Open-Source Fire Science

Blog Post When the Winds Align

How California’s Wildfire Weather is Evolving

By Pyregence Team

California’s wildfire seasons are no longer just “bad” – they’re becoming exceptional. Half of the state’s largest wildfires have ignited in just the last few years, with 2020 alone rewriting the record books. But while forest management and climate change are part of the story, there’s another powerful driver of megafires hiding in plain sight: the weather.

A new study by researchers from the Pyregence Consortium explores the weather types (WTs) most closely tied to extreme wildfire growth in California—and how they are changing.

Decoding Fire Weather Patterns

Instead of relying on general climate metrics, the researchers focused on synoptic-scale weather patterns – the large-scale configurations of wind, pressure, and moisture in the atmosphere that shape fire behavior on a daily basis.

They found that just a handful of distinct fire weather types are responsible for the vast majority of burned areas in California. These include:

  • Santa Ana and Diablo Winds – fierce, dry, downslope winds that dominate fall fire seasons in Southern and Central California.
  • Thermal Lows – summer heat-driven pressure systems that create hot, dry, and unstable air masses.
  • Trough-driven patterns – atmospheric dips in the jet stream that supercharge fire behavior, especially in mountainous regions.

What’s Changing?

Using satellite fire records, climate reanalysis data, and large-ensemble climate model simulations, the study shows a clear trend:
  • Santa Ana and Diablo-type events increased during the 20th century, likely due to natural variability, but are projected to decrease in frequency this century under climate change.
  • Thermal low-pressure systems, on the other hand, are increasing—and fast. These patterns are forming more often in coastal and Sierra regions during hot summer months.
What’s behind these shifts? The answer lies in the temperature gradient between land and ocean—as the land warms faster than the sea, low-pressure systems develop more frequently. Changes in greenhouse gases and aerosol levels also play a role, altering moisture and air pressure across the state.

Why This Matters

Wildfires are driven by ignition, fuel, and weather. While we can’t prevent lightning or control population growth, we can improve how we forecast and prepare for fire-conducive weather patterns.

This study offers fire managers, utilities, and policymakers anticipate the types of days that can unleash massive fires. And it underscores the need to prepare for a future where extreme fire weather becomes more common in the summer, and less confined to traditional fall seasons.

Want to Dive Deeper?

Check out the full paper:

Prein, A. F., Coen, J., & Jaye, A. (2022). The Character and Changing Frequency of Extreme California Fire Weather. Journal of Geophysical Research: Atmospheres, 127, e2021JD035350. DOI: 10.1029/2021JD035350