Here is McMaster's Storm Emergency Policy:
http://www.mcmaster.ca/opr/html/opr/...nts/storm.html
Basically, the provost makes a decision to close the school by 5:30am, but also specifies the duration of the closure. If they announce that the school is only closed until 12:00pm, your 12:30pm class is still on. In all of my years at McMaster, a "snow day" announcement (there have been 5 or 6 since 2007) cancelled all day classes, and night classes were only cancelled if specified.
It's best to check dailynews.mcmaster.ca Monday morning. If there is a snow day, there will be a giant red banner indicating it on the dailynews home page.