FIRE WEATHER DANGER TERMS AND EXPLANATIONS
Background - Daily, the LACoFD processes wildland fire weather data from manual and Remote Automated Weather Stations (RAWS) that it operates, or acquires data from other agencies located in Los Angeles County. These agencies include the City of Beverly Hills, National Park Service and Angeles National Forest. These observations are reported electronically to the Weather Information Management System (WIMS) between 1 and 1:30 p.m. Local Standard Time (LST) and are processed by National Fire Danger Rating System (NFDRS) algorithms. A few of the stations are seasonal and do not report during the off season. The LACoFD queries WIMS each afternoon and generates the Fire Weather Calculations worksheet. Typically during the late spring, summer and fall, Fire Weather Forecasters view these local observations and issue trend forecasts for fire weather forecast zones at approximately 2 p.m. LST. WIMS processes these forecasts into next-day forecasts. The LACoFD acquires this information and posts it into its fire danger rating areas. Daily, LACoFD staff reviews the quality of the data received from each weather station. Data that is known to contain significant errors is not posted into the daily fire weather calculations to maintain data quality. The Daily Fire Weather Calculations are typically posted to the Internet at approximately 3 p.m. LST.
Forecast or Observed Data – Fire Weather Danger is described as either "Observered" or "Forecast". All Fire Weather Calculations are based on once-daily, mid-afternoon observations. During late fall, winter and early spring, observations are generally provided as the fire hazards are typically low. In late spring, summer and fall, the weather service office provides forecast data. The exact timing of when forecasts become available is dependent upon several factors including but not limited to weather patterns, rainfall, and wildland fire activity.
Date – The data represents the observation or forecast day that is displayed.
Threshold – Represents the numeric area average Burning Index (BI) that the LACoFD considers for augmented staffing, response and or deployment of engines, crews, and or other resources to high hazard areas. The BI is one of several National Fire Danger Rating System (NFDRS) outputs that is related to the difficulty of containment or control of wildfire. LACoFD, based on historical fire weather, climatology and fire suppression effectiveness, sets the threshold breakpoints. These breakpoints help guide decision making. The two fire danger rating areas for which calculations have been established are in the Santa Clarita Valley and Antelope Valley. For the LA Basin, Malibu and High Country, the Department uses the Santa Clarita threshold.
Area – The area column identifies each of the five fire danger rating areas of LA Basin, Malibu, Santa Clarita Valley, High Country or Antelope Valley
Station Name – The name of the reporting weather station is shown. For RAWS stations, the word RAWS occurs. If the word RAWS is not shown, the station is a manual weather station.
Nearby Fire Station – The closest LACoFD fire station to the weather station. This information is useful to LACoFD staff, its volunteers, and other organizations.
Station No./Model – The unique six number weather station identifier followed by the primary NFDRS fuel model that is used for the daily calculations. The models are B – Brush, F – Young Open Mixed Chaparral, and T – Sagebrush Grass.
Burning Index (BI) – A NFDRS output that represents the difficulty of containing and controlling wildfire in the selected fuel type. The BI is also related to potential flame lengths for the selected fuel type. The higher the number, the greater the difficulty in extinguishing a wildfire. A BI represents current and antecedent weather, fuel types, and the state of both live and dead fuel moisture.
Temperature – The temperature reported in degrees Fahrenheit at 1 p.m. LST. Humidity – The relative humidity (RH) represented as a percent of maximum for the reported temperature. Fires become more difficult to control when RH is low. Readings of 15 percent or less are cause for great concern.
Wind – The 10-minute average wind speed reported in miles per hour (MPH) for the reporting weather station. This weather factor is the most critical component to fire containment and control. During higher wind speeds, fires are more difficult to control and spot fires occur more readily. Extreme conditions are generally when wind speeds exceed 25 MPH.
Fuel Stick – The percent of moisture contained in dead fuels of 1/4 to 1" diameters. Measurements of 5 or less are critical to fire control.
Adjective Rating – Refers to the public narrative rating for the BI. This is often used in public relations efforts to broadly reflect threats posed by wildland fire. The ratings consist of low, moderate, high, very high and extreme. The adjective is provided for fire danger area and also as a County-wide average.
Number of Stations Reporting – At the bottom of the report, the number of weather stations reporting from the total pool that are available in each rating area. The higher the number of reporting stations, the higher the confidence in the data for making decisions. Generally, more stations report during the late spring, summer and fall as manual weather station observations are more available.