UPDATE: Christmas Winter Storm Expected to Bring Widespread Snowfall and Potential Ice Storm Risk Across Southern Ontario; Heavy Rainfall for Eastern Ontario

There continues to be very high confidence in an impactful winter storm across Southern Ontario starting Christmas Eve and continuing into Christmas Day. The latest trends in the data have shifted the storm track slightly to the east so as a result, this has pushed the heavy snowfall potential into the GTA and parts of Central Ontario. Between 10-25cm of snowfall accumulation by the end of Christmas Day is expected but some areas may see ice pellets mixed in especially near the orange zone which would reduce the accumulation.

The risk of prolonged freezing rain now extends from the Northwestern GTA through the Kawartha Lakes and north into Algonquin Park and Bancroft. Ice accretion in this area may exceed the level to be considered an ice storm (6-13mm of freezing rain) so there will likely be significant ice accretion on surfaces including the possibility for power outages - some of which may last for several days. It would be a good idea to have an alternative plan in place if you are having a Christmas dinner (within your household only) as the lack of power would make it difficult to cook.

Eastern Ontario is still on track to see a Green Christmas with heavy rainfall being the predominant precipitation type from this system in that area. Flooding could be a concern with rainfall totals ranging from 20-40mm with locally close to 50mm possible.

Precipitation will begin to enter the province sometime early Thursday morning starting as rain. Temperatures will be quite warm as you wake up on Thursday ranging from 2 to 7°C but that will change through the day. As the system continues to spread across the region, we’ll see colder air start to flow into the province from the west during the late morning or early afternoon causing the rain to transition over to heavy snow through Windsor and along the Lake Huron shoreline.

This transition will continue to occur throughout the rest of Thursday with the switch over to heavy snow complete west of a line roughly from Algonquin Park to Oshawa by the evening. A few hours of freezing rain is possible through the GTA between the transition during the late afternoon to early evening but it shouldn’t linger around too long. Another push of moisture from south of the border will enter our region late Christmas Eve setting the stage for heavy snow throughout the overnight into Christmas. This combined with the stalled out cold air will allow for the development of a fairly intense and persistent zone of freezing rain from Algonquin Park to Oshawa (closer to the lakeshore will see less icing). The freezing rain will continue for 6-12 hours from late Christmas Eve into Christmas morning before clearing up. Unfortunately, there won’t be a chance for the ice accretion to melt away after all the freezing rain with temperatures plunging below the freezing mark so the impacts of it will persist into the weekend.

Conditions will improve for most areas later in the day on Friday (Christmas Day), but with even colder air flowing into the region behind the system, it’ll cause the lake effect snow machine to turn onto full blast. We’re expecting some very intense snow squalls to develop somewhere southwest/west of Georgian Bay and Lake Huron from Friday evening through the weekend. Even could see some lake effect snow through parts of the Niagara region courtesy of a squall that is expected the hammer the Buffalo area.

Who could see the heaviest snowfall and exactly how much that will be is still uncertain as it will depend on how stable and focused these squalls become. We’ll have a better idea in the coming days as the higher resolution models come into range. With that being said, wouldn’t be shocked to see somewhere in the snowbelts come away with over 30cm of snowfall accumulation by the end of the weekend in addition to the system snowfall.

We will have a full detailed forecast for the upcoming winter storm published later Wednesday or early Thursday. This will include more precise accumulation maps and timeline graphics.