This is the 3623rd most frequent Japanese word.
降る
To fall (e.g., rain, snow).
The word '降る' refers to rain falling from the sky.
雨が降る。
It rains.
Here, '降る' is used to describe snow falling from the sky.
雪が降っている。
It is snowing.
In this context, '降る' is metaphorically describing thunder as coming down, enhancing the imagery.