Beginning this week, the mean trough position will shift west. To date the trough has been centered over eastern North America. Over the next couple weeks, that will shift to the center of the continent. That will allow southern branch systems (Colorado lows) to affect the eastern side of the trough, ie Ontario. Sunday will see the first of these systems affecting Ontario with Ottawa catching the northern edge per current model output. The bigger deal, again according to the models, is the system on Wednesday, currently predicted to give us 20-30 cms of snow, possibly mixed with ice rain, a messy situation.