Detail breakdown of Mammoth Jan. 1 conditions over past 37 years:
0-30% open: 16%
50-70% open: 11%
80-90% open: 11%
100% open: 62%
Detail breakdown of Mammoth Jan. 1 conditions over 15 years <20% open Nov. 15:
0-30% open: 40%
50-70% open: 7%
80-90% open: 13%
100% open: 40%
This provides a good illustration of the degree the odds move by having the first quarter of the 11/1-12/31 time period be dry. And why you increase your odds of success by holding off booking trips until the season starts.
I would guess the figures above would be a little better at Kirkwood and Sugar Bowl, about the same at Alpine Meadows, and worse elsewhere in the Sierra.