Oh... my... this is awesome. 0.0
Well, to start, you perfectly captured the mood of a driving rainfall with muted brush strokes. =)
And I love the subtle outlining of the foreground tree's bark, that adds depth and detail without being distracting. ^^
Your rain is amazing, you managed to find the balance between over-sketchy and laboured, which is very hard to do. The rain has very realistic speed and strength without looking sloppy.

Also, the background trees quite nicely add the necessary feeling of dimension without being distracting. The image balance overall is very good. :3
I have not words to describe how epic this is. ^^