National Fire Danger Rating System (NFDRS)

The National Fire Danger Rating System (NFDRS) was developed in the early 1970s. It was designed around four basic guidelines:

1. Scientifically based.
2. Adaptable to the needs of local managers.
3. Applicable anywhere in the country.
4. Reasonably inexpensive to operate.

In 1972, the National Fire Danger Rating System was released for general use by agencies throughout the United States. Modifications to the original system were made in 1978 and 1988. The most recent update, NFDRS Version 4, was implemented in NC in 2021, with full national implementation expected by Summer 2022. The current system is based on the physics of combustion and laboratory developed constants and coefficients reflecting the relationships between various fuels, weather, topography and risk conditions. The National Fire Danger Rating System tracks the effect of previous weather events through their effect on live and dead fuels and adjusts them accordingly based on future or predicted weather conditions. The outputs are expressed in simple terms easily understood by users. The current National Fire Danger Rating System is utilized by all federal and most state agencies to assess fire danger conditions.

Key Assumptions within the National Fire Danger Rating System
There are four fundamental assumptions associated with the National Fire Danger Rating System that must be understood if the system is to be properly applied and interpreted. They include:

1. NFDRS outputs relate only to the potential of an initiating fire, one that spreads, without crowning or spotting, through continuous fuels on a uniform slope.
2. NFDRS outputs address fire activity from a containment standpoint as opposed to full extinguishment.
3. The ratings are relative, not absolute and they are linearly related. In other words if a component or index doubles the work associated with that element doubles.
4. Ratings represent near worst-case conditions measured at exposed locations at or near the peak of the normal burning period.

In summary, fire danger rating is a numeric scaling of the potential over a large area for fires to ignite, spread, and require fire suppression action. It is derived by applying local observations of current or predicted conditions of fuel, weather, and topographic factors to a set of complex science-based equations. The outputs of a fire danger rating system are numeric measures of fire business that provide a tool to assist the fire manager in making the best fire business decisions.