National Weather Service

NFDRS

ZONE

and

POINT

FORECASTS

 

The National Fire Danger Rating System (NFDRS) is a complex set of algorithms that provide Federal and State land management agencies with indices and components that are used to describe fire danger in a particular area. This is done on a daily basis during the fire season for each fire weather station using data supplied by the 1400 LDT fire weather observation along with catalogued information on the fuels and topography for a given area.

The National Fire Danger Rating System can also produce forecasts of fire danger for the following day. However, instead of using current fire weather observations, NFDRS uses predicted weather conditions for the next 24 hour period. Forecast values of NFDRS indices and components give fire managers an estimate of tomorrow's fire danger in their area of responsibility. Preplanned management actions are then tied to Decision Classes (Staffing Levels, Adjective Classes, and IFPLs) based on forecast NFDRS output.

NFDRS weather forecasts used in this process are are provided once daily in the afternoon by NWS fire weather offices during the fire season and entered into WIMS by 1530 LDT. Forecasts can be in the form of NFDRS zone trend forecasts or NFDRS point forecasts. Zone trends are applied to each weather observation within a given fire weather zone, to derive forecasted fire danger indices for th following day. In general, land management agencies do not see the actual zone trend forecasts unless they are included in the afternoon narrative forecast. Some fire weather offices append NFDRS zone trend forecasts to the end of their afternoon narrative fire weather forecast.

STEP 1 - QUALITY CONTROL
Fire danger ratings are no better than the data used to derive them. Unfortunately, several sources of error can exist in the daily fire weather observations, depending on the type of weather station (manual or RAWS) or data collection system used. These include instrument errors, observational errors, WIMS catalog errors, transmission errors, transcription errors, etc.

Fire weather forecasters utilize current fire weather observations as a starting point in the preparation of NFDRS zone trend forecasts. Therefore, the first step in preparing NFDRS zone trend forecasts should be to quality control today's fire weather observations. Problems with individual data elements within the observation could result in large errors in the zone average. The table below shows a portion of a typical AFOS file that lists the available fire weather observations for zone 659 in the Olympia Fire Weather District for August 11, 1997.

NMCFWOOLM:
SXUS45 KMWI 112145
Weather Information Management System
USDA National Computer Center
11-Aug-97 21:45:45 GMT
Listing of Observations for (OLYMPIA)
ST NME STAT'N DATE HR T W DBT DPT RH Y M DIR WS 10 TMX TMN HMX HMN PD PPAMT
Zone: 659
KOSMOS 451105 970811 13 O O 81 55 42 1 1 194 5 6 85 61 63 29 0 0.00
LONGMI 451106 970811 13 O 1 80 53 40 1 1 135 6 4 80 47 0 0.00
HAGAR 451115 970811 12 O 1 80 55 43 1 1 289 6 81 55 80 30 0 0.00
OHANAP 451119 970811 13 O O 87 52 31 1 1 225 10 6 93 45 0 0.00
LESTER 451705 970811 13 O O 86 50 30 1 1 237 8 90 46 100 4 0 0.00
STAMPE 451711 970811 13 O O 75 47 38 1 1 270 6 75 58 0 0.00
GREENW 451718 970811 13 O O 90 76 63 1 1 228 10 92 53 94 25 0 0.00
ORRCR 451919 970811 12 O 1 89 54 31 1 1 283 6 89 48 100 23 0 0.00
Count: 8

Zone Average:

 84 55 40 7 5 86 52 87 22 0 0.00
Notice that on the last line of the zone listing shown above, WIMS has calculated zone averages for most of the individual weather parameters. However, the zone average relative humidity is likely in error due to a bad weather observation from Greenwater (highlighted in yellow). Greenwater's 90 degree temperature looks okay when compared with other observations from the zone. However, the 63% relative humidity appears to be too high, especially since Greenwater is one of the warmest and driest locations in zone 659 and the other fire weather stations all report humidities in the 30 to 40% range. This is likely a case where the 6 and the 3 were transposed when the observation was entered into WIMS. The actual humidity was probably 36% not 63%. Notice too that the WIMS-calculated dew point of 76 degrees is about 20 degrees higher than the other sites within the zone.

In this case, the zone average temperature is correct but the zone average relative humidity is too high. To correct the error, recalculate the average relative humidity for the zone excluding Greenwater. This gives a corrected zone average relative humidity of 36% instead of the 40% reported by WIMS. The corrected zone average relative humidity of 36% should then be used as the starting point for any modifications in the zone averaged relative humidity over the next 24 hours resulting from changes in the air mass, the amount of cloud cover, winds, precipitation, etc.

Note: Fire weather observations entered into WIMS are archived in the National Interagency Fire Management Intergraded Database (NIFMID) for later use. Users should always be notified when errors appear in their fire weather observations. Be sure to notify the station's owner and the dispatch center that entered the observation into WIMS of the incorrect data. The observation can then be edited before it is archived in NIFMID.

STEP 2 - FORECAST ZONE TRENDS
Fire weather forecasters use a wide range of meteorological data when making zone trend forecasts. A forecaster must consider changes expected in the air mass over the fire weather zone during the next 24 hours and the effect those changes will have on the average temperature, relative humidity, and wind speeds reported in the zone at 2pm tomorrow. The forecaster should then estimate tomorrow's average temperature, relative humidity, and wind speed for each fire weather zone in the fire weather district. An example of this step is shown in the table below. Notice that the corrected value of 36% is being used for the zone average relative humidity. In the example below, temperatures are expected to be up 3 degrees, relative humidity down 6 percent, and wind speed down 2 mph.

f

TODAY'S ZONE AVERAGE TOMORROW'S ZONE FORECAST
TMP RH WS TMP RH WS
84 36 7 87 30 5

 

X
STEP 3 - NFDRS ZONE FORECAST
The next step in the process is to use the zone trends calculated above to complete the NFDRS zone forecast. In the table below, the actual trends calculated in step 2 are shown in red. The zone forecast is completed by entering the expected state of the weather (WX), today's lightning activity level (AL) from 1300-midnight, tomorrow's lightning activity level (TL) from midnight to midnight, the 10hr timelag fuel moisture change (10HR), the precipitation duration in hours (PD1) from 18z today to 12z tomorrow, the precipitation duration in hours (PD2) from 12z to 18z tomorrow, and wet fuel flag (WET). Note...Some fire weather offices do not enter a trend forecast for 10hr fuel moisture, choosing instead to let NFDRS calculate tomorrow's 10hr fuel moisture using a model that requires temperature, relative humidity, precipitation, and insolation data.

NFDRS ZONE FORECAST WORKSHEET
ZONE WX TMP RH AL TL WS 10HR PD1 PD2 WET
659 0 +3 -6 1 1 -2 -2 0 0 N
Based on the data from the partial worksheet shown above, the format of the NFDRS zone forecast for fire weather zone 659 would be....

ZONE,659,970812,13,0,3,-6,1,1,-2,-2,,,,,0,0,N
Explanation of individual elements of the NFDRS forecast....

1. WIMS command...ZONE

2. Fire weather zone number...659

3. Date/Time group (yymmdd)...970812

4. Time...1300 LST

5. Weather at time of observation (1-9)...0

6. Temperature change (24hr at 1300 LST)...+3

7. RH change (24hr at 1300 LST)...-6

8. Lightning Activity Level 1300-midnight (1-6)...1

9. Lightning Activity Level midnight-midnight tomorrow (1-6)...1

10. Wind Speed change...-2

11. 10hr timelag fuel moisture change...-2

12. Maximum temperature...not forecast

13. Minimum temperature...not forecast

14. Maximum RH...not forecast

15. Minimum RH...not forecast

16. Precipitation duration in hours from 1300-0500 LST...0

17. Precipitation duration in hours from 0500-1300 LST...0

18. Wet fuels (Y=wet fuels, N=dry fuels)...N

Note that all parameters must be comma delineated, even the ones that are not forecast!

Example NFDRS zone forecast file
x
STEP 4 - FORECAST FIRE WEATHER OBSERVATIONS
NFDRS zone trend forecasts are produced once per day during the fire season for every fire zone that has at least 2 to 3 current fire weather observations. Each observation in the zone is modified according to the trends specified in the NFDRS forecast for that zone. This will produce a forecasted fire weather observation for the following day, which in turn is used to generate forecasted fire danger indices for the station and the zone. The table below shows the forecasted fire weather observations for zone 659 for August 12, 1997. Notice that the NFDRS zone trend forecast has been applied to each fire weather observation in the zone.

NMCFWOOLM:
SXUS45 KMWI 112245
Weather Information Management System
USDA National Computer Center
11-Aug-97 22:45:45 GMT
Listing of Observations for (OLYMPIA)
ST NME STAT'N DATE HR T W DBT DPT RH Y M DIR WS 10 TMX TMN HMX HMN PD1 PD2
Zone: 659
KOSMOS 451105 970812 13 F O 84 55 36 1 1 194 3 6 0 0
LONGMI 451106 970812 13 F O 83 51 34 1 1 135 4 4 0 0
HAGAR 451115 970812 12 F O 83 54 37 1 1 289 4 0 0
OHANAP 451119 970812 13 F O 90 50 25 1 1 225 8 6 0 0
LESTER 451705 970812 13 F O 89 48 24 1 1 237 6 0 0
STAMPE 451711 970812 13 F O 79 46 32 1 1 270 4 0 0
GREENW 451718 970812 13 F O 93 75 57 1 1 228 8 0 0
ORRCR 451919 970812 13 F O 92 52 25 1 1 283 4 0 0
Count: 8

Forecast Zone Average:

 87 54 34 5 5
The forecast zone average (shown in red above) should match the forecasted zone averages from Step 2. The zone average temperature of 87 degrees and the zone average windspeed of 5 mph look good. However, the zone average relative humidity is 4% higher than the the original forecast of 30% from Step 2.

Once again, the problem appears to be with the predicted relative humidity for Greenwater (highlighted in yellow). Unfortunately, NFDRS applies the zone trend forecast to every current observation in the zone - even if the data is bad. In this example, the relative humidity trend of down 6%, when applied to the observed relative humidity of 63%, gave a predicted relative humidity of 57%.

From the discussion above, it was felt the observed relative humidity for Greenwater should have been 36% instead of 63%. In this case, the predicted relative humidity for Greenwater should have been 30% instead of 57%.

STEP 5 - NFDRS POINT FORECAST
The NFDRS point forecast is a weather forecast for a single point or station. It is often used to override a zone trend forecast when there is an error in the original observation. WIMS applies the zone trend forecast to each fire weather station first. It then looks for point forecasts to adjust individual station forecasts within the zone.
The format of the NFDRS point forecast for Greenwater would be....

FCST,451718,970812,13,0,93,30,1,1,228,8,6,95,57,90,27,0,0,N
Explanation of individual elements of the NFDRS forecast....

1. WIMS command...FCST

2. Fire weather station number...451718

3. Date/Time group (yymmdd)...970812

4. Time...1300 LST

5. Weather at time of observation (1-9)...0

6. Temperature at 1300 LST...93

7. RH at 1300 LST...30

8. Lightning Activity Level 1300-midnight (1-6)...1

9. Lightning Activity Level midnight-midnight tomorrow (1-6)...1

10. Wind direction...228

11. Wind Speed ...8

12. 10hr timelag fuel moisture change...6

13. Maximum temperature...95

14. Minimum temperature...57

15. Maximum RH...90

16. Minimum RH...27

17. Precipitation duration in hours from 1300-0500 LST...0

18. Precipitation duration in hours from 0500-1300 LST...0

19. Wet fuels (Y=wet fuels, N=dry fuels)...N

Note that all parameters must be comma delineated!

STEP 6 - CORRECTED FORECAST FIRE WEATHER OBSERVATION FILE
After the individual station forecast for Greenwater has been adjusted by the point forecast, the corrected forecast file list should include the corrected weather forecast for Greenwater. Notice that the zone averages now match what was originally forecast in Step 2.

NMCFWOOLM:
SXUS45 KMWI 112245
Weather Information Management System
USDA National Computer Center
11-Aug-97 22:45:45 GMT
Listing of Observations for (OLYMPIA)
ST NME STAT'N DATE HR T W DBT DPT RH Y M DIR WS 10 TMX TMN HMX HMN PD1 PD2
Zone: 659
KOSMOS 451105 970812 13 F O 84 55 36 1 1 194 3 6 0 0
LONGMI 451106 970812 13 F O 83 51 34 1 1 135 4 4 0 0
HAGAR 451115 970812 12 F O 83 54 37 1 1 289 4 0 0
OHANAP 451119 970812 13 F O 90 50 25 1 1 225 8 6 0 0
LESTER 451705 970812 13 F O 89 48 24 1 1 237 6 0 0
STAMPE 451711 970812 13 F O 79 46 32 1 1 270 4 0 0
GREENW 451718 970812 13 F O 93 57 30 1 1 228 8 6 95 57 90 27 0 0
ORRCR 451919 970812 13 F O 92 52 25 1 1 283 4 0 0
Count: 8

Forecast Zone Average:

 87 52 30 5 5

Disclaimer....As required by 17 U.S.C. 4023, third parties producing works consisting predominantly of the material appearing in NWS Web pages must provide notice with such subsequently produced work(s), identifying such incorporated material and stating that such material is not subject to copyright protection.

Please send comments or questions about this fire weather training module to John Werth, WSO(FW) Olympia, Wa.

email: john.werth@noaa.gov

phone: (206) 526-6095 ext 251