CIMIS (California Irrigation Management Information System)
CIMIS is a California-only weather service used primarily by farmers and researchers. It is not recommended you use this service unless you are an advanced user.
To use CIMIS, you will need an account with them. Go to their website and register. Once registered, log back into the account page (top right), scroll to the end, and select the "Get AppKey" button. Copy that key and save all changes.
Then from the web interface at https://my.rainmachine.com you can go to Settings, Weather, Weather Services, and click on "CIMIS." Here you can click "Enable" and enter your "AppKey."
Now enter your local station ID. Here you can find the list of all the stations used by CIMIS.