Extreme Diablo Wind Events in Berkeley: Duration and Risk—Summary
The general expectation in Berkeley is that, while the Berkeley Wildland Urban Interface (WUI—“Berkeley Hills”) is dangerously exposed to wildfire, the flatlands in Berkeley are reasonably safe from it. Conventional wisdom is that average Diablo wind events in the East Bay average 8 hours, which could allow some of the city core to avoid the brunt of a wildfire penetrating from the east Berkeley ridge.
The severity of a wildfire coming from the East Bay Regional Park District (Tilden Park) into Berkeley can depend upon the duration of the Diablo wind event that accompanies it. The longer the event (before a change in weather), the more destructive the fire may be. Recent reanalysis of past Diablo wind events in the past 40 years, however, indicates that extreme Diablo events are likely to last 12-24 hours, sometimes much longer, with an average duration near 20 hours.
Destruction could be significantly more extensive than previously expected, potentially reaching the coast. The risk to the Berkeley flatlands needs to be reevaluated upwards.
Extreme Fire Weather
Once an ember fire turns into a structure-to-structure fire, in unfavorable weather circumstances firefighters cannot stop it until the weather changes. For us, in the Bay Area, unfavorable weather is also called, specifically in Berkeley, Extreme Fire Weather: very strong Diablo winds coming roughly from the NE, very low humidity and generally high temperatures. This combination of factors, statistically, accompanies the large majority of uncontrollable fires in California:
Speed of wildfire progression
The 1923 Great Berkeley Fire, which remained, for many decades, the worst ever fire in California, started in what is now Tilden Park and came over the ridge. We know that it took the fire 2 hours and 20 minutes to cover a mile, downslope, from the edge of the city to Shattuck and Hearst:
source: City of Berkeley
This represents a rate of 0.4mile per hour. In reality, it is possible that the fire would progress more slowly today, since, in 1923, it was able to run down along Codornices Creek as a very fast grass fire. The area is now built up and would not allow such a fast pace. At the same time, both Palisades and Eaton fires, in 2025, registered progression speeds between 1 and 2.3 miles per hour (downslope for Pacific Palisades) during critical moments of expansion (Fire Spread, Intensity, and Emissions Observations by Multiple Satellites: The Southern California Wildfires of January 2025 - Li - 2026 - AGU Advances - Wiley Online Library). In this context, a downslope speed of 0.4 miles per hour does not appear outlandish.
Diablo wind events in the Bay Area: the data
UC Davis scientists have conducted excellent research on Bay Area wildfire weather patterns. In “Fire Season,” between September and February, they counted 226 Diablo events between 1979 and 2018:
source: UC Davis
Of these events, approximately 37 (16%), marked in black in the diagram, are extreme Diablo wind events per their Fosberg Fire Weather Index. These extreme Diablo wind events roughly correspond to the incidence of Extreme Fire Weather.
Average duration of Diablo events
source: UC Davis
The distribution of Diablo Wind Events shows 65% of them lasting less than 12 hours, and the duration average across all events to be approximately 11.5 hours. The exact magnitude of this number depends upon the definition of Diablo events. Other assessments with slightly different definitions come with slightly different but similar numbers (8 hours is frequently quoted).
Average duration of extreme Diablo wind events
source HelpBerkeley.org (counts approximated)
If we isolate extreme Diablo events, however, the situation is quite different, and most extreme Diablo events last much longer:
- Just over 15% of extreme Diablo wind events are under 12 hours old.
- More than 80% of extreme Diablo wind events last more than 12 hours
- Almost 50% of extreme Diablo wind events last more than 24 hours!
In fact, the average duration of extreme Diablo events is almost 20 hours.
We need to reassess wildfire risk for the Berkeley Flats
In most places there are only 3 to 4 miles between the eastern edge of Berkeley and the coast. Almost half of extreme Diablo wind events last more than 24 hours. Even with a low progression speed on the ground for a wildfire moving west, in that time an uncontrolled wildfire could could easily cover the whole ground between the ridge and the coast.
Comparing Berkeley and the Pacific Palisades Fire (scale approximated)
Berkeley, in area, as a whole (17.7 square miles), represents no more than half of the burnt area of the Palisades Fire (36.7 square miles). A similar scenario in Berkeley to that of Pacific Palisades is possible, and not unlikely. We need to reassess our perception of wildfire risk for the Berkeley Flats.
Michel Thouati is the CEO of HelpBerkeley.org, a wildfire thinktank and FireSafe Council