Can it REALLY rain cats and dogs? The phrase “raining cats and dogs” is used to describe a very heavy rainfall. Other phrases describing the same type of rain are “frog strangler” and “gully washer”. The last term might be the most accurate description. After all, heavy, intense downpours can bring flash floods – a wall of water washing down a dry streambed or gully. On the other hand, it would take an enormous amount of rain to unsettle an amphibian such as a frog, let alone “strangle” it. As for “raining cats and dogs”, while strange things have rained from the skies - including frogs and fish – there haven’t been any records of cats or dogs. The phrase might have originated in medieval London when heavy rains would flood the narrow streets drowning stray cats and dogs. People emerging after the storm would note that it must have been raining cats and dogs.
Mary Knapp, Weather Data Library