Southwestern Ontario is in for a spate of nasty weather. A warm front is moving across Lake Huron, shadowed closely by a cold front with lots of associated lake-effect snow. And the forecast is for thundersnow.
Thunderstorms usually form when the air is unstable, with strong ground heating and a cold clashing air mass. The same kind of instability can embed a thunderstorm into a regular snowstorm, but it's a lot harder to have enough heat energy while also having enough of a temperature differential. It's not enough for the upper part of the cloud to be below freezing. It's already below freezing on the ground!
That's why lightning and thunder during snow storms is nothing new for this part of the Great White North. There's lots of thunderstorms in this part of the world year-round. The Great Lakes hang onto a lot of heat energy, so as long as they're unfrozen, part of that heat warms up the lowest part of the airmass. When a storm comes in from the west, it sweeps in really cold air from the Prairies, and that cold air keeps on pulling up lake-effect squalls even after the storm has passed. That makes enough of a temperature difference in the cloud for lightning to happen.
There's a few other places in the world where thundersnow's fairly common. All you need is open water to keep the ground-level air warm, clashing with a rush of bitterly cold, stormy air. Wintery ocean peninsulas are great for this, so Nova Scotia's another thundersnow hotspot. Mountains also have the same kind of temperature differential, so you'll sometimes see thundersnow in the Rockies or in the snowy parts of Japan. If a sharp cold front is dragging a blizzard over unfrozen ground, that can do it too, although that's a lot less common.
Of course, the incoming storm has to have enough strength in it for the kind of strong vertical mixing that creates instability. Your average snow storm doesn't have enough volatility.
Like normal thunderstorms, the parts of the snow storm which have lightning in them are usually a lot more intense than the average snowfall outside that area. Or, looked at the other way around, the parts of the snow storm which are the most intense is where the lightning's going to be, if there's going to be any lightning at all. In a standard snow storm, the strongest temperature differentials are usually just to the northwest of the storm's centre, in the head of the comma formed by the low pressure system and its trailing front.
The downwind regions of the Great Lakes have some or all of these conditions nearly half the winter. On average, storms come about once a week or so. After the storm passes, the northwest winds set up lake-effect snow that's pretty much the definition of cold air meets open water. It's really amazing how intense the snow can get when you're under a snow squall, but just a few kilometres away, there'll be blue sky. Many lake-effect snow squalls are so narrow that one side of the city can be deluged with snow while the other is bone-dry.
People think thundersnow is rare because of the unique acoustical qualities of snow. Snow's a great sound insulator. A thunderclap that would normally travel for dozens of kilometres probably won't be heard very far at all when it's inside a snowstorm. That means a lot less people get to experience it for themselves. However, if you're lucky enough to experience this phenomenon, you'll never forget the way the snow-lit sky lights up, followed by the soft crack of thunder.