A strong cold front passed through the area Saturday afternoon. Heavy rain showers started, but they quickly changed to sleet and then snow.
Thunder was heard, and snowfall rates greatly increased.
Thundersnow is not that common, but it does occur in some situations. Ahead of the cold front, surface temperatures were in the 40s, and temperatures between the surface and about 3 miles up dropped over 40 degrees. The cold front helped lift the air.
Behind the front, there was a sharp temperature drop, both at the surface and a few miles up. Surface air temperatures were in the low/mid 30s, but dropped to -30 degrees about 3 miles up — a drop of about 60 degrees!
This very unstable atmosphere allowed the thunder to occur. This set-up is similar to spring & summer thunderstorms, except the temperatures are colder and the focused heavier precipitation is snow.