What to wear during a cold and wet triathlon
One of my worst triathlon races was being stuck at the bottom on a canyon in the hills out Nice, France, at the 2001 Nice Triathlon. It had rained, I was shivering with cold in a lycra tri singlet and begging fellow competitors for a spare tube. It ruined my race (I finished but was exhausted before I even got to the run) but I learned my lesson. While cold water gets a lot of attention, the conditions once you get out on the road - air temperature, wind, rain, long descents, and time you'll be exposed on the bike - are the real race killers. Of course, it doesn't help if you get a freezing start. So I don't have an easy formula to calculate what combination of the above factors will require you to wear more than your triathlon race suit (which in 95% of your races will be enough) so let's just put your options in increasing order of weather magnitude.
Before we get started, let's see what the rules are if you "overdress" and don't want to carry your extra protection layers with you:
Any item that needs to be discarded, including but not limited to water bottles, gel wrappers, bar wrappers, or clothing items, may be discarded only in the designated trash drop zones at each aid station. Discarding any item outside of the trash drop zones is prohibited.
not discard any equipment on the course, except at the approved, dedicated locations.
USAT rules are less clear. Races rarely mention in their literature "approved locations" for dropping your stuff. Can we assume transition areas and aid stations? Aid stations could be few or far between on a short bike course. You'll probably have to be proactive with the race referee to find this out.
Here's my levels of protection I would consider:
Level 1: Cycling Wind Vest
It's not going to do much from the rain but will protect your main vital area (torso) from rain and a layer against the cold. Is also easy to take off and store should you heat up and have to discard at some point
You'll waste no time slipping these on in T1 around your wrist and then you can just hitch them up as you ride. Easy also to store and discard without stopping. Or just roll down to your wrists again if you get warmer later.
Level 3: Rain Jacket
Especially if it's raining a thin, but water proof jacket can be a life saver in really bad conditions. You will heat up in it pretty quick particularly in a short race if conditions improve and it's harder to stash if you want to get rid of it. So you have to be pretty sure it's going to be horrible out there (or that there's ample opportunity to discard). You might want to pick something up at Goodwill for the occasion if you're worried you'll never see it again.
Long fingered gloves: a pain to put on, but if you're particularly susceptible to cold hands.
Shoe or Toe covers: something you can have on your cycling shoes already, so there's not much time loss here. Although nice in the rain, they probably won't help you much in terms of heat generation.
I personally wouldn't attempt to cover my legs in all but the worst conditions and longest races. Even leg warmers would take a while to get on and off and don't do anything to keep your core warm. I might have some argument here but I don't think a head covering is worth it either, your helmet will serve most of that purpose.
So experiment with all that and take all the elements of the forecast into account, along with importantly race topography. Are there going to be periods of non-pedaling where you can't generate your own heat and will be blasted with wind? With a little bit of prep it is a good opportunity to beat some fitter athletes who make bad wardrobe decisions like Holly Lawrence did at the last 70.3 World Champs. Good luck and stay warm.