Most people have an over-simple idea of how wildfires burn into urban areas. To this way of looking at it, fires cross open land — whether treed or scrub — making their way forward as a so-called "flaming front". If such an advancing front approaches structures (i.e., homes, businesses, etc.), such buildings can be overrun by the fires, thus becoming involved themselves.
So far, so good, right? No, not necessarily. Because what this naive model misses is the dynamic nature of how wildfire propagates.
Most wildfires are wind-driven, even if the wind is partly of their own creation. It drives the fire forward, sometimes at speeds as high as 10 km/h and faster. At the same time, however, the wind floats burning embers well ahead of the fire front. In the recent Guejito/Witch Creek fire in San Diego County, such embers from the approaching fire began arriving in communities in eastern San Diego almost an hour before the main fire. Given the know location and speed of the main fire that means the embers traveled as much as 9 km, which should upend ideas about who is at risk (and when) in such systems.
The advancing embers do more than simply warn of a fire’s advance, however. They also can cause ignition themselves, whether in vegetation, or in homes or buildings directly. In the case of the Guekito/Witch fire, a new NIST report shows ember-related burning of homes well before the front front arrived at 3:45am. Check the following figure to see the timelines and ignition sources in a more visual way:
It is a useful perspective on how wildfires arrive earlier than a simple model would suggest, and how fires leap past urban perimeters. These are, after all, dynamic systems, so we should not be surprised at how they confound expectations.
NIST Technical Note 1635
A Case Study of a Community Affected by the Witch and Guejito Fires
Alexander Maranghides / William Mell