Here's an article I found on SI.com about Holmgren and how the fact that this being his last season is going to affect our team
"It's raining cats and dogs!"
In the 1500s, houses had thatched roofs-thick straw-piled high, with no wood underneath. It was the only place for animals to get warm, so all the cats and other small animals (mice, bugs) lived in the roof.
When it rained it became slippery and sometimes the animals would slip and fall off the roof. Hence the saying "It's raining cats and dogs."