There is a new plugin for the Python Interval program named california_monthly.
It is a modified version of the monthly_adj plugin and uses Evapotranapiration (ET) data from the California Irrigation Management Information System (CIMIS).
The plugin will automatically adjust the run time by a percentage for all irrigation programs according to the average conditions for your particular location.
I usually like to make things that are usable anywhere in the world but the plugin system allows for very specialized features. As a resident of California I am using the plugin on my system. I am also developing a more advanced plugin that can adjust the run times for each program individually based on the ET values for the days since the last run.