December 10, 2024
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How Santa Ana winds fan the Malibu fire
Dry weather and an extreme Santa Ana wind event have contributed to the explosive growth of the Franklin Fire in Malibu, California.
A firefighter pulls on a water hose near a building where the Franklin Fire is burning palm trees on December 10, 2024 in Malibu, California.
The Franklin Fire, which broke out Monday night north of Pepperdine University in the Malibu area of Los Angeles County, grew rapidly within hours of burning more than 2,200 acres due to extremely dry weather — and an unusually violent storm in the region. Santa Ana winds.
Santa Ana winds are a common driver of fast-moving and damaging wildfires in the area, as they can quickly fan and spread flames. Some of the gusts near the last fire have reached more than 50 kilometers per hour.
Santa Ana’s winds are the result of a particular meteorological setup: “What usually happens is: there’s a low-pressure (atmospheric) system” that moves up from California and into Washington State and Oregon and then down south through Nevada and Arizona, he says. Mike Wofford, a meteorologist at the National Weather Service’s Los Angeles office. “We call that the ‘inside slider.’ As this low pressure system moves out, an area of high pressure moves in behind it. “And it’s that high pressure in Nevada and that lower pressure in California that’s driving those winds,” Wofford explained, “because the winds are going from top to bottom.”
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The greater the gap between the high and low areas, the faster the wind will be, and right now the high pressure system over Nevada is quite strong, Wofford says. As the winds move into southern California, they are channeled through its many narrow mountain canyons; this causes the wind to speed up. “It’s like squeezing a balloon and letting the air out,” says Wofford. (The Santa Ana winds take their name from the Santa Ana Canyon, which lies between the Santa Ana Mountains and the Chino Hills.) In some parts of the mountains around Los Angeles, gusts of over 70 miles per hour have been reported. A gust of 93 miles per hour was recorded on the Magic Mountain Truck Trail. Today’s wind event is similar to the one in early November that fanned the flames of the Mountain Fire that burned hundreds of structures in Ventura County.
Wind events in Santa Ana are common at this time of year, but the winds yesterday and today are unusually strong. And, because they are hot and dry by nature, these winds especially help to fuel the fires. As winds move downslope over deserts, the air is compressed and heated, thereby drying out. These dry winds can further dry out already dried vegetation, igniting from the tiniest spark.
“Even in normal years we’re a desert,” Wofford says. The vegetation in the Franklin Fire area was already dry after months of almost no rain. Cold rains tend to arrive after the region’s hot, dry summers, reducing fire risk. But this year’s rains have not yet started in this area.
Forecasters issued an “extremely hazardous condition” warning for the area Monday night into Tuesday evening due to wind and dry conditions. According to Wofford, the relative humidity was as low as 3 percent. “It’s all very crisp.”
Winds are expected to die down Tuesday and into Wednesday, but conditions are still ripe for more fires—and for the Franklin Fire to spread further. Evacuation orders have been issued for parts of the coast, and part of the Pacific Coast Highway is closed, as are schools in Malibu. The fire is currently 0 percent contained.
