I know we are not hoping for rain... but I wanted to chime in from my experience with it last year. My daughter and I put on a garbage bag in the corrals at the start of sprinkles. She dumped hers on the side of the road at the start. I kept mine on for 1-2 miles, and
then I folded it up and just tucked it into the back of my spi belt, folded over. I am so glad I did, by mile 8 back on the overpass going into DHS it started coming down pretty hard and I was getting cold. I put it on again and was warmed up quickly. I took it off by the back entrance of IG. My daughter was wearing a tank and her shoulders were freezing by the time we left DHS. Made a quick call to the boys and DH & DS were waiting at YC with a running jacket I left in the room for her.
If the forecast shows rain, pack a trash bag or cheap $2
walmart poncho and try to hold onto it during the race, if you can. It really made a huge difference for me last year, I am one of the few that will say the rain did not ruin it for me in '14.