Maybe some snow, cold rain for sure, I'd guess 25, 30 degrees. It's right by the coast so it's not going to get that cold, even being up that high. If a good pacific storm from the north hits, you can get some really good weather, storms, wind.
With that said, I broke down and finally stopped using cottons/wools when backpacking up there on a winter trip where I almost got hypothermia up there, not quite but it was close.
I used to head out there during winter when storms were approaching, I did get a 20 degree synthetic bag to handle the humidity/cold, and was never even remotely cold using that. But that was a real 20 degree bag, not the current practice of calling 30 degree bags 20 degree which some bag makers sadly have started doing.
Aiming to get back there in a few weeks health permitting, wish the weather wasn't so nice, but can't have everything I guess.
Your instinct is spot on however, winter is the best season there in my opinion. The colder the better, then nobody is there at all, especially up in the higher areas.