Thunderstorms are fairly frequent meteorological events. Brilliantly illuminating darkened skies with the flash of lightning and shaking our windows with the subsequent thunder. Frequent squalls of heavy rain inundate the land below them. But on rare occasions in winter, instead of raindrops, thunderstorms drop snow upon our heads. When this occurs, that winter thunderstorm or thunder snowstorm is called thundersnow. If the precipitation is the mixture of rain and snow we call sleet, it is called thundersleet.
Even in winter though, thunderstorms are far more common than thundersnow. The electrical discharges that are lightning, can occur within clouds, between cloud formations, between clouds and ionized air or between clouds and the ground; the last commonly called lightning strikes. The term "lightning strike" is, however, something of a misnomer; as only a relatively mild electrical discharge makes a path from the clouds to the earth, the much stronger electrical discharge follows that path back up from the ground to the clouds.
Regardless of the path taken, lightning superheats the air it traverses to approximately 50,000 degrees Kelvin (70,000 degrees Fahrenheit), causing the air to explode ultrasonically. That explosion soon decays into the sonic boom discernible to our human ears. Many animals, particularly our dogs, can hear much higher into the ultrasonics than ourselves, so that they hear an approaching thunderstorm before we do. The sonic boom is distorted and slowed further by the air and the underlying topography (shape) of the land, becoming the claps and rumbles we recognize as thunder. When lightning is frequent, the general air temperature can become quite warm; making rain far more likely than snow, by the time the precipitation reaches the ground.
Thundersnow is thought to occur in three possible ways:
1/. When a normal thunderstorm forms under winter conditions on the leading edge of a weather front, either cold or warm, or a thunderstorm on the leading edge of a front runs into an area of cool air, it is possible that the resultant precipitation will reach the ground as snow rather than rain. For this to occur, the strength and frequency of lightning discharges generally need to be mild and few respectively.
2/. When a heavy synoptic (widespread) snowstorm occurs in the center of a mid-latitude low pressure weather system, often referred to as an extratropical cyclone, sometimes the vertical air pressure disparity can result in strong air flows that produce strong positively and negatively charged regions. If these regions are strong enough to overcome the air resistance between them, lightning can discharge across the skies, producing thunder. This type of thundersnow might be more considered a snowstorm that produces lightning, rather than a thunderstorm that produces snow. They only rarely produce lightning between the clouds and the Earth's surface.
3/. A thunderstorm that forms as the result of cold air passing over comparatively warm water can quite often produce heavy snow showers or snow squalls. This is the most frequent type of thundersnow. Such thunderstorms are called lake effect or ocean effect thunderstorms depending on the body of water they form over. The Great Lakes region of the US and Canada is the most likely place to experience thundersnow for this reason.
Because specific conditions need to be met and sustained, thundersnow events are relatively short-lived. The longest period of thundersnow detected at a single, specific weather station is nine hours, recorded at Eau Claire in Wisconsin on the 23rd of March, 1966. The snowfall was not continuous during the full nine hours of thunder and lightning, but returned repeatedly as snow squalls.