The answer is; the water vapor in the westerlies condenses as it rises over mountain ranges
When the westerlies pass over the warm pacific ocean, the winds carry with them the vapor from evaporation of the oceans. As the winds blow and encounter large mountain ranges, the winds are forced upwards where it's cooler and begin to cool. The moisture in the winds condenses and precipitates as rain. On the leeward side of the mountain the winds that dry is therefore low on moisture content hence bring little rain.
