Extremely High Fire Danger for Southern California as Santa Ana Winds Develop

Tom Moore
Published: September 24,2016
The first significant Santa Ana wind event of the season will keep residents of Southern California, who reside in areas vulnerable to fires, on edge this weekend.
The National Weather Service in Los Angeles issued a red flag warning for the mountains of Los Angeles and Ventura counties through 3 p.m. Monday for the valleys and coastal mountains of those two counties.
A Fire Weather Watch is in effect for a larger area of Southern California from Sunday morning through Monday afternoon.
A combination of heat, low humidity levels and Santa Ana winds are responsible for the elevated fire threat.
(MORE: A Complete Rundown of All the Times Summer 2016's Weather Turned Weird and Violent)
Santa Ana winds are strong dry, downslope winds that often heat up and dry out Southern California. These winds can lead to fires.

Timeline For Santa Ana Wind Event

Red flag warnings and fire weather watches are in effect parts of Southern California through Monday afternoon.

Saturday
Hot temperatures will prevail, with highs in the 90s in the lower elevations, while the mountain areas will see highs in the 70s and 80s. The air will be very dry with relative humidity levels in the 10 to 15 percent range.
It will be a relatively weak Santa Ana event, but wind gusts could reach 35 mph in the mountains and 25 mph at lower elevations. The strongest wind gusts should remain confined to the mountains and valleys of Los Angeles and Ventura counties.
Saturday Night-Sunday
Conditions will be much more favorable for Santa Ana winds to increase and expand over a wider area. Sunday will be very hot and dry across Southern California. High temperatures will be well into the 80s in the mountains, while valley areas will see highs from the upper 90s to over 100 degrees.
Relative humidity levels are likely to dip into the single digits, so atmospheric conditions will be extremely dry. Northeast winds will continue, and gusts in the valleys and coastal mountains could reach 40 mph. Gusts near 50 mph may occur in mountain areas.
The highest wind gusts should be in Los Angeles and Ventura counties. It is during this period that the fire danger is highest; any fire that does ignite will likely spread very rapidly.
By Monday, winds will begin to subside, but it will remain hot and dry.

Forecast High Temperatures

(MORE: What Was Your Hottest Temperature This Summer ?)

Setup For Santa Ana Winds

Strong offshore winds (Santa Ana) develop across parts of Southern California this weekend.

Over the weekend, a high-pressure system will build into the Great Basin, while a trough of low pressure will establish itself along the coastal area of Southern California.
The general wind flow is from higher pressure to lower pressure. The greater the difference in pressure (pressure gradient), the stronger the winds will be.
In this case, the winds will be blowing from the northeast, across Southern California and toward the ocean. These are known as Santa Ana winds.
The winds will come down the west side of the mountains that are situated to the east, along the border with Nevada. As a result, the air sinks and warms and becomes very dry, so relative humidity levels drop significantly.
On Saturday, the Santa Ana winds will be modest, but the pressure gradient will increase on Saturday night and Sunday morning, resulting in increased winds.
MORE: California Fires-September 2016