provides monthly weather data for Cumberland, MD, the approximate midpoint of the full trip from Pittsburgh to DC. Admittedly, as John W points out, the weather can vary widely both with time and location. The US hurricane season is given as June 1 through November 30. If you are unfortunate enough to encounter remnants of an Atlantic hurricane travelling up the east coast during your trip, it could be rather uncomfortable and warrant adjusting your itinerary. Notwithstanding that scenario, a day or partial day of rain should be expected for a week long trip. Yes, it can make portions of the trip a little sloppy, but that should not dissuade you from making the journey. Just grin and bear it and enjoy all the other days. With resurfacing of many miles of the C&O recently along with the paved Western Maryland Rail Trail that parallels the C&O for over 20 miles, that trail is much improved making wet weather less of an issue. For what it is worth, my last trip was bone dry the entire length making dust rather than mud the issue to contend with.
If you are flexible on your schedule, just watch for 10-day forecast and pick some forecasted dry days for your journey. But most of all, enjoy the journey.