We only had to say it was going to rain to one of our Ridgebacks, and he would look up at the sky, shake himself, and head off rapidly for home.
So, if they truly hate the rain... dont force them into long walks getting wet, let them just "do the necessary" and have a longer walk on a fine day!
One more plea... make sure your pets get their chips!
A micro-chip is such a good idea, and will give peace of mind... even though we all hope it will never be needed in reality.