December 21st, Øvre Årdal
Average Temperature (620m ASL) -7.7°C
Extrapolated using SALR
gives me a figure of around -21°C at 2500 metres. This could be read as -25°C for an overnight low with -17°C for a high.
Average windspeed extrapolated to the summit would come up to around 12-14 metres per second, giving an average windchill of around -35°C.
During storms, temperature might rise as high as -10°C for a while, but 50m/s wind speeds aren't uncommon, still producing wind chills of -35°C, with the additional hazard of white-out conditions. As long as you make sure you're packed for -50°C, and you have knowledge of Arctic survival, most importantly, being experienced in how to make a snow shelter, as well as food for three times the time you're planning to stay out, you should be fine.