Unseasonably cold air will sweep across eastern Canada Monday into Tuesday behind a strong Arctic front. As the very cold air moves over the relatively warmer Great Lakes there will be an outbreak of heavy, lake-effect snow Monday night into Tuesday.
Before the lake-effect sets up, there will be an initial band of rain showers along the cold front Monday which will change over to snow right behind the front as the temperature drops steadily.
Untreated roads and especially bridges/overpasses could get slippery Monday evening from much of southern Ontario to southern Quebec as any wet, slushy roads get icy as the temperature drops below the freezing mark. So keep that in mind if you are heading out on the road after dark Monday.
The map below shows the general areas (outlined in purple) where I believe there is a chance for accumulating lake-effect snow, especially Monday night into Tuesday.
The red areas are the regions that have the best chance of seeing the heaviest snowfall rates with poor visibility and difficult travel.
As is always the case, lake-effect snowfall forecasts are difficult due to the narrow bands and how quickly they shift. At this point, I think the red outlined area south of Lake Huron has the best chance of seeing localized snowfall amounts in excess of 15 cm, while the area south of Georgian Bay could see localized amounts in excess of 10 cm.
It will be a close call for London, Ontario, as I could see the heavier band setting up either over or just to the west of the city later Monday night.
The Lake Huron band will likely be narrow, but intense for a time with the potential for thunder snow as the atmosphere will become unstable due to the cold air (12 degrees C below zero at 5,000 feet) moving over the 10-degree-C lake surface.
The lake-effect snow bands will begin to shift more toward the east and gradually weaken Tuesday afternoon as high pressure tries to approach from the west.
I will update this again later Monday.
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