Friday will be act as the calm before the storm. A pleasant day with single digit highs, light winds, and some sunshine. Beginning after midnight rain will start in the Windsor area. With a mixed bag of precipitation spreading throughout Southern Ontario during the pre-dawn & morning hours.
A moisture laden system will slide towards the lower Great Lakes late this evening, following along a boundary separating cool air across Southern Ontario with warm air south of Lakes Erie & Ontario. Concurrently, an arctic cold front will be descending southwards heralding the arrival of deepening trough over Ontario & the Great Lakes region for Sunday. Temperatures will be running much below seasonal Sunday, daytime highs will struggle to reach the freezing mark. Additionally, Lake-effect flurries or snow is likely across the northwesterly snow-belts.
The challenging aspect of this system will be determining how the air-masses interact. In forecasting situations like this, thickness (a function of the average virtual temperature between 1000 and 500 millibars; read more: https://bit.ly/2WykrG4) can be useful in determining precipitation types. Anywhere, along and north of the blue line will likely see rain change to wet snow. With a brief period of freezing rain or ice pellets possible. South of the red lines will see mostly rain. These lines will move southwards as we progress through the day Saturday. Meaning areas who see mostly rain near Lake Ontario and Lake Erie, may end as wet snow as the system departs.
OKAY SO WHAT DOES THAT MEAN – SNOW, RAIN, OR FREEZING RAIN?
Simply put, all of the above. Depending where in Southern Ontario or Quebec you reside.
The greatest threat for freezing rain will exist across the Ottawa and Montreal region. There’s potential for several hours of freezing rain for this area. And even a threat for appreciable accretion on untreated surfaces. The winds will be light, with temperatures teetering around freezing – largely mitigating any power outage threat.
Next, the snow. Any snow that falls will be wet, the greatest accumulations will be across the Dundalk and Haliburton highlands/highway 60. 10cm or more expected South of Georgian Bay. 10-20cm+ for the Haliburton Highlands. Across much of Southern Quebec – well north of Montreal. Mont-Tremblant National Park could see accumulations as high as 20-30cm. Highways 117, 309, and 105 will be snow covered and slick through Saturday afternoon and evening.
Finally, the rain. 20-40mm will fall along the lower Great Lakes. Including the GTA, Golden Horseshoe, Southwestern Ontario, & Eastern Ontario.