I was living in Hilo the Big Island of Hawaii where it rains as compared to the other side which is dry Kailua-Kona. You can check under your cowl area under your engine hood, there is a rubber that suppose to seal water from getting into the engine bay. It could be something as simple as replacing the rubber or silicone the cowl screw holes. Check to see if you could see any rust around the EGR area. Water was splashing around my (Throttle Position Sensor),therefore giving my the Check Engine Light. Being Cheap & all used silicone from my closest auto store and the problem was fixed. Hope this helps.

My guess is you have bad wires or something in the ignition system that causes misfire, bad wires will do this in damp conditions and not in dry.

only way to help is to tell us what the codes are!

goto autozone / advance autoparts for a free code scan