On Friday, a strong surface low pressure system will develop ahead of an upper level trough over the Great Plains in the US, tracking northeastward through the forecast period. A warm front extending from the low to the east to southeast will slowly approach southern Ontario from the south on Saturday, though it isn't expected to pass through the region until Sunday morning. Moist air being lifted by the front will bring a brief chance for showers ahead of the front around mid-day. Partly to mostly cloudy conditions are expected otherwise, concentrated mainly in the mid-to-upper levels. Light surface winds from the east to southeast will cause slightly cooler temperatures during the day.
As I mentioned before, the warm front is expected to move through the area Sunday morning. Southerly/southwesterly winds will bring warmer temperatures to the region during the day, resulting in the high being the warmest of the weekend. Conditions will be partly to mostly cloudy with a mix of low and high-level clouds. The surface low will continue to approach the region from the west during the day. After the track sessions have concluded for the day, there is a chance of thunderstorms ahead of a cold front from the system.