One of these years (or every year) it’s going to be raining during your event. It does’t matter if it’s a bike race, triathlon, running race, or a Twister Tournament (don’t steal my idea) you’ll have to be prepared with proper rain gear. Buy your self a proper rain coat, rain pants, and water proof footwear. The cost is well worth it.
The best rain gear in my opinion is pvc backed nylon. Sure, it does’t breath like goretex. But unlike goretex, it doesn’t stop being waterproof when it gets dirty. For maximum comfort it is recommend to wear some moisture wicking underlayer with a top layer like a cotton t-shirt. The wicking layer will pull your sweat up to the cotton layer and will keep you much more comfortable and warm.
Some other items that are key to a good days work in the rain are: a good rain hat, a waterproof bag for your phone and wallet (a ziplock bag does the trick), solid waterproof boots, and a full change of clothes.