Harvey wrote
From what I've seen Alta rarely closes because the base is gone.
Yep, the Alta closing date is set well in advance and has nothing to do with snow coverage during late season. But in a low snow year getting down off the High T the last few weeks of the season can get a bit tricky in spots. Have only had that problem once in the last ten years though.
In recent years, Alta opens for a few final weekends after 7-day operations ends. Supreme closes a week early. For the weekends, best to check which lifts are running.