cold weather Tri?

I don't think you need to worry about hypothermia at those temps.

My experience has been that the excitement and the adrenalin of the race and the fact that I am pushing really hard means that I don't feel the cold as much during a race as I would on a regular training ride.

I would personally dress as if the temp for the race were going to be 10 degrees warmer than the forecast.  No tights, but possibly a long sleeve bike jersey.  If the temp were in the 40's, I'd go with full finger bike gloves as well.

In lieu of putting on gloves, if the air temp is <50, just take a couple of zip ties and those hand warmer packets (like for camping/hunting -find at the sporting goods store in the camping section), and strap them to the aerobars, hoods of the bike, whereever you normally keep your hands..
I've done that for the really code races and the warmth on the palm of the hand helps without sacrificing T-1 time.

Agree with what someone else posted, the race day adrenaline is worth about 10 degrees and your core will be warm by the time you reach the bike.

Whatever you choose, practice getting on the gear in T-1 by having wet arms/legs to see how hard it will be.  Gloves, while easy to slip on when dry, are a real bear when wet so if gloves are needed, hopefully they are loose....