Compound Atmospheric Drivers of the Catastrophic 2025 Los Angeles Urban Firestorm

December 29, 2025

A new paper entitled “Compound Atmospheric Drivers of the Catastrophic 2025 Los Angeles Urban Firestorm” was recently published in npj Natural Hazards. The study was led by Kristen Guirguis (CW3E), with co-authors Benjamin Hatchett (NOAA); Rachel Clemesha, Rosana Aguilera, Ian Campbell, and Mark Merrifield (Scripps Institution of Oceanography); and Alexander Gershunov and Dan Cayan (CW3E). The study identifies the atmospheric and surface conditions that contributed to the devastating January 2025 Los Angeles urban firestorm, which destroyed more than 16,000 structures and caused 31 fatalities. The research shows the event was driven by a rare “jet-forced Santa Ana” wind subtype, featuring an amplified upper-level ridge over the West Coast and a retrograding trough over the central and eastern US, that produced extreme northerly flow, strong subsidence, and mountain-wave activity that impeded aerial suppression efforts (Figure 1).

At the time the fires ignited, Southern California had received only ~4% of typical October-January precipitation, and the onset of the first wetting rain did not occur until January 28, which was the second latest arrival date in the record (Figure 2). While surface winds during the event were not unprecedented, their occurrence ahead of the first winter rain was highly unusual, leaving fuels critically dry and allowing the fires to spread explosively. Despite accurate multi-day forecasts, response systems were overwhelmed, illustrating how compound upper-level and surface atmospheric processes can create catastrophic fire weather even in well-developed urban areas.

The findings underscore the importance of long-range forecasting of AR variability, not just for flood preparedness and water resource management, but also for wildfires. This is particularly important given California’s increasingly variable hydroclimate and the growing exposure of communities at the wildland-urban interface.

The paper additionally highlights new understanding of the synoptic-to-mesoscale processes that drive Santa Ana-related firestorms and provides insight for improving fire preparedness, communication, and hazard mitigation.

Figure 1. Atmospheric drivers of the January 2025 Los Angeles firestorm. (a-c) An amplified upper-level ridge and retrograding trough produced a highly meridional jet and strong northerly flow over Southern California at 500-250 hPa. (d-e) Subsidence and mountain-wave activity further intensified surface winds and impeded aerial fire suppression. (f) A wind-rose comparison shows that the 2025 event (Firestorm’25) exhibited a distinct northerly “jet-forced Santa Ana” signature that differs from typical historical Santa Ana wind events, but is similar to a previous high-impact Santa Ana windstorm on December 1, 2011 (Windstorm’11). Figure 6 from Guirguis et al. (2025).

Figure 2. Timing of extreme offshore flow events overlain on seasonal accumulated precipitation. The timing of extreme offshore-flow events (orange dots) overlain on October-February cumulative seasonal precipitation (yellow-blue shading) shown as a percentage of a typical season’s total. Large red circles mark major Los Angeles fires discussed in the text (e.g., Sherwood/Newton, Bel Air, Topanga, Bradbury-Duarte, Kinneloa, Calabasas, Buckweed, Porter Ranch, Thomas, Woolsey); triangles indicate the 2024-2025 Mountain, Franklin, and Eaton/Palisades fires. Precipitation onset was extremely delayed in the 2024-2025 season, with the second latest arrival date on record (after 1962-63 with on onset date of January 30). Figure 8e from Guirguis et al. (2025).

Citation:

Guirguis, K., Hatchett, B., Clemesha, R., Aguilera, R., Gershunov, G., Campbell, I., Cayan, D. & Merrifield, M. (2025). Compound atmospheric drivers of the catastrophic 2025 Los Angeles urban firestorm. npj Nat. Hazards, 2, 103. https://doi.org/10.1038/s44304-025-00155-7

Additional References:

Cayan, D. R., DeHaan L. L., Gershunov, A., Guzman-Morales, J., Keeley, J. E., Mumford, J., & Syphard, A. D. (2022). Autumn precipitation: the competition with Santa Ana winds in determining fire outcomes in southern California. International Journal of Wildland Fire, 31(11), 1056-1067. https://doi.org/10.1071/WF22065

Guzman-Morales, J., Gershunov, A., Theiss, J., Li, H. & Cayan, D. (2016). Santa Ana Winds of Southern California: their climatology, extremes, and behavior spanning six and a half decades. Geophysical Research Letters, 43(6), 2827-2834. https://doi.org/10.1002/2016GL067887

Gershunov, A., Guzman Morales, J., Hatchett, B., Guirguis, K., Aguilera, R., Shulgina, T., Abatzoglou, J. T., Cayan, D., Pierce, D., Williams, P., Small, I., Clemesha, R., Schwarz, L., Benmarhnia, T., & Tardy, A. (2021). Hot and cold flavors of southern California’s Santa Ana winds: their causes, trends, and links with wildfire. Climate Dynamics, 57, 2233-2248. https://doi.org/10.1007/s00382-021-05802-z

Hatchett, B. J., Smith, C. M., Nauslar, N. J. & Kaplan, M. L. (2018). Brief Communication: Synoptic-scale differences between Sundowner and Santa Ana wind regimes in the Santa Ynez Mountains, California. Natural Hazards Earth System Science, 18(2), 419-427. https://doi.org/10.5194/nhess-18-419-2018