Will skies be clear over southern Ontario Sunday night?
Skies will most likely become mostly clear before sunset, after the cloudiness that's over southern Ontario Sunday afternoon moves eastward and a high pressure ridge spreads eastward over Ontario during Sunday and Sunday night.
Then, skies are likely to become partly or mostly cloudy during mid-evening as mid-level moisture (illustrated below) and lift ahead of an upper-level trough spread southward over southern Ontario during the evening.
So, in summary, skies will most likely be mostly clear during the early evening and then become mostly cloudy.