The threat of damaging winds for Mon across nearly all of southern ON started to be communicated as early as Thursday afternoon. Most substainal weather here so far this year probably.
Regarding the severe threat in the south for Sunday, I'll believe a major tornado outbreak when we see one. IMO the last single day was April 28 2014.
Large geographic area of high potential is a big indicator for how likely we'll see one for the books and Sun has that. Even with my default non-scientific feeling always leaning on bust due to persistence, I think 1 or 2 strong tornadoes will occur. I wouldn't want to be down there in Dixie at a time like this, for multiple reasons but if so I'd be looking at my tornado plan and making sure I have a safe spot ready. Dixie is trending more as the main region for tornado alley.