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Previous Chapter 6 Fire Weather Products
This chapter describes the fire weather products that will be supported by AFPS. It gives examples of each product and describes its parameters.
Table17 summarizes the product parameters for the fire weather products.
Table 17 - Fire Weather Product Parameters
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Parameter Fire National Land Smoke Fire
Weather Fire Manage- Manage- Suppression
Pre- Danger ment ment Forecast
suppression Rating Forecast Forecast (FWS)
Forecast System (FWL) (SMF)
(FWF) Forecast
(FWM)
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Synopsis X X X X
Sky and Weather or Weather Code X X X X
Probability of Precipitation X X X
Quantitative Precipitation X X X
Chance of Wetting Rain X X X
Lightning Activity Level X X X X
Precipitation Duration X X X X
10-Hour Time-lag Fuel Moisture X X X X
Wind Direction (7 meters) X X X X
Wind Speed (7 meters) X X X X
Wind Gust (7 meters) X X X
Free Air Wind X X X
Wind Shift X X X
Maximum Temperature X X X X
Minimum Temperature X X X X
Maximum Relative Humidity X X X X
Minimum Relative Humidity X X X X
Temperature X
Relative Humidity X
Temperature Trend X
Relative Humidity Trend X
Wind Speed Trend X
Maximum Temperature Trend X
Minimum Temperature Trend X
Maximum Relative Humidity Trend X
Minimum Relative Humidity Trend X
10-Hour Fuel Moisture Trend X
Inversions X X
Freezing Level of the Free Air X X
Maximum Transport Winds X
Minimum Transport Winds X
Maximum Mixing Depth X
Minimum Mixing Depth X
Maximum Stability X
Minimum Stability X
Haines Index X X X
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Currently, fire weather forecast product formats and content vary considerably from site to site. The examples shown in this chapter are taken from the cited references.
The fire weather forecast (FWF) is a zone-based product that consists of both structured text and tabular formats. It is formatted period by period and covers the following periods: today (the rest of the day), tonight (12-hour period), and then 24-hour periods up to 6 days from the initial time. The zones are a combination of National Fire Danger Rating (NFDR) zones and counties. Combining zones which have similar forecast parameters is acceptable. The FWF is issued once or twice per day depending upon local requirements. Examples of the structured and tabular formats of the FWF are shown in Figure11. The information content of the fire weather forecast varies across the NWS. For example, some sites include freezing level and winds aloft information while others do not.
Figure 11 - Fire Weather Forecast
The synopsis is a narrative description of the synoptic weather pattern including high pressure centers, airmass characteristics (e.g., dry and stable), and frontal position and movement.
The sky and weather is a brief description of the sky coverage and precipitation. The description is based on a weather code as described in Table18. Note that the FWF product contains a description of the sky and weather and does not specify the weather code.
Table 18 - Weather Codes for Fire Weather Forecasts
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Code Description
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9 Thunderstorms likely or definite.
8 Rain showers likely or definite.
7 Snow, snow showers, or sleet likely or definite.
6 Rain likely or definite.
5 Drizzle likely or definite.
4 Fog
3 Cloud cover greater than or equal to 9/10ths.
2 Cloud cover greater than or equal to 6/10ths.
1 Cloud cover greater than or equal to 1/10th.
0 None of the above (clear skies).
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The probability of precipitation for various forecast periods is stated as a percentage.
The amount of precipitation expected during the various forecast periods is specified in inches.
The probability of enough precipitation to wet the fuel significantly. The quantity of precipitation is typically 0.10" over a period of 12 hours but is defined site to site.
The lightning activity level is a one-digit number that reflects the forecast lightning levels during the various forecast periods. The meaning of the lightning activity level is explained in Table19.
Table 19 - Lightning Activity Levels for FWM Product
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Code(a) Description
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1 No thunderstorms.
2 Few building cumulus only occasionally reaching cumulus congestus stage; single CB in forecast
area.
3 Scattered cumulus and cumulus congestus; widely scattered CBs. Cloud-to-ground lightning averag
ing 1-2 per minute maximum.
4 Growing cumulus and cumulus congestus over 1/10 to 3/10 of the area. Scattered cloud-to-ground
lightning in area averaging 2-3 strikes per minute.
5 Cumulus congestus common over area, occasionally obscuring the sky. Moderate to heavy rain asso
ciated with CBs; light to moderate rain preceding and following lightning activity. Lightning flashes
occurring steadily at some place in or during storm period. Maximum cloud-to-ground flash rate
greater than 3 per minute.
6 Scattered towering cumulus with few at thunderstorm stage. Very limited horizontal extent with high
bases (15000-17000 feet MSL), virga in most prominent hydrometeor form. Lightning flash rate is
low, averaging less than 1 to 3 per 5-minute period each storm.
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- (a)
- Levels 4 6 are generally not forecast since their verification requires cloud-to-ground lightning and radar observations.
Precipitation duration is calculated in hours for the various forecast periods. Precipitation duration is typically derived from 3-hour rain probabilities. For each 3-hour period, the duration is set to 3 hours if the probability is definite, 1.5 hours for likely probability, 0.5 hours for chance, and 0.25 hours for slight chance. The hours are summed.
This parameter is a measure of the amount of moisture in the wood and timber, which affects how well they will ignite and burn. Fuel moisture is calculated for the tomorrow period only. Potential fuel moisture is calculated from the forecast temperature and relative humidities at the fuel level, based on surface temperatures, surface relative humidities, and the weather code, corrected for precipitation. The 24-hour forecast of 10-hour time-lag fuel moisture is then calculated from the observed fuel moisture (at 1300/1400 local time) and the forecast potential fuel moisture for 0500/0600 and 1300/1400 the next day. The resulting fuel moisture is represented as a percentage.
The forecast 10-minute average 7-meter height wind direction for each period is specified to eight compass directions. If the wind speed is light, then the direction may be given as VARIABLE.
The forecast average 7-meter wind speed for each of the periods is specified in miles per hour. Typically a range of speeds is given, e.g., 10 TO 15 MPH. The word LIGHT is used to describe speeds below 5 mph.
The forecast 7-meter wind gust for each of the periods is specified in miles per hour.
The wind direction (to eight compass points) and speed (mph) for the wind above the boundary layer.
Significant wind shift information, consisting of both direction and speed changes, is specified in the product. The time of the wind shift is given.
The expected maximum (minimum) temperature for the daytime (nighttime) forecast periods.
The expected maximum (minimum) relative humidity for the nighttime (daytime) forecast periods.
The Haines Index, more formally known as the Lower Atmospheric Severity Index (LASI) for Wildland Fires, is calculated for the daytime forecast periods. This index improves over the National Fire Danger Rating System and the Fire Behavior Prediction System in forecasting extreme fire behavior or "blowup" conditions.
The National Fire Danger Rating System (NFDRS) forecast is a tabulation of fire weather elements and parameters for various sites and zones. The U.S. Forecast Service receives this information on the Administrative and Forecast Fire Information Retrieval and Management System (AFFIRMS) which is being upgraded to the Weather Information Management System (WIMS). The FWM product is usually issued daily at around 13001400 local time and is valid for a 24-hour period. An example of the FWM product is shown in Figure12.
Figure 12 - National Fire Danger Rating System Forecast Example
The weather code reflects the thunderstorm probability, precipitation, and forecast cloud cover, as shown in Table18 on page48. There is a single weather code for the 24-hour period.
The lightning activity level is a one-digit number that reflects the forecast lightning levels during the forecast period. There are two forecast periods during the 24-hour time. The first period is for the first 10 hours or until midnight tonight, and the second period is from midnight tonight to midnight tomorrow. Lightning codes are shown in Table19 on page49.
Precipitation duration is calculated in hours for two forecast periods. The first period covers the first 16 hours of the 24-hour forecast period and the second covers the last 8 hours.
This parameter is a measure of the amount of moisture in the wood and timber which affects how well they will ignite and burn. See Section 6.1.2.8 on page49.
The forecast 7-meter wind direction and speed for the next day at 1300/1400 local time are given in degrees and miles per hour, respectively.
The expected maximum and minimum temperature for the 24-hour forecast period.
The expected maximum and minimum relative humidity for the 24-hour forecast period.
The forecast surface temperature for the next day at 1300/1400 local time in degrees Fahrenheit with a resolution of one degree.
The forecast surface relative humidity for the next day at 1300/1400 local time.
Trends are forecast for temperature, relative humidity, wind speed, maximum and minimum temperature, maximum and minimum relative humidities, and 10-hour time-lag fuel moisture. Each of these is the forecast difference between the current observation and the 24-hour forecast.
The land management forecast (FWL) is usually issued during the nonfire season to assist land management agencies with frequent and/or large scale hazard reduction operations, such as prescribed burning. The format of the FWL is similar to the presuppression forecast (FWF) but with several additional parameters. It is issued once or twice per day depending upon local user requirements.
The synopsis is a narrative description of the synoptic weather pattern including high pressure centers, airmass characteristics (e.g., dry and stable), and frontal position and movement.
This is a brief description of the sky coverage and precipitation, based on the weather code (see Table18 on page48). A typical phrase is CLOUDY WITH 40 PERCENT CHANCE OF RAIN.
The probability of precipitation for the various forecast periods is stated as a percentage.
The amount of precipitation expected during the various forecast periods is specified in inches.
The probability of enough precipitation to wet the fuel significantly. The quantity of precipitation is typically 0.10" over a period of 12 hours and is tailorable from site to site.
The lightning activity level code reflects the forecast lightning levels during the various forecast periods. The meaning of the lightning activity level is explained in Table19 on page49.
Precipitation duration is calculated in hours for the various forecast periods.
This parameter is a measure of the amount of moisture in the wood and timber which affects how well they will ignite and burn. Fuel moisture is calculated for the tomorrow period only.
The forecast average 7-meter height wind direction for each period is specified to eight compass locations. If the wind speed is light, then the direction may be given as VARIABLE.
The forecast average 7-meter wind speed for each period is specified in miles per hour. Typically a range of speeds is given, e.g., 10 TO 15 MPH. LIGHT is used to describe speeds below 5 mph.
The forecast 7-meter wind gust for each of the periods is specified in miles per hour.
The wind direction (to eight compass points) and speed (mph) for the wind above the boundary layer.
Significant wind shift information, consisting of both direction and speed changes, is specified in the product. The time of the wind shift is also given.
The expected maximum (minimum) temperature for the daytime (nighttime) forecast periods.
The expected maximum (minimum) relative humidity for the nighttime (daytime) forecast periods.
Height above ground level of base and top of inversions expected during the forecast periods.
Height above ground level of the 0 degree Celsius isotherm during the forecast period.
The Haines Index is calculated for the daytime forecast periods.
The smoke management forecast (SMF) contains meteorological guidance for the management of smoke from controlled burns. The information contained within the product is designed to help user agencies determine the horizontal and vertical extent of smoke and its persistence over the forecast area. Figure14 is an example of a smoke management forecast.
Figure 13 - Smoke Management Forecast
The synopsis is a narrative description of the synoptic weather pattern including high pressure centers, airmass characteristics (e.g., moderately unstable), and frontal position and movement.
Height above ground level of base and top of inversions expected during the forecast periods.
Height above ground level of the 0 degree Celsius isotherm during the forecast period.
The maximum and minimum expected mean wind speed and direction for the forecast period within an atmospheric layer bounded by the surface (averaged through the mixing layer). Transport winds are for multiple levels which are determined to be significant for local user agency smoke management.
The maximum and minimum mixing depth expected for the forecast period as calculated above the terrain surface level.
The maximum and minimum stability of atmospheric layers expected during the forecast period as indicated by standard measures of stability. Stability is forecast in a five-category system which is based on the temperature change with height. The stability categories are specified in Table20.
Table 20 - Stability Categories
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Category Pasquill(a) Description
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Extremely Unstable A less than -1.9 degrees Celsius per 100 meters height change
Moderately Unstable B -1.9 to -1.7 degrees Celsius per 100 meters change
Slightly Unstable C -1.7 to -1.5 degrees Celsius per 100 meters change
Neutral D -1.5 to +0.5 degrees Celsius per 100 meters change
Slightly Stable E +0.5 to +1.5 degrees Celsius per 100 meters change
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- (a)
- Pasquill categories are used to classify atmospheric stability.
The fire suppression forecast (FWS) is also known as the Spot Fire Weather Forecast. It is a site-specific, localized forecast of weather, winds, temperature, and humidity. It normally consists of a detailed 12-hour forecast and an additional 12-hour outlook. Spot forecasts are initially issued only upon request. During an extensive fire or burn, spot forecasts are routinely issued twice daily. Forecasts have a resolution down to one square mile. Figure14 on page56 is an example of a fire suppression forecast.
Figure 14 - Fire Suppression Forecast
The synopsis is a narrative description of the synoptic weather pattern including high pressure centers, airmass characteristics (e.g., dry and stable), and frontal position and movement.
This is a brief description of the sky coverage and precipitation, based on the weather codes listed in Table18 on page48. Note that the FWS product contains a description of the sky and weather and does not specify the weather code.
The probability of precipitation for the various forecast periods is stated as a percentage.
The amount of precipitation expected during the various forecast periods is specified in inches.
The probability of enough precipitation to wet the fuel significantly. The quantity of precipitation is typically 0.10" over a period of 12 hours and is tailorable from site to site.
The lightning activity level reflects the forecast lightning levels during the various periods. The meaning of the lightning activity level is explained in Table19 on page49.
Precipitation duration is calculated in hours for the various forecast periods.
This parameter is a measure of the amount of moisture in the wood and timber which affects how well they will ignite and burn. Fuel moisture is calculated for the tomorrow period only.
The forecast average 7-meter height wind direction for each period is specified to eight compass locations. If the wind speed is light, then the direction may be given as VARIABLE.
The forecast average 7-meter wind speed for each of the periods is specified in miles per hour. Typically a range of speeds is given, e.g., 10 TO 15 MPH. The word LIGHT is used for speeds below 5 mph.
The forecast 7-meter wind gust for each of the periods is specified in miles per hour.
The wind direction (to eight compass points) and speed (mph) for the wind above the boundary layer.
Significant wind shift information, consisting of both direction and speed changes, is specified in the product. The time of the wind shift is also given.
The expected maximum (minimum) temperature for the daytime (nighttime) forecast periods.
The expected maximum (minimum) relative humidity for the nighttime (daytime) forecast periods.
The Haines Index is included for the daytime forecast periods.
Height above ground level of the base and top of inversions expected during the forecast periods.
Footnotes
- (1)
- Information obtained from August, 1992 Service Transition Plan for Fire Weather Operations and Services, TSP 91-10, Format Fire Weather Forecasts, and WSOM Chapter D-06, Fire Weather Service Program.
- (2)
- Information obtained from TSP 90-27.
- (3)
- Information obtained from the WSOM Chapter D-06, Fire Weather Service Program, dated August 22, 1991.
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