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Posted on: October 17, 2023

Wildfire Weather: Exploring Weather Conditions Contributing to Wildfires, Red Flag Warnings, & More

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The risk of wildfires varies throughout a calendar year. Locally, in the City of Santa Rosa, the start and stop of Fire Season is declared to initiate compliance with local weed abatement requirements and during the time of the year when the risk is elevated. Favorable weather conditions and property owners’ preventative measures have led to a less active 2023 season. Even with the decrease in fire activity, weather conditions can change, so we will continue to monitor the forecasts and encourage our community to stay Wildfire Ready. Several environmental factors contribute to wildfire ignition. It is helpful to understand the role weather and vegetation can play in the development and containment of wildfires and how those factors affect fire weather.

Contributing Factors

Drought: Prolonged periods of drought are common in Northern California, reducing soil moisture and increasing the flammability of vegetation. Dry conditions make it easier for wildfires to ignite and spread rapidly. Additionally, Northern California experiences hot, dry summers with high temperatures, which further desiccate the landscape, making it susceptible to ignition. 

Diablo Winds: These are powerful, dry winds that can blow in from the desert and mountain regions, particularly in the San Francisco Bay Area, respectively. According to the National Weather Service, the ‘Diablo Wind’ occurs below canyons in the east bay hills (Diablo Mountain Range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central and northern California coast, affecting Sonoma County along the way.  

Low Humidity: Low relative humidity levels, often associated with California's ‘Mediterranean’ climate, contribute to the dryness of the region, making it more susceptible to wildfires. Santa Rosa Fire Department has implemented 6 permanent Remote Automatic Weather Stations (RAWS) in key locations throughout the city in 2021 to better track weather and climate conditions directly in the city.  

Vegetation Types: Northern California is home to various vegetation types, including chaparral, grasslands, and forests, which can fuel wildfires. Evergreen trees, like the ponderosa pine and Douglas fir, are especially prone to burning during dry spells. To learn more about fire safe landscaping please visit HERE.

Lightning Strikes and Dry Lightning: Thunderstorms can occur in the region during the summer months, and lightning strikes can ignite wildfires in areas with dry vegetation. "Dry thunderstorms" is a phrase used quite often in the wildland fire fighting community. It is usually meant to describe thunderstorms that produce little or no precipitation at the surface. The "drier" the thunderstorm, when combined with dry vegetation (or fuel), the more efficient it is in terms of fire ignitions by cloud to ground lightning. 

Thunderstorms can be considered dry if they don’t produce any rain at the surface. However, for the fire weather community, a "dry thunderstorm" may be used to describe a storm producing very little rainfall, such that it is not effective in extinguishing fire spread after lightning ignition. The second scenario is more common, though there are rare instances when thunderstorms produce no sensible rainfall. In the purest sense, “dry” thunderstorms produce rain just below the cloud base but due to a very dry atmosphere below the cloud base, the rain evaporates at some point between the cloud base and the ground. To read more about dry thunderstorms, click HERE.

Red Flag Warnings and Weather Alerts

Fire weather advisories are issued by the National Weather Service (NWS) and mean warm temperatures, very low humidity, and stronger winds are expected to combine to produce an increased risk of fire danger. This also alerts fire officials to the increased risk for fire due to the weather conditions. Here are definitions for the various types of fire weather advisories:

Red Flag Warning: Take Action. Be extremely careful with open flames. NWS issues a Red Flag Warning, in conjunction with land management agencies, to alert land managers to an ongoing or imminent critical fire weather pattern. NWS issues a Red Flag Warning when fire conditions are ongoing or expected to occur shortly.

Fire Weather Watch: Be Prepared. A Watch alerts land managers and the public that upcoming weather conditions could result in extensive wildland fire occurrence or extreme fire behavior. A watch means critical fire weather conditions are possible but not imminent or occurring.

Extreme Fire Behavior: This means a wildfire is likely to rage out of control. It is often hard to predict these fires because they behave erratically; sometimes dangerously. One or more of the following criteria must be met:

  • Moving fast - High rate of spread 
  • Prolific crowning and/or spotting 
  • Presence of fire whirls 
  • Strong convection column

Please make sure you sign up for Emergency Alerts to receive important Red Flag/Critical Weather Alerts from the City and the Santa Rosa Fire Department. Review important alert information here (en español aquí).

Visit to learn more about Wildfire Preparedness and Weather Warnings.

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