I have done Jtree in Nov, Dec. and Feb.
Here's what i recall:
I have been coldest in feb., with the wind making it very difficult to function.
Dec. (right around xmas) was real cold at night (down jacket is key), but every day was warm enough to climb, no snow that year.
early nov was the best weather- shorts on some days, little wind, warm enough to climb every day and night's were very comfortable.
so, it just depends on the year, but its always worth a shot!