Synoptically-driven snow will impact southwestern Ontario on Thursday, but on the backside of the departing system, snow squalls should bring very localized heavy snowfall across Ontario. This is due to the coldest temperatures of the year moving across southern Ontario on Thursday night and Friday, which should bring temperatures in the minus double digits.
Seen above are the cold 850mb temperatures (-13C) going over Lake Huron, which is still fairly warm. This will enhance snowfall amounts downstream, especially in higher terrain of southwestern Ontario.
Localized snowfall could range in the 20-30 cm range from Thursday evening/night through Friday for areas near Goderich, Hanover, Mapleton and surrounding regions in Ontario, dependent on where the snow squalls set up.