I am having problems actually understanding what exactly I shoudl delete here :

1. baseline rain
All the layers will be colored with one color only. I use a not-too-bright blue, RGB:100.100.150. Brighter colors will make the end result hard to read.
For the baseline rain you just select one very big area (the whole map if you want) and bucket-fill it with this color. Then toggle off the layer and toggle on atmospheric features. Make a selection (using the lasso tool) around the high pressure centers (stretch it further east and west than the actual centers drawn before). Don't loose the selection, toggle off that layer and toggle on the dominant winds. Adding to the selection choose coasts where the wind direction is offshore and the areas behind mountains that winds cross (normally called the rain shadows of those mountains). Still, keep the selection. Toggle off the winds. Go back to the baseline rain layer and just press delete. Now your initial baseline rain has gaps big and small. Toggle it off again so you can clearly see the land. Make a selection encompassing all land that is far away from the coast or protected from coastal influence by mountains. Back to the baseline rain layer and delete that as well. The end result, for me, was this: