It is looking more and more likely that the first measurable snow of the season will fall across southern Quebec this upcoming weekend. This system has been on the weather models for several days now, showing up as early as last week. However, as with all potential winter storms, the devil is in the details, and that is often hard to pin down here in the St. Lawrence Valley. Warm air at the onset of the precipitation may keep the snow totals down somewhat for Montreal. That being said, here is my best guess at our first snowfall.
High pressure will be in place for Wednesday and early Thursday, with fair skies and fairly cold temperatures. Overnight lows will be the coldest of the season so far, dropping down to -7C (19F) by Wednesday morning, and rising to 1C (34F) by late afternoon. Milder air will arrive on Thursday, with daytime highs going above the freezing point to 4C (39F). That mild air intrusion will remain in place in Montreal through mid-day Friday, assuring us a rainy start to the potential storm.
An upper level low is forecast to move across the Great Lakes and south of Montreal, accompanied by a rather strong cold front on Friday. Precipitation will start late Thursday persisting into Friday morning, and likely starting as rain. As cold air filters into the St. Lawrence Valley on Friday, precipitation should change to snow by late in the day. I think this will be a relatively light storm for valley locations, with a better chance for significant snowfall across the higher elevations, especially in the Eastern Townships and Green Mountains.
At this time a general 5-10cm of snow is possible for metro Montreal through Saturday morning. Northwest winds will become rather gusty as well, up to 50km/h, along with dropping temperatures. All in all, driving should be less than ideal Friday night. The snow should taper off to flurries in Montreal early Saturday, with a chilly high of -1C (30F). Snow will persist south and east of Montreal on Saturday.
I will post updates as the forecast becomes a little more precise.