One of my first races was at the Central Valley Classic last year where we did the RR in a complete downpour. It was actually worse than that because halfway through we started getting hail !
The RR was completely flat with wide open roads which made things a bit easier. What is the course like for your race? If the roads don't involve insanely twisty descents on bad pavement or roads with potholes, etc. then I would still go. Who knows? Maybe you will have a good time and do well. At the very least it will count towards your CAT5-4 upgrade. I'd prepare accordingly if you do go and get one of those clear rain jackets so you don't get soaked.