Actually, I started this controversy, and it wasn't to cause trouble, just to try to understand the weather that night. My thought was exactly what you stated: that the weather was rather localized. Perhaps the airliner came out of a thunderstorm into mist, or even clear air. It might have given Cooper a chance to get his bearings and jump at a spot at least familiar to him, if not his ideal jump spot.
I saw a comment from Jo that got me to thinking, and I checked the weather reports for that day, at three specific airports: SEATAC, PDX, and Olympia. I added Olympia in, because it is approximately midway between the others. It could have been a very bad night in some areas, and nearly clear in others. I believe you related just such localized weather. Belive me, I am not trying to cause trouble, just trying to understand possibilities.