Sort:  

It's the rain shadow. Both the Rockies and the Cascades (the western coastal mountains) have big rain shadows on the eastern slopes. The moisture comes in off the Pacific and 'rains out' before it crosses. Then it re gathers and rains out later.

Thank you @bigtom13, I've never heard of that, most interesting... never too old to learn hey ;)
I don't think we have anything like that here in South Africa.