I would say keep your tires on the softer side, if they are kind of worn maybe replace them before the race, bring one more tube than you normally would because the chance of getting a flat is a lot higher in the rain, be careful of the areas where the car tires go in the lane because the pavement tends to be smoother and the water pools up there. Allow some extra distance for braking and back off on the corners a bit. Careful of metal stuff like railroad tracks and utility covers, don't cross them with turning or braking force on the bike.

55 and rain for 5 hours is going to mean keeping the heat in your core, dress in layers you can remove, maybe wear a bike jersey with pockets to stuff clothes in if the sun pops out.

I ride a lot in difficult weather and would respect that temperature if it's rainy, given the length of the event.