As an air parcel reaches the windward side of the mountain, it rises upwards and quickly expands and cools off. With the right conditions, clouds will form and rain will occur. As the air parcel passes over the top, it’ll being to sink and compress along the leeward side. This is why you have the ran shadow effect on the leeward side!
In Southern California, our Santana winds are due to the same physical processes. As the warm winds blow from the desert and into the mountain canyons, the air parcel is compressed and super-heated. This leads to intense drying of flash fuels, high wind, and with our overgrown forests, it becomes the perfect condition for the fire storms we experience.