GUI isn’t progressing currently as I find it less interesting than other things I could be working on. I finally managed to get the rain shadow calculation time under control it now only takes a couple seconds to calculate instead of a couple minutes. The results seem to be better too. Here is a description the algorithm:
Previously I was examining ocean points and then simulating where the wind would carry the water. In this version I instead start by examining each land point. For each land point I use the wind direction and reverse it and keep following the vectors until I reach a water point. This creates a path from the land point to a source of water. This path is then examined and I find the highest point along it. If that point is higher than a specified cutoff there is mountains in between this land cell and its source of water indicating that it is in the rain shadow. A value is assigned to the land cell so that there is a slight falloff as the distance from the highest point is increased making the rain shadow less intense over longer distances. Some scaling and normalization are then performed as well as adding a little bit of noise to make it more interesting.
Elevation images following by their corresponding rain shadow: