National Weather Service United States Department of Commerce

Definitions For Fire Weather Forecasts


During the fire season the Fire Weather Planning Forecast will normally be issued by 300 PM Guam Local Time, seven days a week. A headline may be added to the top of the forecast, denoting significant weather, or for the issuance of a Red Flag Warning or Fire Weather Watch. The discussion will briefly cover locations of fronts and systems which produce the weather along with highlighting significant trends or changes that the forecaster anticipates. The 2 day tabular forecast will cover specific weather elements mentioned below. The narrative extended forecast portion of the forecast will pick up where the short term left off and continue out through day seven. The extended portion is a general forecast which mentions the possibility of precipitation, expected high and low temperatures for each day, and wind speeds and direction.

Elements of the tabular and narrative sections are described below.

A. Clear (or Sunny) -- < 1/8th cloud cover.
B. Mostly Clear/Mostly Sunny -- 1/8th to 2/8ths of cloud cover.
C. Partly Cloudy/Partly Sunny-- 3/8ths to 5/8ths of cloud cover.
D. Mostly Cloudy -- 6/8ths to 7/8ths cloud cover.
E. Cloudy -- 8/8ths cloud cover.
F. Increasing Cloudiness -- the clouds are increasing in amount (this also implies thickening of clouds).
G. Decreasing Cloudiness-- A progressive decrease in the amount of sky covered with clouds.
H. Variable Cloudiness-- A constant variation in the amount of clouds covering the sky with respect to time and space.

A. Rain--General, not showery, usually in a stable atmosphere. Small to medium sized water droplets.
B. Drizzle--General precipitation in a stable atmosphere. Very small water droplets that appear to float in the atmosphere.
C. Showers--Rain/snowfall of short duration and varying intensity, usually beginning and ending abruptly.
D. Thunderstorms--Downpour of rain, often with strong gusty winds..

The temperature will be in degrees Fahrenheit. The maximum and minimum temperatures are forecast for the 30-hour period from 1:00 PM the day of the forecast until 7:00 PM the next day.

The Relative Humidity (RH) is the ratio, in percent, of the amount of moisture in the air compared to the amount the air could hold if fully saturated (100%). The range of RH is from 0% to 100%. Usually, the minimum RH occurs at the time of the maximum temperature and the maximum RH occurs at the time of the minimum temperature. Because of the dependency of the relative humidity upon temperature, it should be noted that if the temperature is under forecast (the actual temperature is higher than forecast), then the forecasted relative humidity will likely will be too high.

The wind direction applies to the direction from which the wind will blow. The direction will be listed using the 16 point compass (e.g. NE, S, WSW, etc.). Any significant changes expected during the forecast period will be mentioned in the narrative. The wind speed will be in miles per hour (mph). The speed is the forecast for the 20-foot level. Speeds pertain to the two minute averages while gusts pertain to the maximum instantaneous value expected.

If a shift in wind direction associated with a shear-line passage is expected during the period, the new direction and wind speed will be forecast. Wind shifts may also be mentioned in the synopsis. Because a shear-line may take several hours to move through a zone, the approximate time of the wind shift will be encoded (i.e. Northeast 10 to 15 mph after midnight).

The probability of precipitation, or POP, expresses the chance that measurable rainfall will occur at any given point within a county zone group. Measurable rainfall is 0.01 inches or greater. Probability is expressed in percent. A forecast of the predominate type of precipitation will accompany a probability of precipitation forecast (i.e. 40 percent chance of showers, 60 percent chance of rain, 100 percent chance of thunderstorms).

The forecast parameters include mixing height, and transport wind. Note: One consequence of the Clean Air Act is that land managers must practice principles of careful smoke management. This is done by combining favorable meteorological conditions with a variety of prescribed fire techniques so that smoke will be readily dispersed. The Clean Air Act has established 500 meters (1700 feet) as a minimum for mixing height for permitting prescribed burning.

Mixing height is the extent or depth to which smoke will be dispersed by means of turbulence and diffusion. The forecast of mixing height is expressed in feet above ground level.

Transport wind is the average wind speed in mph in the mixing depth above the surface. These winds are a good indication of the horizontal dispersion of suspended particles. The transport wind is the forecast wind at the time of maximum mixing of the atmosphere, normally during the mid afternoon. Usually a wind of less than 8 mph restricts an agency from burning. Transport wind directions are typically given to eight compass points (e.g. northeast, east southwest, etc.)