A Fire Weather Watch means the National Weather Service expects critical fire-weather conditions are possible in about the next 12 to 72 hours. A Red Flag Warning means those conditions are happening now or expected within about 24 hours.
The Watch is the earlier, lower-confidence alert; the Warning is the more urgent, higher-confidence alert. Neither means a wildfire has started. Both describe weather, mainly low humidity and wind over dry vegetation, that would let a new fire spread fast. NWS Los Angeles/Oxnard and San Diego issue both for Southern California, zone by zone.
What Is the Difference Between a Fire Weather Watch and Red Flag Warning?
A Fire Weather Watch and a Red Flag Warning differ in timing and confidence. The NWS issues a Fire Weather Watch when critical fire weather may develop but its timing, location or strength is still uncertain, giving residents and fire agencies time to prepare. The NWS issues a Red Flag Warning when those conditions are happening or expected within about a day, with enough confidence to commit to the alert.
A Watch often becomes a Warning as a storm, wind event or dry spell becomes clearer in forecasts and observations. In fast-developing situations, the NWS can issue a Warning with no Watch first.
A Red Flag Warning is a weather alert, not a fire alert. The National Weather Service does not track active fires, report containment or order evacuations; other agencies do, as explained below. A Red Flag Warning means wind, humidity and fuel dryness have reached a level where any ignition, from lightning, equipment or people, would likely spread fast and be hard to control.
| Feature | Fire Weather Watch | Red Flag Warning |
|---|---|---|
| What it signals | Critical fire-weather conditions are possible | Critical fire-weather conditions are occurring or imminent |
| Typical lead time | Roughly 12 to 72 hours ahead | Generally within about 24 hours |
| Forecast confidence | Moderate; timing and intensity still firming up | High; NWS is committing to the alert |
| Key ingredients assessed | Forecast wind, relative humidity, and fuel dryness trending toward zone criteria | Wind, relative humidity, and fuel dryness meeting or exceeding zone criteria |
| Issuing authority | NWS Los Angeles/Oxnard or NWS San Diego, by zone | NWS Los Angeles/Oxnard or NWS San Diego, by zone |
| What it does not mean | A wildfire is confirmed or guaranteed to start | A wildfire has started, or that evacuation is ordered |
What Conditions Create Critical Fire Weather?
Critical fire weather combines low relative humidity, strong or gusty wind and dry vegetation; one factor alone rarely qualifies. Low humidity dries out grass, brush and small dead branches, which then ignite easily and burn hotter. Wind feeds oxygen to a fire and carries flames and embers ahead of the fire front. That process, called spotting, lets fire jump roads, firebreaks and creeks.
Forecasters track fuel dryness two ways: live fuel moisture, the water in living plants, and dead fuel moisture, the water in dead branches, grass and leaf litter. Both drop through Southern California's rainless summer and fall.
Lightning is a separate ignition source that needs no wind. A thunderstorm with frequent lightning and little rain, called dry lightning, can ignite dry vegetation in a remote area where a fire burns unseen for hours.
The NWS lightning safety program covers lightning hazards generally. Dry-lightning risk peaks in Southern California's mountains and deserts during the July-to-September monsoon. No single statewide humidity or wind value triggers a Watch or Warning everywhere.
NWS Los Angeles/Oxnard and NWS San Diego each set their own numeric criteria for every fire-weather zone, because climate and terrain differ sharply across their areas, as the next section explains.
Why Do Warning Criteria Vary by Zone?
Warning criteria vary by zone because humidity or wind that would be alarming at the coast is nearly normal in an inland valley or the desert. NWS Los Angeles/Oxnard and NWS San Diego divide their areas into dozens of fire-weather zones based on terrain, elevation and climate, not city or county lines. A coastal zone near Downtown Los Angeles usually has higher humidity from the ocean, so its criteria target dryness that is unusual for the coast.
Inland valleys, foothill zones near Pasadena and higher mountain zones are drier on an average day, so their thresholds flag conditions unusual even for their drier baseline.
Zone criteria also reflect vegetation and season. Grassy foothill zones dry out and become flammable earlier in the season than chaparral at higher elevation, and desert zones account for monsoon storms. Each NWS office reviews its zone criteria periodically, so check the current NWS Los Angeles/Oxnard or NWS San Diego zone forecast instead of relying on a rule of thumb.
For residents, the alert itself, Watch or Warning, matters more than any humidity or wind number, because the numbers differ by zone and can change between reviews.
How Do Santa Ana Winds Fit In?
Santa Ana winds are one of the most common and dangerous causes of Red Flag Warnings in Southern California. Santa Ana winds are dry offshore winds that flow from high pressure over the Great Basin through the region's mountain passes and canyons. They bring strong or gusty wind and humidity that can drop into the single digits as the sinking air compresses and warms. When a Santa Ana event meets dry vegetation, usually in fall after a long rainless summer, NWS Los Angeles/Oxnard and NWS San Diego often issue Red Flag Warnings for many zones at once.
Santa Ana winds are not the only trigger. A dry cold front with little rain can bring gusty wind over dry vegetation.
Offshore wind can dry the air enough to qualify without a classic Santa Ana pattern. Monsoon thunderstorms can prompt a Watch or Warning for dry lightning with little wind, especially in summer in the mountains and deserts. For how Santa Ana winds form and where they blow hardest, see Santa Ana winds and Los Angeles fire weather.
Is a Red Flag Warning the Same as a Power Shutoff or Evacuation?
A Red Flag Warning is not a power shutoff or an evacuation order. A Red Flag Warning is a National Weather Service forecast of fire-weather conditions, issued whether or not anyone else acts. A Public Safety Power Shutoff is a decision by an electric utility, such as Southern California Edison or San Diego Gas & Electric, to cut power to lines in high fire-risk areas so damaged lines or vegetation contact cannot start a fire.
Utilities weigh Red Flag Warnings and their own risk models when deciding where and when to shut off power. The shutoff is the utility's decision, not an automatic result of an NWS warning.
An evacuation warning or order is a third, separate decision, issued by a county sheriff, fire department or local emergency management agency for an actual fire or other specific, imminent threat. Evacuation decisions depend on fire behavior, terrain, roads and population, which a weather product cannot capture. In a serious event, all three can be active within hours: a Red Flag Warning for the region, a power shutoff for specific circuits and an evacuation order for a neighborhood near a fire. Check each one from the agency that issues it.
What Should Residents Do During Either Alert?
During a Fire Weather Watch or Red Flag Warning, remove outdoor ignition sources; a Warning calls for the same steps with more urgency. NWS wildfire safety guidance and CAL FIRE say to avoid outdoor burning, spark-producing power tools, welding and grinding, and towing chains dragging on pavement. Many Southern California cities and counties ban fire pits and other open flames during a Red Flag Warning. Those rules are set locally, so check your city or county fire department.
Around your home, secure or bring in anything the wind could blow into power lines or across a driveway, such as patio furniture, umbrellas and trash cans. This matters most in foothill and canyon areas with strong Santa Ana gusts, including Woodland Hills, where wind exposure runs higher than in sheltered valleys and coastal areas. Use a Watch, while there is still time, to confirm your evacuation zone and ready a go-bag and plan.
Fire-weather alerts describe forecast conditions and do not replace incident guidance once a fire is burning. For weather that active wildfires create themselves, see fire whirls and pyrocumulus clouds.
Before a critical fire-weather period, check your destination's wind, gust and humidity forecast on WeatherLA's Los Angeles pages, confirm whether a Watch or Warning covers your zone and check utility shutoff notices and local emergency alerts for your area.