California Thomas Fire 2017

Situation Update No. 10 on December 16, 2017, 05:53 AM (UTC)

Ever since the Thomas fire erupted Dec. 4, 2017, it has steadily burned its way up the list of California's largest wildfires since the Great Depression. That list, however, does not include what some consider to be California's largest known wildfire - the 1889 Santiago Canyon fire, which scorched 300,000 acres in Orange, Riverside and San Diego counties. The official California Department of Forestry and Fire Protection list of the top 20 wildfires dates back only to 1932 because records before then "are less reliable," the department says. The largest fire on the list is the 2003 Cedar fire, which burned more than 273,000 acres and killed 15 people.

Keely notes that the Thomas fire is a "real anomaly," because it is tied to an unusually long Santa Ana wind event. "Normally when we get a strong Santa Ana wind event, it lasts two or three days and then the winds die down and the fire is then contained," Keely said. "But this fire is occurring during a Santa Ana wind event that apparently isn't even over yet. ... The longer the Santa Ana winds blow, the larger the fire grows. That's an important characteristic of the Thomas fire." The Santiago Canyon fire ignited under Santa Ana wind conditions and that was coupled with the fact that there were very few people on the landscape to fight that fire, Keely said. Contemporary news reports said the fire began in a sheep herder's camp. In a recent paper on fire and climate, researchers evaluated the correlation between the number of Santa Ana wind events each year and the area burned and found no relationship. "We get Santa Ana winds every year.

It's just, some years, somebody ignites a fire during them, and when that happens, they tend to get really large," Keely said. "But the really large ones, we found, were related to prior drought." He pointed to the 2003 Cedar fire, which was preceded by more than a year of drought. "We think that this fire, the Thomas fire, is likely very large in part not just because the Santa Ana wind event is long, but there was this very extreme drought between 2012 and 2014." Often drought is thought of as affecting the fuel moisture of vegetation. But researchers' analysis suggests otherwise, Keely said. "What extreme droughts do is they cause dieback of the vegetation," he said. "Basically a canopy of the vegetation dies and oftentimes the entire plant dies. So you have lots of dead vegetation out on the landscape from the drought. And when the Santa Ana winds blow embers ahead of the fire front, they'll ignite spot fires, but only if they land on dead vegetation." The USGS is now gearing up to start a study to look at the amount of dieback in the Thomas fire prior to this fire event, he said. "I suspect there must have been a lot of dead vegetation and that enhanced that fire," Keely said.