Gouache is a great medium for quick outdoor sketches. It takes less time to pack your stuff and start painting than oils, but gouache gives you the same possibility to start with transparent layers, and then move to opaque painting.
I had to be quick this time, because it started to rain soon after I started painting. Because of that I had to make desicions really fast, and focus mainly on the basics of the composition. I would have liked to stay and add a little more detail here and there, but because of the rain I had to quit.