I rode in the rain one day without a rain jacket which I had along but left in the team SAG vehicle. I will never do that again! No matter what the week’s weather is generally or how warm it may seem on a day when rain is expected or even occurring when you set out it will often turn chilly during a rainstorm and if you are rain soaked you can get quite cold and surprisingly fast. It was nice and warm that morning and the rain seemed little more than annoying. We stopped for breakfast and then had to retreat to a HS when lightning started. After it blew past I was feeling a little chilled but many people were curled up in balls on the gym floor, shivering with hypothermia. I set out figuring that the exertion would warm me back up and it did, eventually. The wind had picked up however which made my rain soaked jersey an even more effective refrigerator. After about a mile I was shivering so badly I almost could not steer the bike and stay upright. After another 3 to 5 miles my internal heat engine had caught up and I was feeling good except for having to pedal into that blasted headwind!
But some of those in the gym were there for quite a while and some SAGed the rest of the day. It is worth carrying a light rain jacket if rain is expected.