A look at how intense gusts rarely before seen in Southern California stoked blazes in urban areas in and around Los Angeles.
Los Angeles has experienced one of its worst disasters as powerful Santa Ana winds, sometimes at hurricane strength, swept down the surrounding mountains and pushed wildfires into several neighborhoods starting Tuesday.
Five blazes were sparked Tuesday and Wednesday: the Palisades, Eaton, Kenneth, Hurst and Lidia fires. The latter three were under 1,000 acres as of Sunday.
With the winds so strong, there was little firefighters could do to control the flames.
Jon Keeley, a research ecologist with the U.S. Geological Survey and adjunct professor at UCLA, explained via UCLA’s The Conversation what causes extreme winds like this in Southern California, and why they create such fire risk.
RELATED: Why is California’s wind blowing so hard in January?
What causes Santa Ana Winds?
“These winds occur when there is high pressure to the east, in the Great Basin, and a low-pressure system off the coast. Air masses move from high pressure to low pressure, and the more extreme the difference in the pressure, the faster the winds blow.”
Topography also plays a role
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“As the winds rush downslope from the top of the San Gabriel Mountains, they become drier and hotter. That’s a function of the physics of air masses. By the time the winds get to the point where the Eaton fire broke out in Altadena on Jan. 7, it’s not uncommon for them to have less than 5% relative humidity, meaning essentially no moisture at all. These strong, dry winds are often around 30 to 40 mph. But they can be stronger. The winds in early January were reported to have reached 60 to 70 mph.
“These strong, dry winds are often around 30 to 40 mph. But they can be stronger. The winds in early January were reported to have reached 60 to 70 mph.”
Have the Santa Ana winds changed over time?
“My colleagues and I recently published a paper comparing 71 years of Santa Ana wind events, starting in 1948. We found about the same amount of overall Santa Ana wind activity, but the timing is shifting from fewer events in September and more in December and January. Due to well-documented trends in climate change, it is tempting to ascribe this to global warming, but as yet there is no substantial evidence of this.
“California is seeing more destructive fires than we saw in the past. That’s driven not just by changes in the climate and the winds, but also by population growth.
“More people now live in and at the edges of wildland areas, and the power grid has expanded with them. That creates more opportunities for fires to start. In extreme weather, power lines face a higher risk of falling or being hit by tree branches and sparking a fire.”
You can read the whole article on UCLA’s website here.
The way terrain and wind impacted the fires:
This 3D map from Cal Fire provides an idea of the steepness of the terrain for the largest of the L.A.-area blazes, the Palisades fire. As of Friday it had burned more than 20,000 acres. This is not the first Palisades fire – a 1,202-acre blaze took place in the area in May 2021, believed to be arson. Cal Fire
What’s ahead?
The Santa Ana Wildfire Threat Index is reporting moderate Santa Ana wind conditions continue Sunday, Jan. 12. There will be somewhat of a lull later this afternoon through Monday morning, then another long duration moderate event will begin. Exceptionally dry fuel conditions are present over Southern California.
End of the wind?
From 1950 to 2020, there were 3,219 Santa Ana wind days, and fires ignited on 12% of them. In this period there were 22,704 nonwind event days, and 12% of those days had ignitions. Winds contributed to much larger burns. The number of Santa Ana wind days will decrease but continue through February. A marginal Santa Ana wind event is expected through Wednesday.
What went wrong last week
The winds originated in the Nevada and Utah regions of the Great Basin, where high pressure produced winds that are flowing into Southern California. The fires were made worse by a phenomenon called mountain waves. Mountain waves are oscillations in airflow that can happen when wind blows across a mountain or hill. If a mountain wave occurs and a fire is present, it may become unexpectedly severe as the strong downslope wind fans the flames. A few areas in Los Angeles County had gusts over 100 mph, sharply limiting suppression efforts by land and air.
A red flag alert is usually issued when a combination of high winds, low humidity and high temperatures is expected.
Winds are created by flow from high to low pressure centers. Wind maps have lines called isobars. “Iso” means “equal,” and a “bar” is a unit of pressure, so an isobar means “equal pressure.” The closer the isobars are drawn together, the more quickly the air pressure changes. The map on the right shows how close the lines were, prompting a red flag alert from the National Weather Service.
Sources: California Public Utilities Commission, Cal Fire, FEMA, NOAA, California Fire Science Consortium, UCLA, Business Insider, International Journal of Wildland Fire
