The HRRR was updated in late October which affected the SST assimilation. This caused the Great Salt Lake and all other sea surface temperatures to be stuck. This was fixed in late December. Below you can see the Great Salt Lake surface temperatures at the Buoy in the HRRR (blue) and observed (red). The rapid drop in SST is a result of the fix which brought lake temperatures closer to it's actual temperature.
For more information, see the announcement here: