FIRE WEATHER PLANNING FORECAST
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.
1. SKY COVER
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.
2. PRECIPITATION TYPE
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..
3. TEMPERATURE
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.
4. RELATIVE HUMIDITY
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.
5. WIND - DIRECTION AND SPEED
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.
6. WIND SHIFT
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).
7. POPS AND TYPE
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).
8. SMOKE MANAGEMNET FORECAST PARAMETERS
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.
A. AFTERNOON MIXING HEIGHT
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.
B. TRANSPORT WIND
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.)
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