GIS software is used to interpolate the weather data between stations to produce gridded weather maps. The FWI System components are then calculated on a cell-by-cell basis according to the equations in Van Wagner and Pickett (1985) to produce the FWI maps.

Elevation - The elevation grid was derived from the US Geological Survey's 1 x 1 km hydrologically correct digital elevation model (DEM) for North America, downloaded from the USGS web site. The DEM is a hydrologically corrected version of GTOPO30, a 30-arcsecond global DEM assembled by the USGS. In Canada, the data for GTOPO30 were taken from two sources, Digital Chart of the World and Digital Terrain Elevation Data, both produced by the US National Imagery and Mapping Agency (formerly the Defense Mapping Agency).

The elevation grid was derived from GTOPO30, a 30-arcsecond global digital elevation model assembled by the United States Geological Survey (USGS)

The CWFIS currently uses weather data from about 900 stations in Canada and the northern United States. About 700 of these stations are operated by Environment Canada (EC) or other federal departments under contract to EC. Sixteen stations are operated by the National Weather Service of the United States, and the remainder of the stations are operated by the provincial governments of Nova Scotia, Quebec, and Saskatchewan.

The EC and NWS data are obtained via Telesat Canada's Anik satellite. Weather observations are collected from the Canadian stations by the Canadian Meteorological Centre in Montreal and transferred to Telesat Canada in Toronto to be uplinked to the Anik satellite. The data are then downlinked to the Northern Forestry Centre in Edmonton for processing and storage.

The provincial data are collected by fire management agencies and transferred daily to the Northern Forestry Centre by FTP.

FWI Calculation Start-up

To determine spring start-up dates and starting fuel moisture code values, the procedures described in Turner and Lawson (1978) are used. There are two methods, depending on snow cover.

Method 1
For stations that report significant snow cover during the winter, start-up occurrs when the station has been snow-free for three consecutive days. Significant snow cover is defined as a mean snow depth of more than 10 cm, with snow cover present at least 75% of the days in January and February. Start-up values are as follows:

• Fine Fuel Moisture Code (FFMC) set to 85
• Duff Moisture Code (DMC) set to 6
• Drought Code (DC) set to 15*

• FFMC set to 85
• DMC set to 2 times the number of days since precipitation
• DC set to 5 times the number of days since precipitation*

*Where overwinter precipitation data are available, the DC is set according to an alternative, more rigorous method, Turner and Lawson (1978), which takes into account the fact that saturation of the deep fuel layers may not occur over the winter. In areas with low overwinter precipitation, the starting value for the DC can be significantly higher than the default.

Daily Weather Processing

Weather observations are received in raw format and must be decoded before being saved in the weather database. The FWI System requires observed temperature, relative humidity, and wind speed at noon local standard time, as well as 24-hour precipitation. Once the noon observations have been received for all the time zones, the 24-hour precipitation is calculated for stations that report every hour, three times per day, four times per day, or irregularly. Various other observations, such as wind direction, dew point, and atmospheric pressure, are also saved in the database to be used for interpolation.

The FWI System requires an unbroken record of daily weather. If a station fails to report or reports missing data, the missing values are estimated from nearby stations by means of inverse distance weighted (IDW) interpolation. For temperature and relative humidity, the IDW interpolated value is corrected for elevation.

Lastly, daily FWI system output values are calculated for each station and saved in the database.

Daily Weather Grids

Daily raster maps (grids) of temperature, relative humidity, wind speed, and precipitation are created by interpolating values between weather stations using IDW interpolation. Values are assigned to each grid cell by calculating a weighted mean of the values of the nearest 12 stations. For each cell, the station values are weighted by the inverse of the square of the distance to the cell.

The weather grids are then used as inputs to the Fire Weather Index (FWI) and Fire Behavior Prediction (FBP) grid calculations.

Elevation Correction

Temperature and relative humidity grid values are adjusted for elevation using the elevation grid (see above). For temperature, the adjustment is based on the United States Standard Lapse Rate of -6.5°C/km; that is, for every kilometer gain in elevation, the temperature is assumed to drop by 6.5°C. For relative humidity, the mixing ratio (ratio of water vapor to dry air by weight) is assumed to be constant with elevation. The mixing ratio is calculated for each station and interpolated to each grid cell location. The relative humidity is then calculated an a cell-by-cell basis using the elevation-adjusted temperature grid.

Daily Grid Production

The grids for the fuel moisture codes (FFMC, DMC, and DC) are built using both interpolation and calculation. Because the fuel moisture code calculations require the previous day's values as inputs, values for areas where new stations are starting up are interpolated rather than calculated. In areas where the previous day's values are available, the fuel moisture codes are calculated on a cell-by-cell basis using the previous day's grids, together with the current day's weather grids, as inputs. In the output maps, the non-calculating areas are assigned a null value, as are areas above the arctic treeline.

The FWI System fire behavior indices (not to be confused with the Fire Behavior Prediction System outputs) are calculated from the fuel moisture codes. These calculations are done on a cell-by-cell basis to produce the ISI, BUI, and FWI grids. Lastly, the Daily Severity Rating (DSR) grid is calculated from the FWI grid. The DSR gives a relative rating of fire control difficulty, or the amount of work required to suppress a fire.

References

Turner, J.A.; Lawson, B.D. 1978. Weather in the Canadian Forest Fire Danger Rating System. A user guide to national standards and practices. Environment Canada, Pacific Forest Research Centre, Victoria, BC. BC-X-177.

Van Wagner, C.E.; Pickett, T.L. 1985. Equations and FORTRAN program for the Canadian Forest Fire Weather Index System. Canadian Forest Service, Ottawa, ON. Forestry Technical Report 33.

Additional Resources:

• For more information, please see Forest Fires in Canada from Natural Resource Canada - http://fire.cfs.nrcan.gc.ca/index_e.php
• http://cwfis.cfs.nrcan.gc.ca/en/index_e.php
• Satellite Imagery can be found at http://cwfis.cfs.nrcan.gc.ca/en/current/cc_img_e.php
• The Green Pages
• The Canadian Interagency Forest Fire Centre (CIFFC) provides an online fire reporting system
• Daily fire maps and up-to-date season summaries (including archived maps and data)
• http://www.ciffc.ca/