Skip to main content

PMS 437

CFFDRS: Fire Weather Index FWI System

  1. Introduction
  2. Fire Weather Index – NFDRS Crosswalk
  3. FWI Fuel Moisture Codes
  4. FWI Fire Behavior Indices
  5. Grass Fuel Moisture (GFM)
  6. Other Considerations

Introduction

Analogous in concept to the National Fire Danger Rating System (NFDRS), the FWI System depends solely on weather readings. Resulting fuel moisture codes and fire behavior indices are based on a single standard fuel type that can be described as a generalized pine forest, most nearly jack pine and lodgepole pine.

The Fire Weather Index System calls for weather observations to be collected from a standard observation site and time. Location standards can be found in the, Weather Guide for the Canadian Forest Fire Danger Rating System (Lawson and Armitage, 2008). The system calls for observations to be taken at solar noon, when the sun is at its peak directly overhead.

Reference tools and current conditions are available to US users through:

FWI Process Flow Chart

Image
The Fire Weather Index System. This process flow chart outlines the system inputs, as well as the system array of output codes and indices.

Return to Top

Fire Weather Index – NFDRS Crosswalk

This table provides the crosswalk between FWI weather inputs, fuel moisture codes, and fire behavior indices with their NFDRS counterparts.

Image
Comparison of Fire Weather Index (FWI) and National Fire Danger Rating (NFDRS) Systems. Establishes crosswalks of comparable elements of inputs and system outputs.

Return to Top

FWI Fuel Moisture Codes

There are 3 fuel moisture categories, or codes, in the FWI system, compared to 7 found in NFDRS. These are generally represented as unitless codes instead of fuel moisture content (represented as a % of dry weight). They can be converted to moisture content, and in fact, are converted each day as part of the daily or hourly calculations.

The Fine Fuel Moisture Code (FFMC) represents fuel moisture of forest litter fuels under the shade of a forest canopy. It is intended to represent moisture conditions for shaded litter fuels, the equivalent of 16-hour timelag. It ranges from 0-101. Subtracting the FFMC value from 100 can provide an estimate for the equivalent (approximately 10h) fuel moisture content, most accurate when FFMC values are roughly above 80.

The Duff Moisture Code (DMC) represents fuel moisture of decomposed organic material underneath the litter. System designers suggest that it is represents moisture conditions for the equivalent of 15-day (or 360 hr) timelag fuels. It is unitless and open ended. It may provide insight to live fuel moisture stress.

The Drought Code (DC), much like the Keetch-Byrum Drought Index, represents drying deep into the soil. It approximates moisture conditions for the equivalent of 53-day (1272 hour) timelag fuels. It is unitless, with a maximum value of 1000. Extreme drought conditions have produced DC values near 800.

This example plot of all three moisture codes through a fire season demonstrates how fuel moisture codes rise as fuels dry out, and falls with precipitation and (primarily with FFMC) with moderating weather.

Image
Example plot of fuel moisture trends from 2015 season at the Hogatza RAWS in Alaska. It shows that FWI system fuel moisture codes rise as fuels dry and fall as they are wetted by precipitation.

In this way, these fuel moisture codes can more easily provide visual comparisons with the associated fire behavior indices. Among them, the DMC and DC are effectively open ended codes with little chance environmental conditions will produce maximum values.

The FFMC, representing moisture conditions in shaded forest litter, is analogous to 10-hr fuel moisture estimates. This table suggests the conversion for both shaded (10h-Sh) and unshaded (10h-Unsh) fuel bed conditions:

Return to Top

FWI Fire Behavior Indices

There are 3 fire behavior indices in the FWI system:

  • The Initial Spread Index (ISI) is analogous to the NFDRS Spread Component (SC). It integrates fuel moisture for fine dead fuels and surface windspeed to estimate a spread potential. ISI is a key input for fire behavior predictions in the FBP system. It is unitless and open ended.
  • The Buildup Index (BUI) is analogous to the NFDRS Energy Release Component (ERC). It combines the current DMC and DC to produce an estimate of potential heat release in heavier fuels. It is unitless and open ended. In Alaska and the Lake States, it is the primary indicator of season severity during the growing season.
  • The Fire Weather Index (FWI) integrates current ISI and BUI to produce a unitless index of general fire intensity potential. It is analogous to NFDRS Burning Index. With dry fuel conditions, it is a key indicator of extreme fire behavior potential. Again, unitless and open ended.

As shown in this graph, the FWI integrates the influences of spread (ISI) and fuel flammability (BUI) to produce a unitless index of potential fire intensity and prospect for extreme fire behavior.

Image
Example plot of fire behavior indices from 2015 season at the Hogatza RAWS in Alaska. It shows that FWI system of indices represent day to day changes in spread (Initial Spread Index), fuel consumption (Buildup Index), and fire intensity (Fire Weather Index).

Return to Top

Grass Fuel Moisture (GFM)

The GFM is a fourth fuel moisture category for grass fuel moisture specifically (Wotton, 2009). It is not part of the daily FWI system. Research in Ontario (Kidnie et.al, 2010) quantified the fuel moisture trends for grass fuels and established a grass fuel moisture model that is produced only with hourly data. Its corresponding Grass Fuel Moisture Code (GFMC), along with the FFMC, provides hourly estimates to represent diurnal and event based changes in fine fuel moisture as they occur.

Image
Grass Fuel Moisture (GFM) Estimation Table. Though not part of the daily FWI system, grass fuel moisture is an important characteristic of diurnal changes to flammability in grass systems.

Return to Top

Other Considerations

Diurnal Variations

There is an hourly version of the Fine Fuel Moisture Code that reflects variability influenced by temperature and humidity changes throughout the day and night. Using the corresponding, locally observed windspeed, updated values for Initial Spread Index and Fire Weather Index may also be produced.

Seasonal Start-up and Resumption after Interruption in Observations

Daily records are generally started as soon as there is measurable fire danger in the spring. Where winter snow cover is continuous, this is defined as the third day after snow has left the area to which the fire danger rating applies. Default seasonal start-up values are 85 for FFMC, 6 for DMC, and 15 for DC.

If daily observations are interrupted during the season and missing observations cannot be estimated, fuel moisture codes must be estimated for the last day of missing observations and used as “yesterday” fuel moisture codes for the newly resumed weather observation.

Return to Top

Print PDF

NWCG Latest Announcements

The Incident Position Standards and Next Generation Position Task Book are now available for Status/Check-In Recorder (SCKN)

Date: August 27, 2024
Contact: Incident Planning Subcommittee 

NWCG is excited to announce that the NWCG Incident Position Standards for Status/Check-In Recorder, PMS 350-32, NWCG Position Task Book for Status/Check-In Recorder (SCKN), PMS 311-32, and Checking In Resources Customer Service Job Aid, J-111 are now available.

The Performance Support Package, which for SCKN includes the Incident Position Standards, Next Generation Position Task Book, and job aid were developed through the Incident Performance and Training Modernization (IPTM) effort. The Performance Support Package will support trainees, those qualified in the position, and evaluators.

References:

NWCG Status/Check-In Recorder Position Page

NWCG Incident Position Standards for Status/Check-In Recorder, PMS 350-32

NWCG Position Task Book for Status/Check-In Recorder (SCKN), PMS 311-32

Checking In Resources Customer Service Job Aid, J-111

The Next Generation Position Task Book and Incident Position Standards are now available for Safety Officer, Field (SOFF)

Date: July 26, 2024
Contact: Risk Management Committee 

NWCG is excited to announce that the NWCG Incident Position Standards for Safety Officer, Field, PMS 350-81 and NWCG Position Task Book for Safety Officer, Field (SOFF), PMS 311-81 are now available.

The Safety Officer, Field (SOFF) is responsible for monitoring operations on an incident from a risk management perspective to provide for the welfare of incident resources and the public. The new Incident Position Standards and Next Generation Position Task Book are developed through the Incident Performance and Training Modernization (IPTM) effort.

References:

NWCG Safety Officer, Field (SOFF) Position

NWCG Incident Position Standards for Safety Officer, Field, PMS 350-81

NWCG Position Task Book for Safety Officer, Field (SOFF), PMS 311-81

Updated NWCG Standards for Electronic Documentation (eDoc), PMS 277

Date: July 25, 2024
Contact: Incident Planning Subcommittee 

The Incident Planning Subcommittee has updated the NWCG Standards for Electronic Documentation (eDoc), PMS 277.

The NWCG Standards for Electronic Documentation (eDoc) establishes the standards for collection and retention of records on wildland fires. This July 2024 update will provide incident management teams the most current standards required to maintain incident records and submit them to host units at the close of an incident.

References:

NWCG Standards for Electronic Documentation (eDoc), PMS 277

eDoc Box Directory (zip file)

NWCG Off-Highway Vehicle Typing Standard Request for Comment

Date: July 24, 2024
Contact: Mobile Fire Equipment Subcommittee 

The Mobile Fire Equipment Subcommittee has released Equipment Bulletin 24-002 NWCG Off-Highway Vehicle (OHV) Typing Standard - Request for Comment. This bulletin outlines the proposed NWCG OHV typing standard, as well as the business need for establishing the standard. Comments on the proposed standard will be accepted through August 15th using the comment form linked below.

References:

ETC-EB-2024-02: NWCG Off-Highway Vehicle (OHV) Typing Standard - Request for Comment

NWCG Off-Highway Vehicle (OHV) Typing Standard Comment Form