'12 Felt F75X, '11 Scott CX Team, '08 Specialized Singlecross
Posting while waiting for my son to wake...
I rode Greylock the last weekend in October. I started my ride from the Mohawk Trail Campground, which turned the trip into a metric century. It was in the low forties at the campground when I left. I dressed for the climbs so as to not overheat. The summit was 32F, but luckily my wife and son met me there with some warmer clothes. I still froze on the descent. I descended the southern route, which I believe is the longer of the two.
It was miserable and lovely at the same time. BZ to you.
I've descended in cold rain a lot in the PNW hills and mountains, with the same clothes I had on the way up, just zipped up. The only thing to have on your feet is winter MTB boots. Put dry suit leg seals over the tops of the boots so water doesn't run down your legs into them. You need a decent set of tights, like Pearl Izumi Amfib or Performance Triflex. On top, you need a long sleeve Craft undershirt, a Pearl Izumi Elite or PRO softshell jacket, and a Voler Jet wind jacket. For gloves, Gore-Tex Xenon. Balaclava on head.