We've all been there when we've arrived to the course and confronted with a large downpour of rain. The best way to deal with this is through thorough preparation. Ensure you always have at least a rain jacket stored in your bag and an umbrella. An extra pair of rain pants and glove can also be very useful depending on the severity of the rain.
The next element is how to deal with the rain on the course. The biggest aspect is trying to keep your equipment dry as well as your grips at all times. What people don't realise is that then water goes onto the clubface, it loses friction with the ball. This can cause the ball to come out with less spin and therefore difficult to judge distance. A great tip is tying a towel inside the umbrella at the top and ensuring the club head is dry before each shot. Ofcourse when on the putting green and before teeing off you can make sure your ball and equipment is dry.
Another important element to remember is that the greens will be quite soft after a considerable downpour. What you would want to do is take a longer club on most occasions as the balls will stop a lot quicker. This is particularly true with wedges. If you do spin the ball a lot, you may want to land the ball past the pin to ensure you don't end up too short of the hole. Most pros love to come onto the course after a downpour because they generally make more birdies because of the softness of the greens.
Another great tip when playing in wet weather is keeping your swing shorter. You want to make sure that the swing is as simple as you can make it particularly if there are strong winds also. Just like any shorter swing, you may want to choose a longer club also to make up for the distance.
If you'd like any further help in strategizing during wet conditions just let me know.