I got out fishing this morning in light rain. It had also rained hard last night. There were fish all over the surface and in close feeding. That's because the rain tends to send a lot of food into the water in the form of seeds, bugs, worms, etc. and there were carp just about everywhere I looked. It was simply a matter of tossing a freelined doughball in front of a feeding fish, and most likely it would grab it. It was that simple. The score for just a couple of hours of fishing in the morning was 11 carp with several other takes and misses. I experienced similar fishing last week on those rainy and cool mornings.
Compare this to the last two sunny days. I got out both days and BLANKED. There was just nothing going on and no fish to be found.
Carp are advantageous feeders. I have always had my best luck in some of the worst weather imaginable. Yes, rainy and stormy days can be very productive!