Wiktionary
n. A thunderstorm accompanied by snow, rather than rain or hail.
Wikipedia
Thundersnow, also known as a winter thunderstorm or a thunder snowstorm, is an unusual kind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorms, but the top of the cumulonimbus is usually quite low. In addition to snow, graupel or hail falls.
Usage examples of "thundersnow".
Win-terlight and Stormcloud, Brightmoon and Daystar, Earthsong, Thundersnow and Firedance - she matched names with faces, with smiles shy or bold, with personality quirks.
The winters have steadily grown worse in this land, and I want us all inside where it is safe and snug before the thundersnows begin in earnest.
The only overt sign of oncoming trouble was the increasing cold of the climate, with short summers and heavy winters characterized by thundersnows, to which the people had adjusted and accepted.