Here, a storm is something to love rather than fear. The speaker watches the lightning, feels the thunder, and describes it as a rush moving through their veins, as if the storm has gotten inside the body and become part of them. The repeated address, "Thunder, beautiful thunder," makes it sound like a greeting to a person, maybe even a lover.
The most striking line is "You're the most wonderful catastrophe." It puts two opposite ideas side by side: something destructive and something wonderful. That suggests the speaker is drawn to intensity, even when it carries danger or chaos. The last line shifts the mood. After all the excitement, the speaker asks, "when will you be coming back to me?" and suddenly sounds lonely and waiting. The poem leaves us with a mix of thrill and ache. Judging by the language, the thunder could stand for a real storm, but it might also stand for a person or feeling the speaker misses.