A couple of years ago I was out in California and went to Mammoth over the weekend and they had some type of odd polar air masss move in and the temp ws like -12 F. My toes could only handle (no heaters) about an hour at a time before I had to come in and take my boots off. Probably the most uncomfortable I've ever been.
That would be MLK weekend 2007. I was there and had to come in for foot thawing 2-3 times each day. Living in SoCal and skiing mostly the Sierra and Utah I don't see below 0F often, probably less than 10 days lifetime. Since I also go to Canada in early Feb. I need to be prepared for it though. I've found that if I start underdressed and start losing heat, I wind up having to take breaks all day even after I've added more layers.
Ironically 2007 was the lowest Mammoth January snowpack in 18 years. The upper steeps were largely inaccessible as the wind had stripped most of the entrances. With 2 months of snowmaking the groomed lower runs were in excellent shape.
MLK 1987 at Mammoth was worse. That one was only -5F but the wind was stronger, reported -55F wind chill. I rode chair 3 once, had no feeling below my wrists when I got off the chair, skied awkwardly to mid-station where ski patrol inspected my hands and I stayed inside for an hour. That was an even worse drought year, and Mammoth had no snowmaking back then. You had to watch for rocks on anything steep or with traffic on the reported 12-18 inch base. So we skied the quieter and more protected June Mt. the next day.