The weather set-up is key to the Santa Ana winds blowing. They occur when a large area of high pressure becomes established over the interior of western US, centred around the Great Basin, which includes much of Nevada and parts of Utah, Idaho and south-east Oregon. These are generally dry, desert areas, which means the winds that arrive in California are lacking in moisture.
With winds flowing clockwise around high pressure systems, they move south and west into southern California, where they descend over the inland Sierra mountains. As the air descends, it dries further and accelerates in speed. Wind speeds can increase further as they funnel through the valleys and gorges in the area, much like water increases its speed as it passes through a hose.