How does rain affect fishing for trout?
Cold Rain/Hail: If the rain is significantly colder than the water temp, it could trigger some serious feeding activity, or it could shut them down completely. It is very common for these drastic and sudden water temp changes to trigger a huge hatch as well during, or just after the storm. Even large amounts of hail can often trigger fish to go haywire shortly after it has finished falling.
Very Heavy Rain: If you can see your dry fly on the water and can keep it floating during major rain, you could catch a lot of fish. If it has been raining for a while and you aren’t seeing any activity on the surface, you should put on a nymph rig. The heavy rains will have washed a lot of food for the trout off the banks and trees into the river and it won’t be floating on the surface.
You could always try the San Juan worm, because worms are always out and about during the rain. If the rain water has changed the color of the river, you might also try to throw some brighter colored flies.
What Flies to use?
Streamers Streamers will often become very effective during the rain, but it really depends on the day. If the hatch shuts down and fish stop hitting the surface, streamer fishing might be your best choice. In a dark rain storm use black, but olive colored patterns have also been affective in dark weather.
Nymphing Running a double or triple nymph rig can be very good choice in the rain. You might have to completely change the bugs that the fish were hitting prior to the weather coming in, but something will probably work.