From Glossary of Meteorology
In the United States, a dry, foehnlike desert wind in southern California, generally blowing from the northeast or east, especially in the pass and river valley of Santa Ana, California, and other nearby passes, where it is further modified as a mountain-gap wind.
It is driven by strong pressure gradients from an anticyclone over the Great Basin of the western United States. It blows, often hot and sometimes with great force, from the deserts to the east of the Sierra Nevada Mountains and may carry a large amount of dust. The combination of heat, dryness, and strong winds make it an especially hazardous fire weather condition. It most frequently occurs in late fall and winter (October–March); when it comes in spring, however, it can do great damage to fruit trees.