Where exactly will it be placed? I would worry about the possibility (depending on cord orientation) for water to run into the outlet cover. I haven't been known as the smartest dude, but I would be inclined to drill a small hole on the face of the cover for water to drain (if it ever gets in) and not worry.
And actually, as I typed this I remember that I have an outlet without a cover in my soffit on the front porch facing down. The electrician (I had it placed for security cameras when the place was built) didn't want to do it for some reason, but did and it all passed inspection. I can post a pic when I get home from work if that helps.