A recent lack of rainfall fueled the ferocious Los Angeles wildfires of January 2025 by leaving large stocks of vegetation dry and easily ignited. How unusual is that? Is it a new phenomenon? Here are some rainfall statistics for the area which may help us understand how this season differs from years past.
For records of monthly totals of Los Angeles rainfall, I looked at data for the US California Climate Division #6, South Coast Drainage. This is a narrow strip along the coast from California's southern border to about half way to the Bay Area. Data is from the National Centers for Environmental Information's Divisional Time Series page.
Southern California is notoriously dry, with annual rainfall of about 17 inches. That's about half of what the East Coast gets. However, not every month is dry. As the figure shows, rainfall in the LA region is extremely seasonal. The winter peak is about 4 inches a month. If sustained all year, that would be a very wet total of 48 inches. The dry summers have much less than an inch of rain a month. Sunny California.
The figure also shows the great year-to-year variability. While average rainfall in December, for instance, was about 3 inches, the 6th wettest December (80th percentile precip) had almost 5 inches, and the 6th driest (20th percentile) had less than an inch.
Because of the great variability, Los Angeles weather is much more interesting than what's portrayed in Steve Martin's LA Story. No two years look alike. Both winter 2023 and winter 2024 had very high rainfall, but in 2023 the heavy rain came in January and March, whereas in 2024 it came in February. Earlier, December 2021 started off the winter with heavy rainfall, but January-March was a bust.
In contrast, Fall 2024 had barely any rain, even in the normally wet month of December. Since summer is the dry season, that means that the southern California coast had almost no precipitation since last April. The combination of a wet winter followed by drought is especially dangerous, because the rains stimulated more plant growth, which meant more fuel when the plants dried out.
Bars are color-coded for month of the year, with Jan-Feb-Mar in dark blue, etc. When all months are shown, the winter peaks tend to overshadow the rest of the year. How frequently are [light-blue colored] October, November, and especially December, this dry? The figure below shows these months ("OND") for each year going back to 1971. We can see that a dry OND is unusual but not unheard of. 2017 had even less rain than the miniscule amount in 2024, and a handful of other years (1999, 1990, 1989, 1980) were almost as dry.
Rising temperatures driven by societal emission of greenhouse gases are likely increasing the variability of rainfall throughout much of the world. Rain events are more intense but more rare, thus driving more frequent droughts. Are the dry OND's in 2017 and 2024 a result of global heating? To answer this question, I went back all the way to 1905 to compare recent climate to the past. For each of the 6 15-year periods between then and now, I plotted the minimum, maximum, and average rainfall.
The resulting plots don't have any trends, which is confirmed by linear fits. It is true that the year with the minimum OND rainfall had less rain in the last 15-year period compared to most other years, but 1920-1935 also had a such a low value. Note that the minimum rainfall in the most recent 15-year period came in 2017 (0.12 in); 2024 had 0.52 in, similar to the minimum in several other periods.
Does this rule out an influence from climate change? Not exactly. It is possible that the probability of having a dry OND was increased by climate change by a small amount, and that further heating of the Earth will make the effect more noticible. It is also possible that drying of the vegetation was made worse by the higher temperatures associated with climate change. Finally, it is possible that there is some link between global heating and the other weather component of the fire: the unusually strong Santa Ana winds that fanned the flames. I am not aware of any such mechanism, but I do not know too much about such winds.
What I can say is that the data suggests that the kind of extreme lack of rainfall that helped set the stage for the Los Angeles fires this month could have happened even without global heating.
Last modified: 12 Jan 2025