Did a Mexican General Cause the California Wildfires?

Steven B. Newman, Ph.D.
Central Connecticut State University Weather Center

              
     The wildfires that have brought death and destruction to many communities in southern California during late October, 2003 are finally under control.  Over 3500 homes were lost, and more than 20 people were killed by the fires, which have also destroyed more than 743,000 acres of brush and timber.  Who (or what) is responsible for starting the conflagration?

The fires began, as many southern California wildfires do, as a result of hot, dry late summer conditions.  These are not unusual in California, as summer tends to linger into late October.  However, the persistent lack of rainfall along with a particular type of wind associated with a long-dead Mexican Army general turned the area into a giant pile of kindling, just waiting for any kind of spark to set it all burning.

During the summer months in the southwest United States, the heat of the desert floor in Arizona and New Mexico can cause air to rise, creating a low pressure system, called a "thermal" low.  The counterclockwise circulation around this low will draw air in from the Gulf of California.  This air will cross the San Gabriel mountains, east of the Los Angeles/San Diego area and flow across these regions from the northeast.  Any moisture in that hot air will condense out over the northeast slopes of the mountains, then descend the southwest slopes, becoming hotter and drier.

This very dry downslope wind, which can attain speeds in excess of 75 mph,  and temperatures near 100^oF, is locally known as the "Santa Ana" wind.  It gets its name from the great Mexican general, who commanded the Mexican Army in the 1840's Mexican War against California and Texas.  When this wind develops and persists, the fire danger in southern California reaches very extreme levels.  Once fires begin, the winds make it easy for sparks to jump from one group of burning trees and homes to another.  The satellite image shown above was taken during the height of the fires.  Note that the smoke plumes east of Los Angeles and San Diego are moving toward the southwest--a clear indication of the presence of Santa Ana winds (photo courtesy of NOAA).

Until the Santa Ana subsided, and the weather changed from hot and dry to cooler and rainier, the firefighters had a tough time gaining the upper hand.  Now that the fires are just about out, California officials are left to consider how they can face the threat of similar fires in the future, and what steps they need to take to ensure that any such fires are quickly brought under control.