Fire Danger Rating Signs
The purpose of the fire danger rating sign is to prevent wildfires by increasing awareness of wildfire ignition and spread potential. Wildfire ignition potential is determined by the following factors:
- Size and shape of fuels
- Compactness or arrangement of fuels
- Fuel moisture content
- Fuel temperature.
In general, these factors vary over a geographic area, change each day, and are not practical to continually assess. Therefore, the signs will display the Fire Danger Rating using air temperature. Wind is the most influential factor affecting wildfire spread.
Any reliable weather forecast can be utilized and when in doubt, round up to the next higher rating. Windy periods with low fuel moisture and relative humidity usually result in Red Flag Warnings. Red flags will only be displayed when the National Weather Service declares a “Red Flag Warning” for any portion of Sonoma County. The rating will be “Extreme” when red flags are posted regardless of other weather conditions.
Signs will change to “low” when there is sustained rain (over 4 hours).
May 1 - September 31: Highest predicted air temperature
October 1 -November 30: Highest predicted temperature
December 1 - April 30
Sign is kept at low unless weather associated with increased wildfire ignition and fuel conditions support changing. This will be a discretionary decision by the fire agencies and will be considered during significant dry periods during the winter months where presribed burning increases wildfire ignition potential.
| Fire Danger Rating and Color Code | Description |
|---|---|
| Low (L) (Green) | Fuels do not ignite readily from small firebrands although a more intense heat source, such as lightning, may start fires in duff or punky wood. Fires in open cured grasslands may burn freely a few hours after rain, but woods fires spread slowly by creeping or smoldering, and burn in irregular fingers. There is little danger of spotting. |
| Moderate (M) (Blue) | Fires can start from most accidental causes, but with the exception of lightning fires in some areas, the number of starts is generally low. Fires in open cured grasslands will burn briskly and spread rapidly on windy days. Timber fires spread slowly to moderately fast. The average fire is moderate intensity, although heavy concentrations of fuel, especially draped fuel, may burn hot. Short-distance spotting may occur, but is not persistent. Fires are not likely to become serious and control is relatively easy. |
| High (H) (Yellow) | All fine dead fuels ignite readily and fires start easily from most causes. Unattended brush and campfires are likely to escape. Fires spread rapidly and short-distance spotting is common. High intensity burning may develop on slopes or in concentrations on fine fuels. Fires may become serious and their control difficult unless they are attacked successfully while small. |
| Very High (VH) Orange | Fires start easily from all causes and, immediately after ignition, spread rapidly and increase quickly in intensity. Spot fires are a constant danger. Fires burning in light fuels may quickly develop high intensity characteristics such as long distance spotting and fire whirlwinds when they burn into heavier fuels. |
| Extreme (E) (Red) | Fires start quickly, spread furiously, and burn intensely. All fires are potentially serious. Development into high intensity burning will usually be faster and occur from smaller fires than in the very high fire danger class. Direct attack is rarely possible and may be dangerous except immediately after ignition. Fires that develop headway in heavy slash or in conifer stands may be unmanageable while the extreme burning condition lasts. Under these conditions the only effective and safe control action is on the flanks until the weather changes or the fuel supply lessens. |
Source: https://www.nwcg.gov/sites/default/files/products/pms932.pdf