Santa Ana and Diablo Winds: Why Fall Is California's Most Dangerous Fire Season

Most of the country thinks of fire season as summer. In coastal California, the most destructive fires tend to come later — in the fall and early winter, when the hills are at their driest and strong, dry winds blow from the interior out toward the ocean. In Southern California they're called Santa Ana winds; in the Bay Area and Northern California, Diablo winds.

What makes these winds different

On most days, California's air flows in from the Pacific: cool, damp and slow. A Santa Ana or Diablo event flips that. High pressure builds over the Great Basin — Nevada and Utah — and air spills downhill toward the lower pressure along the coast. Three things happen on the way down:

• It speeds up. Air is funnelled through mountain passes and canyons, so gusts are often far stronger there than in open terrain a few miles away.
• It warms up. Sinking air is compressed and heats as it descends, so a Santa Ana day can feel unusually warm for the season.
• It dries out. Warming air holds more moisture without gaining any, so relative humidity can fall into the single digits.

Fire behaviour follows directly. Wind drives flames forward and throws embers well ahead of the fire front, low humidity makes those embers far more likely to start new spot fires, and the speed leaves crews and residents with very little time. Several of California's most destructive fires — including the January 2025 fires in the Los Angeles area — were driven by this kind of offshore wind.

Why fall and early winter

Santa Ana conditions can occur from roughly September through spring, but the danger peaks when two things overlap: the strongest wind events of the year and vegetation that has had all summer to dry out. Once winter rains have soaked the hills, the same winds are much less dangerous. In a year when the rains arrive late, that risky window stays open longer — which is why a dry November or December worries fire agencies far more than a windy one alone.

How to read a Santa Ana forecast

You don't need to be a meteorologist. Watch for these signals in the days before an event:

• Fire Weather Watch, then Red Flag Warning from the National Weather Service. A Watch gives one to three days' notice; a Warning means the conditions are imminent. See our guide to Red Flag Warnings for what each one means.
• "Particularly Dangerous Situation" wording. Some NWS offices add this to a Red Flag Warning when the forecast is unusually extreme. Treat it as the top of the scale.
• The Santa Ana Wildfire Threat Index (SAWTI). Developed by the U.S. Forest Service with San Diego Gas & Electric, it rates each event from Marginal to Extreme by combining wind, humidity and how dry the vegetation is.
• Wind direction from the northeast or east in the forecast, paired with humidity below about 15%.
• Public Safety Power Shutoff notices from your utility. Utilities may cut power to lines in high-risk areas during strong winds, sometimes for a day or more.

What to do before the wind arrives

The day before

• Charge phones, power banks and any medical devices — a power shutoff can last longer than the wind.
• Fill your car's tank. Gas stations can't pump fuel without power.
• Put your go bag by the door and look up your evacuation zone so you'll recognise it in an alert.
• Bring in patio cushions, doormats, brooms and anything else that burns from around the house. Embers land in exactly these places.
• Make sure your phone will alert you overnight. Emergency alerts should come through Do Not Disturb, but check your settings.

During the event

• Avoid anything that can throw a spark outdoors: mowing, welding, grinding, towing with dragging chains, parking on dry grass.
• Keep a manual way to open your garage door in mind — automatic openers stop working without power.
• If a fire starts nearby, don't wait for a knock on the door. With offshore winds, conditions can change in minutes. Learn what Warning and Order mean before you need to.
• If smoke reaches you, keep windows closed and use a filter if you have one. Our smoke and air quality guide covers what the AQI numbers mean.

Watching the wind on a map

Wind is the variable that changes fastest during these events. On FireRadar's map, you can turn on the animated wind layer to see whether air is moving offshore across your area and how a fire's likely direction lines up with where you live. Save your home as a location, and the app will push an alert if a fire starts or grows within your chosen radius — useful precisely because these fires so often start at night.