Basically, when the West was flat, the airflow was zonal, meaning along latitude lines. With a solid 2 mile high barrier in place, it is easier for the westerly air currents to "go around" the Sierra Nevada then to continue zonal flow.
We still get winter rain because the whole atmospheric circulation pattern in the Northern Hemisphere moves south in the winter, and the Jet Stream occasionally finds its way over Southern California.
Comments and feedback: Tom Chester
Last update: 6 November 2005.