Sounds like you had an anoxic layer form in the deepest parts and when the cold rain came and caused runoff it flipped, mixed and killed a lot of fish. Combination of the hydrogen sulfide, low DO and Temp shock is not good.
As Shorty suggested, I'd bet there are some smaller, healthy individuals that made it through.
If that is the cause, would you expect to see fish gasping at the surface? (Higher DO, warmer water.)
If a pond owner did observe that (or just had an event similar to what Golfnut described), would there be any preventative measures that could be taken to avoid or lessen a fish kill?
