Rain can make the belt lose some traction. If the alternator isn't producing enough power, because of the electrical design of the car, all those lights will come on. They'll flicker as the alternator dips into the low end of output. It's most likely due to rain causing the belt to slip a little on the alternator. Nothing much to worry about if it doesn't happen all the time. I get the same effect sometimes when at idle with the A/C on. There is a new length belt and belt routing method to correct this problem.