Weather is always tricky to predict, that being said, last year's 8 Dec launch produced frozen solid nalgine bottles the first 3 days but after that it was moderate enough to take a few full immersion baths (mid day)
Not from personal experience, but know people who have done it in late fall/winter. Temps sounded great when they were there. Not to say you could get unlucky, but I don't think there's any reason to be scared off by the cold.
the link was posted to the Grand Canyon Private Boaters Assoc. (GCPBA) list, a very good site to visit if you're planning a GC trip. I'd expect that if you asked that group, you'd get lots of detailed info. try GCPBA-dot-com
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