[morscuba]

I "Think" Mike had the solution at least for me. I use the term think because I have not had a hard rain since I had the problem. It turned out to be the lower wiper seal, the hard rubber gasket that goes over the channel. It takes a 32" seal/gasket and mine was about 1/2 inch short. A new gasket is being sent by Herr but in the mean time I stuffed a plastic sandwich bag in the gap and covered it with silicone.. If your gasket is long enough I would coat with a thin coat of silicone where it meets the sill.
Good luck
Sam