snow that falls at the same time as there is a storm with thunder and lightning (= flashes of bright light in the sky produced by electricity)

For thundersnow to occur, the air layer closer to the ground has to be warmer than the layers above, but still cold enough to create snow. 要形成雷雪，离地面较近的空气层必须比上面的空气层暖和，但温度仍然必须低至能够形成雪。