Residents of several Eastern Massachusetts towns reported unusual activity during last week’s snowstorm — rolling thunder and cracks of lightning. The phenomenon is known as “thundersnow,’’ a thunderstorm while it snows. Thunderstorms are significantly more likely in the summer, when instability is in the air, along with lift and moisture, according to AccuWeather. For thundersnow to occur, the air below needs to be cold enough for snow, yet warmer than the air above it.