TT def closes in the rain.
part of the ride is outside and its the part that you hit close to 65mph.
i've been at that part when its just starting to rain and it doesn't feel all that great.
if you go at RD you shouldn't have much to worry about since MOST of the time its not raining at 8am-9am.
it usually holds off until later in the day.
had your plan been to ride at 2pm i would have advised you to change it to near RD.