Los Angeles County is experiencing a once-in-a-generation wildfire event, spurred on by a once-in-a-decade windstorm. A true catalog of the destruction is almost impossible to assemble at this date, since any figures are quickly rendered obsolete by the ongoing infernos.
What is possible is to trace the natural factors that conspired to produce such an apocalyptic result, and explain how these factors will likely continue to cause devastation across the country in the coming days.
The first ingredient in this deadly cocktail was the recent drought. Despite intense rainfall in the northern part of the state, Southern California has been bone dry, with only 0.03 inches of rain falling at LAX in the last three months of 2024. This is a negligible amount compared with the more than 3.5 inches that fall during that period in an average year. As a result, vegetation that would typically be full of water by midwinter instead remained parched. Dry fuel is much easier and quicker to ignite than wet fuel, since the fire needs to expend less energy on evaporation.
The next ingredient was a high-pressure weather system perched over southern Idaho. In the Northern Hemisphere, high-pressure systems create winds that rotate in a clockwise direction around their center.
Since Southern California was located somewhere near 6:30 on this imaginary clock face, the result was winds traveling from east to west — the opposite of their normal direction. When these winds descended from the higher altitudes of the Mojave Desert toward the Pacific coast, the air warmed due to compression — the same effect that makes a bicycle pump hot when it is used to inflate a tire — and accelerated as it traveled through narrow mountain passes. These hot, dry, east-to-west winds caused by a high-pressure system are typical of the Santa Ana winds, which can afflict Southern California more than a dozen times per year.
So what made these Santa Anas so exceptional? The final ingredient, which elevated the winds and wildfires from risky to ruinous, was a low-pressure system spinning up over the Gulf of California. Winds rotate in the opposite direction around low-pressure systems, and since Los Angeles was located smack in between the zones of high and low pressure, the result was like a baseball stuck into a pitching machine.
With the combined force of both systems, wind gusts reached 100 miles per hour in the San Gabriel Mountains. High winds increase fire danger immensely by feeding oxygen to the blaze, carrying embers, and even bending the flames forward to ignite material in the fire’s path. These hurricane-force winds, combined with the unusually dry conditions, led to the unprecedented explosion of wildfires across L.A. County as even the tiniest spark was rapidly fanned into an inferno. Moreover, the treacherous weather conditions also hampered efforts to fight the fires, as air crews were unable to operate due to strong gusts.
And the destructive effects of this low-pressure system will not be over once it leaves Southern California behind. It is predicted to become a winter storm somewhere in East Texas before continuing through a string of Southern states, depositing ice and snow in a volume unusual for the region. This wintry mix can lead to dangerous travel conditions and significant power outages.
The fact that these two dangers share a single cause is not just a curiosity — “fire and ice,” “hot and cold” — but rather an illustration that many different forms of extreme weather are controlled by one phenomenon: the jet stream.
The jet stream is a band of strong winds that travel from west to east across the globe. Normally, the jet stream resembles a real stream, slicing directly across the country with perhaps a few small meanders. But if the jet stream starts to twist and turn like a winding river, it produces powerful high- and low-pressure systems that are responsible for heat waves, snowstorms, floods and strong winds.
Worryingly, some scientists believe that climate change could produce a more “curvy” jet stream, which would lead to more instances of back-to-back disasters like the U.S. is experiencing now and like the world experienced during the summer of 2021, when heat waves and floods seemed to ricochet across the globe. So far, this effect has largely been observed in climate model projections rather than observations, but if it does emerge in the real world, it is expected that it will be most significant in the winter, when the jet stream has the most energy.
Whether this theory is correct or not, climate change has been intensifying a variety of extreme weather events, including droughts. In the case of the disaster that is now unfolding, the unusually dry conditions played a crucial role in setting the stage for extreme wildfires; a similarly powerful windstorm in Pasadena in December 2011 was preceded by a damp November, helping to prevent any significant fires.
While we can never attribute a given event to climate change with 100% certainty, it is impossible to ignore that the main source of change over the intervening decade is the ever-increasing amount of carbon dioxide entering Earth’s atmosphere.
If this trend is not rapidly reversed, we can expect that these extraordinary wildfires will become horribly more ordinary.
Ned Kleiner is a scientist and catastrophe modeler at Verisk. He has a doctorate in atmospheric science from Harvard.
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