Complex Prolonged Winter Storm Takes Aim at Southern Ontario to Start Off December; Up to 20–40cm of Snow Possible by Wednesday

DISCLAIMER: This is a very preliminary forecast for a complex winter storm over 3 days. It is subject to change as we get closer to the event and the situation evolves. We’ll have a full detailed forecast for Monday and a preview of Tuesday’s forecast out sometime Sunday afternoon or evening - including our regional timeline graphic.

It might have been a slow start to the wintery weather across Southern Ontario after we saw the season’s first significant snowfall (for most areas) last week, but Mother Nature is wasting no time! A very difficult forecast is ahead for Southern Ontario as we say goodbye to November and hello to December with an approaching low-pressure system that is expected to intensify as it moves into the Great Lakes region. The precipitation will start sometime Monday morning or afternoon depending on the location. The usual complexity this time of the year which is dealing with the freezing line and determining who sees rain and who sees heavy snow will certainly exist for this system. But there is an additional layer of uncertainty with this system as we expect it will stall out somewhere over our region and continue to pump moisture through Tuesday and into Wednesday. As we head into Tuesday, colder air will start to flow into the province which will allow the lakes particularly around Lake Huron and Georgian Bay to enhance the system and add even more moisture.

With that being said, the best approach to a multi-day storm like this with vastly different conditions day-by-day is to look at the expected impacts on each day. Now, we’ll have a more detailed forecast for each day later on but at this point, we can provide a broad look at each day.

MONDAY

Precipitation will start to affect Southern Ontario sometime during the late morning or early afternoon hours on Monday. The exact precipitation type is a little uncertain but it’s a fair bet that most areas will start with some rain as temperatures are hovering several degrees above the freezing mark. Later in the day on Monday, we’ll see colder air slowly begin to invade from the west causing the rain to switch over to some moderate to heavy snowfall through much of Southwestern and Central Ontario especially in the higher elevations around the Dundalk Highlands. This includes Windsor, Kitchener/Waterloo, York Region and the Barrie/Muskoka area. Heavy precipitation will continue through the evening with the mixing line extending from around Hamilton to the northeast away from the Lake Ontario shoreline to Peterborough and up into the Pembroke area. Those around the Lake Ontario shoreline and into Eastern Ontario will continue to see heavy rainfall past midnight and overnight into Tuesday.

Tuesday

As we head into Tuesday morning we’ll see the system begin to stall out and the heaviest precipitation bands will move out over Quebec but less intense bands will rotate back in. Light rainfall will be ongoing through Eastern Ontario and light to moderate snowfall for the rest of Southern Ontario. We do expect some lake enhancement around Lake Huron and Georgian Bay to develop as the precipitation associated with the system wraps back around. This will result in locally heavier snowfall particularly east of Lake Huron and Georgian Bay throughout the day on Tuesday and into early Wednesday.

Wednesday

Wednesday morning is where we have the biggest amount of uncertainty as we believe the low-pressure system that was stalled out over our region for over 24 hours will begin to move out. While it’s moving out there are some indications that it could rapidly intensify yet again over Quebec just after midnight. Depending on when and where this happens it could wrap in some heavy bands of snow that may affect Central Ontario (especially around the Quebec border around North Bay) with intense snowfall during the early morning hours on Wednesday. Some of the latest data suggest this occurs as the low pressure is further north into Quebec which would have less of an effect on our region. We’ll be watching this one closely and have a better idea as we see what happens as the system moves into our area.

Putting together the potential accumulation over the three days, some areas may see between 15-30cm of snowfall accumulation or even more by Wednesday. We should note however that this is over three days so there is a big difference in the impacts of a storm that brings this amount of snow in 12-24 hours vs. over a 72 hour period. Also, some of the snow will be coming down onto ground that is quite wet after earlier rainfall so it may take a few hours until the snow begins to accumulate which would throw off the data we base our forecast on. Those in Eastern Ontario are expected to see little snowfall accumulation but could see a fair amount of rain with totals between 15-30mm by Wednesday. As we mentioned above, there’s still lots of uncertainty so take any accumulation amounts with a grain of salt. We’ve decided to put this map out to give you a rough idea of potential impacts but it’s far from a final forecast. Check back in the coming days for more detailed information as we get a better handle on what will happen.