For thundersnow to occur, all of the elements need to be in place at exactly the right time. It’s basically a winter thunderstorm but with snow falling as the primary form of precipitation instead of heavy rainfall.
The factors that need to come together are:
1. Moisture, so you can have precipitation
2. Cold air, so the precipitation is in the form of snow
3. Strong upward motion in the atmosphere to get the particles charged
When all of it comes together just right ... BOOM! Thundersnow!