Not wanting to sound like grandma, but have you done any major climbs in cold wet weather? After doing a few of them, this is what I've figured out:
The uphill part is fine, but the downhill part is usually miserable and possibly dangerous if you're not well-prepared and careful.
Make sure you take every piece of warm cycling clothing you have. And if you don't have a full set of it (baselayer, jacket, arm and leg warmers, booties, beanie, thick winter gloves, etc. etc. etc.) get it before you go.
Obviously, the road is slippery, and your brakes don't work as well as they normally would. So be really careful downhill. Incidentally, if you don't fit them, consider the Kool-stop pads, which are a big improvement on at least Shimano's stock brake pads in my opinion.
And if it's marginal, and you don't have a support car, think long and hard about whether you really want to go.
All that said, enjoy your trip!