I was always under the impression that evaporated water from the Atlantic Ocean traveled west to hydrate the eastern U.S. and that not only did the western U.S. have to deal with rain shadows from tall mountains, but the wind barely blew in that direction anyway.
Apparently this isn't the case because the prevailing wind direction is almost always west to east. So where the heck is all the precipitation coming from? If it's coming from the west, why isn't it wetter westward than eastward?