Sorry about your (hopefully small) fish kill.

I would definitely keep posting with esshup on this topic, because his advice will help you avoid a repeat in the future.

Were you at the pond close to dawn on the day after the storm? That time of day is generally the absolute low for dissolved oxygen. Once the sunlight hits the pond, all of your photosynthesizing organisms got to work trying to save your fish. You pumping some water probably helped too if you were able to agitate some air into it.