The fire danger signs posted along highways and outside communities across Southwest Colorado read simple: low, moderate, high, very high or extreme.
Drivers pass them daily going through Dolores, between Mancos and Dolores or upon entering and leaving the city of Cortez. It could warrant a longer look if the sign reads “high” – or if the arrow points to the fiery, red slot indicating fire danger is “very high.”
Behind each rating is the inter-working of a complex measurement system that provides precise indications for fire danger daily – both predictive and observed values.
What determines if the highway sign changes from “moderate” to “high,” or vice versa, is called the Durango Zone Fire Business Calculator, a regional forecasting tool run by the San Juan National Forest that uses dozens of observed weather points to localize the answer.
San Juan National Forest spokesperson Lorena Williams said the calculator is maintained by the agency. But many local fire districts and federal partners rely on it. Fire Chief Rick Spencer with the Cortez Fire Protection District said the calculator determines local danger and is updated regularly.
Current fire danger across forest elevations is rated very high, meaning fires start easily, spread rapidly, and quickly increase in intensity. These fires are difficult to control and can become large and last for a long time.
When danger is rated high, it means fires start easily, and fine fuels like grass and needles ignite readily. It is in comparison to, for example, “extreme,” which means fires of all kinds start easily and burn intensely. “Low” means fuels do not ignite as easily and need a stronger heat source, such as lightning.
Several data points feed the calculator. It collects information from weather stations across the region, calculating things like the burning index and commuting historical fire trends. The burning index indicates the level of effort required for firefighters to contain a fire.
“It’s a scientific and technical process, measuring things like the energy-release components, which is how much energy a fire would release from a fuel if it were to start,” Williams said.
She said the forest agency takes the process seriously.
It’s used to keep the public safe as an education tool, Williams said. Additionally, the ratings system plays a role in determining fire restrictions – when, if or in what severity to implement them.
It is used by the multi-agency group coordinating fire preparedness and restrictions across several Southwest Colorado counties – called the Durango Zone board – which assesses wildfire risk in Montezuma and Dolores counties all the way to Archuleta County and areas in between.
The calculator has a dedicated staff member called the fire planner, who develops, maintains and manages it.
“San Juan National Forest is quite large ‒ about 1.8 million acres ‒ and we have to generalize a lot more with our fire danger using this calculator than, say, Cortez Fire might,” Williams said.
In areas with elevation change, fire danger patterns vary and that is typical for springtime. Fire officials in mid-May observed lower elevations nearing peak green-up – when grasses and vegetation are lush enough to slow a fire spreading.
However, conditions have now progressed: some higher-elevation spots remain fully green, while in lower forest areas, officials have implemented a Stage 1 fire ban, in response to grasses rapidly drying out.
“Those physical signs in communities are really important to pay attention to because they’re adjusted down to the local area a little bit better,” Williams said.