December 08, 2017

Firefighting crews in Southern California are hard at work battling destructive wildfires that ignited Dec. 4-6 in Los Angeles and Ventura counties. As of Dec. 7, nearly 190,000 residents in both counties were ordered to evacuate ahead of the fast-moving fires. Four large fires of note - the Thomas Fire in Ventura County and the Rye Fire, Creek Fire, and Skirball Fire in Los Angeles County - have burned a combined 46,943 hectares (116,000 acres). While personnel are working to restrict fire growth, containment of these major fires is still in its early stages.

Since Dec. 4, weather conditions in Los Angeles and Ventura counties have hampered the success of firefighters. The US National Weather Service (NWS) maintains a Red Flag warning for both counties, meaning that conditions will be ideal for wildfire ignition and rapid spread. Strong winds, known as Santa Ana winds, are the leading cause of the recent wildfire outbreak.

What are the Santa Ana Winds?

The Santa Ana winds, also known as 'devil' winds, are an annual weather event in Southern California. They typically develop in Northern Hemisphere fall/winter, but can occur at any point throughout the year. Santa Ana winds result from areas of high pressure located over the Great Basin. The wind is generated as air moves toward areas of low pressure near the Pacific Coast.

The current wind event was well forecasted, with the NWS issuing Red Flag warnings prior to the start of the wildfires. This year's Santa Ana winds have historical significance; they are being classified as one of the strongest events, in terms of both wind speed and duration, since 2007. Forecast models indicate that wind speeds should start to decline through Dec. 9; strong gusts of up to 72 kph (45 mph) will still be possible on Dec. 10 before a significant decrease in wind speed Dec. 11-12. Even after the winds die down, smoke could create reduced visibility and health concerns, particularly if large fires continue to burn after the air becomes stagnant.

Winds and Low Humidity: An Explosive Combination

The Santa Ana winds, in concert with ongoing drought conditions in Southern California, create ideal fire conditions. Classified as 'katabatic' winds, Santa Anas are down slope events; as cool air flows from higher elevations in the Great Basin to lower elevations along the Pacific Coast, the air compresses and warms. This process, in turn, leads to a hot and dry wind due to low relative humidity levels. Santa Ana winds encounter mountainous terrain as they transit toward the coastline; following the path of least resistance, the winds funnel through mountain passes and canyons. The channeling of winds through tight corridors can produce wind gusts in excess of 113 kph (70 mph).

Near hurricane-force winds, low humidity, and dry vegetation can lead to explosive wildfire growth, as demonstrated by the current phenomenon in Los Angeles and Ventura counties. Wildfire complexes can grow from a few dozen hectares to a few thousand hectares in just hours. Winds can also transport embers from wildfires and ignite spot fires at new locations a significant distance away from main complexes. The rapidly evolving an unpredictable nature of wildfires during Santa Ana wind events underscore the importance of heeding mandatory evacuation orders.

Daily Intelligence Alerts

Follow our daily intelligence alerts on the wildfire briefings, including an interactive map that shows the affected areas, perimeters, and evacuation zones.


About WorldAware

WorldAware provides intelligence-driven, integrated risk management solutions that enable multinational organizations to operate globally with confidence. WorldAware's end-to-end tailored solutions integrated world-class threat intelligence, innovative technology, and response services to help organizations mitigate risk and protect their employees, assets, and reputation.