Welcome to the board, mila.
Here are some links to previous discussions of this subject on the board. It's fun to read some of the old posts. Here's one from way back:
http://wordsmith.org/board/showflat.pl?Cat=&Board=words&Number=1239
And another of the same vintage:
http://wordsmith.org/board/showflat.pl?Cat=&Board=wordplay&Number=3087
And lastly, something a little more recent that gives less info on "raining cats and dogs" but lots of other weather expressions.
http://wordsmith.org/board/showflat.pl?Cat=&Board=wordplay&Number=84888

Can anyone think of anything to add? Seems like this group always has their two cents in hand