Tens of thousands of people have fled their homes as a brush fire in the Pacific Palisades has grown to nearly 20,000 acres on Thursday.

Santa Ana winds are caused by strong high pressure over the Great Basin region of the West creating a pressure gradient that amps up winds through the mountains, foothills and sometimes valleys of Southern California. In this case, the winds are getting a major boost from a strong upper-level low pushing southward across the state.