Why is thundersnow rare in Chicago?

Dear Tom,
Why are lightning and thunder hardly ever associated with snowstorms in Chicago?
— Jimmy Harland, Morton Grove
Dear Jimmy,
Thunderstorms do occur with snowstorms in Chicago, though only once every few years, on average. There have actually been injuries from lightning in winter snow situations. Thundersnow is relatively rare because cold air well north of a winter storm center usually lacks the buoyancy to build clouds to altitudes that can produce lightning. However, powerful winter storms can produce thunder as much as 140 miles north of the path of the low pressure system causing the precipitation. Researchers have noted an especially powerful kind of lightning when it accompanies snow. Impressive snowfall rates often occur under such thunderstorms. The opaqueness of snowflakes and their sound-muffling fluffiness limits how far away you can see the lightning or hear thunder.