Peter Sheridan from Santa Rosa, CA on 10/3/2015 5:52:36 PM:
I just did the same trip on 9/19 and finished ahead of the rain on 9/28. There was a little drizzle coming into Cumberland where the trail was a little greasy. Overall the GAP seems well drained and good traction for any weather. Any rain on the C&O will make for slower going. You have to focus more because it gets slippery even with cross tires. Allow about a week with no rain for the C&O.