I've done two track days in the rain, one of them also at 40 degrees farenheit on RA-1s. I don't look forward to any more.
The scariest one was at Watkin's Glen...cold, rainy, and with low grip. Not much fun, but I learned a lot about being smooth, and anticipating camber changes and surface type changes (asphalt to concrete and back) and resulting grip levels.
Look ahead, pay attention, and slow down until you feel 110% comfortable, especially if you want to drive your car home from the track, or stay out of the emergency room. Also be aware of others on the track- if you feel comfortable driving like a bat out of hell, others may certainly not.
Best of luck to you.