^ Boulder to Eldora should still be doable at 20ºF, or even 0ºF (it gets cold at Eldora), assuming that the road isn't snowpacked. The grade is steady, no up and down at all. The speeds up Boulder Canyon shouldn't be too high, as always: slower is better for range. And if it doesn't work out one can turn around at LBW ("L
arning") and head back home. I've done a much more challenging winter trip.
Some numbers from Tony Williams' range chart
21 miles @ 45 mph: 21.4 miles/5.2 miles/kWh = 4.1 kWh (I'm guessing that some of the speeds will be lower than that)
Elevation gain: 4000 feet x 1.5 kWh/1000 feet = 6 kWh
Temperature correction for 0ºF = 17.5%
Altitude correction for 5300 feet = -8% (that's just at the starting point, the average altitude is more like 7300 feet or -11%)
Net correction: 17.5% - 8% = 9.5%
Total estimated energy: 4.1 kWh + (4.1 kWh x .095) + 6 kWh = 4.1 kWh + 0.39 kWh + 6 kWh = ~10.5 kWh
That leaves plenty of extra battery for heating, although if dressed for downhill skiing it won't likely be necessary on the way up IME. The return trip, even if at LBW, would be very easy due to going downhill the entire way and regen (although regen will be limited in very cold weather). Been there, done that.
Yes, this trip can be done with some confidence IMO. Doing it from Louisville, Lafayette, or Longmont would be more difficult unless one was willing to take the time to charge in Boulder.
For the record: I lived in Boulder for more than fourteen years and have skied at Eldora many times, so I am familiar with the trip.