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WEATHER FORECAST INTERPRETATION

 

Air Quality Warnings:

1.      Ozone: During Ozone Season (from May through September), Georgia Environmental Protection Division (GAEPD) along with other agencies in surrounding states will issue ozone forecasts.  When an area is forecast to have high level of Ozone (defined as at or above Orange Class), Georgia Forestry Commission will to stop issue burn permits for the area.  The below statement will appear on the weather forecast:

In corporation with the Georgia Environmental Protection Division, the Georgia Forestry Commission will not issue burning permits due to the high probability that open burning would make an unacceptable contribution to ozone formation today for the following counties, (listing of affected counties)”

 

2.      PM2.5 (Particulate Matter smaller than 2.5 micrometer in size):  GAEPD along with other agencies in surrounding states are issuing PM2.5 forecast year round.  When an area is forecast to have high level of PM2.5, the following statement will appear on the weather forecast:

“PM2.5 is forecast to be high (i.e. At or Above Orange Class) for today for the following counties, (listing of affected counties….”

 

CAUTION ITEM:  The following statements will show up if following conditions occurs:

  1. If the manning class forecast for today at stations within or near the district is in Class 4 or Class 5, a warning statement will appear in the forecast. 
  2. If wind direction is shifting clockwise or anticlockwise through a period, a warning statement will appear in the forecast.  It will say, "Wind direction shift occurs in Xth period". 
  3. If wind speed is increasing through a period, a warning statement will appear in the forecast.  It will say, "Wind speed increases in Xth period".  Xth period is the period in which the event is forecast to occur. 
  4. If forecast relative humidity is at or below 25% at any of the periods, a warning statement will appear in the forecast.  It will say, "Relative humidity is below 25% in Xth period". 
  5. If strong gusty wind is forecast for any of the periods, a warning statement will appear in the forecast.  It will say, "Strong Gusty Wind in Xth period".
  6. If occassional gusts is forecast for any of the periods, a warning statement will appear in the forecast.  It will say, "Occassional Gusts in Xth period".

 

Xth period is the period in which the event is forecast to occur.  1st period is the 1st column in the forecast.  It is "Today" for the morning forecast and "Tonight" for afternoon update. 

 

SKY CONDITION: This variable describes cloud amount, and how clouds may increase or decrease.  Tables 1 and 2 list the adjectives used to describe sky condition.

 

Table 1.  Sky condition: The amount of cloud in the forecast area for the period.  The   

following table is a list of descriptors used when cloudiness stays the same in the period.  

Percentage of cloud cover

Day

Night

0%

Sunny

Clear

10%

Sunny to Mostly Sunny

Fair

20%

Mostly Sunny

Mostly Fair

30%-50%

Partly Sunny

Partly Cloudy

60%-70%

Mostly Cloudy

Mostly Cloudy

90%-100%

Cloudy

Cloudy

 

Table 2.  If the cloudiness is forecast to change during the period, the following descriptors are used.

Descriptors 

Meaning

Variable Cloudiness

Cloudiness varies in the period

Increasing Cloudiness

Cloudiness increases within the period

Decreasing Cloudiness

Cloudiness decreases within the period

Becoming Partly Cloudy

Partly cloudy sky is expected at the end of the period

Becoming Mostly Fair

Mostly clear sky is expected at the end of the period

Becoming Partly Sunny

Partly sunny sky is expected at the end of the period

Becoming Mostly Sunny

Mostly sunny sky is expected at the end of the period

Becoming Mostly Cloudy

Mostly cloudy sky is expected at the end of the period

 

TEMPERATURE:  This is the extreme temperature (maximum or minimum) expected for the forecast period.   Daytime maximum temperatures usually occur between 1-4pm.  Nighttime minimum temperatures usually occur at or near sunrise. 

 

RELATIVE HUMIDITY: This is the extreme relative humidity (maximum or minimum) expected for the forecast period.  During the day, minimum relative humidities usually occur between 1-4pm.  Nighttime maximum relativity humidity usually occurs just before sunrise. 

 

HEAT INDEX: This is the heat index as calculated by the National Weather Service.  It is based on temperature and humidity.  The heat index category and effects are in Table 3 and it is as used by National Weather Service. 

 

Table 3 Heat Index category and effects

Category

Heat Index

Possible heat disorders for people in high risk groups

Extreme
Danger

130°F or higher

Heat stroke or sunstroke likely.

Danger

105 - 129°F

Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.

Extreme
Caution

90 - 105°F

Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.

Caution

80 - 90°F

Fatigue possible with prolonged exposure and/or physical activity

 

WIND CHILL TEMPERATURE: This is the Wind Chill Temperature as calculated by the National Weather Service.  It is based on temperature and wind speed. 

 

PROBABILITY OF PRECIPITATION:  This is the probability of precipitation expressed as a percentage.  The phrases associated with various probabilities are detailed in Table 4.

 

Table 4.  Probability of Precipitation: The probabilities and associated phrases associated

with probability of precipitation. 

Probability of Precipitation

Qualifying remarks

10%-20%

Slight chance

30%-40%

Some chance

50%

Good chance

60%-70%

Likely

80% or above

Will have precipitation

 

PRECIPITATION TYPE: The type of precipitation such as showers, rain, thunderstorm, drizzle, snow, and sleet.   This line will not be in the forecast if PROBABILITY OF PRECIPITATION is none. 

 

PRECIPITATION AMOUNT: The amount of precipitation in inches.  This line will not be in the forecast if PROBABILITY OF PRECIPITATION is none.  Forecast precipitation amount of 2 inch or more is rare.

 

PRECIPITATION DURATION: The length in hour of precipitation is expected to last.  This line will not be in the forecast if PROBABILITY OF PRECIPITATION is none.

 

SURFACE WINDS (OPEN): Surface wind speed and wind direction in an open field at 20 feet.  Special notes include variable and occasionally gusty.  Variable means that wind direction will be variable and this usually occurs at wind speeds of less than 5-7 MPH.  Occasionally gusty means that wind speed will be occasionally gusty and this is usually associated with frontal passage.

 

WIND SPEED INCREASE WITHIN PERIOD:  For the periods in which the event occurs, this field indicates the beginning and ending wind speed.  For the periods in which the event does not occur, "None" will appear in the field.  If this event is not forecast in any of the periods, this line will not appear in the forecast.

 

WIND DIRECTION SHIFT WITHIN PERIOD:  For the periods in which the event occurs, this field indicates the beginning and ending wind direction and whether it is a clockwise or counter-clockwise shift.  For the periods in which the event does not occur, "None" will appear in the field.  If this event is not forecast in any of the periods, this line will not appear in the forecast.

 

CANOPY WINDS: Expected wind speed and wind direction inside a forest.  It is assumed this wind is representative of a pine stand of basal area 100, with no more than a 3-year rough.

 

THE SMOKE DISPERSION INDEX: The smoke dispersion index is a measure of the atmosphere’s "diluting power".  Values are relative in that an index of 48 would suggest that the atmosphere should be able to take twice the smoke as would be possible for an index value of 24. The atmosphere over a district or county can be viewed as a covered cake pan in simplistic terms. Even if the dispersion in this cake pan were good, it would still be possible to saturate the container with too much smoke. This index SHOULD NOT BE USED ALONE in the deciding whether to initiate or approve a burn.  Description and interpretation of the range of smoke dispersion values can be found in Table 5.  Caution should be used in initiating or permitting burns when daytime values are below 40.

 

No two cases are ever exactly alike.  Thus, some reasonable judgment and common sense needs to be applied. The dispersion index as used in the GFC fire weather forecast is valid for the middle of the daytime periods and from the middle to last half of the nighttime periods.

Because the daytime dispersion index given is, say 50, does NOT mean that the dispersion index starts out at 50 for the given forecast period. There is a "time window" during the 12‑hour forecast period when, if the forecast is correct, this dispersion index should be at or near 50.

 

At sunrise, dispersion is low normally. As the sun gets higher and induces more heating, the dispersion index will climb. At first, the amount of improvement will be minimal. About 3 or 4 hours after sunrise, the amount of improvement should be significant. On average, the best dispersion will be in the early to mid‑afternoon. After this period, dispersion will start to degrade and towards sunset, it should drop like a "rock". Each case will be somewhat different as many factors enter into the equations.

                

 

Table 5.  Smoke Dispersion Index Interpretation (Lavdas, 1986)

Dispersion Index

Interpretation

> 100

Very good (but may indirectly indicate hazardous conditions; check fire weather)

61-100

Good (Typical-case burning weather values are in this range)

41-60

Generally good (Climatological afternoon values in most inland forested areas of the United States fall in this range)

21-40

Fair (Stagnation may be indicated if accompanied by persistent low windspeeds)

13-20

Generally poor; stagnation, if persistent (although better than average for a night value)

7-12

Poor; stagnant at day (but near or above average at night)

1-6

Very poor (very frequent at night; represents the majority of nights in many locations)

 

 

FOG POTENTIAL:  This is an experience based estimation of the potential for natural fog.

It is a function of cloud cover, probability of precipitation, relative humidity, and wind speed.

 

LOW VISIBILITY OCCURRENCE RISK INDEX :  The Low Visibility Occurrence Risk Index (LVORI) was developed based on  Florida automobile accident reports when fog/smoke was a factor, and the associated weather (Lavdas and Achtemeier, 1995).  It is an index for probability of low visibility, and ranges from 1 – 10, depending on relative humidity and smoke dispersion index.  A 1 means there is almost no chance of low visibility, while a 10 indicates low visibility is likely.  This Index is a function of relative humidity and smoke dispersion index.

 

From operational experience, one should be VERY CAUTIOUS ABOUT BURNING if one of the following situations occurs:

 

1.  When LVORI for a nighttime forecast period is 8,9 or 10

 

2.  When ACTIVE SMOKE from stumps logs, etc. is present during the night

 

3. When there is a roadway within three miles of a burn site with open fields, logging roads, or open streams that can provide an easy transit of the smoke from the burn site to the roadway

 

Although not as dangerous as an index value from 8 to 10,  LVORI of 6 or 7 should be treated with caution if there is active smoke produced during the night, and the burn site is near a roadway.  

 

NWS at Greenville/Spartanburg has an informative page on LVORI and Dispersion Index.

 

MIXING HEIGHT: The mixing height is the thickness of the lower layer of the atmosphere where "mixing" occurs.  Usually, it is in the order of a few thousand-feet during the day and on the order of a few hundred-feet at night. . It is available in the forecast as both metric and non‑metric units. Most federal Environmental Protection Agency’s (EPA) models use the metric values.

 

In general, mixing height will be highest during daytime periods and lowest during nighttime periods. Heating, winds, etc. are some of the factors that determine how thick the mixing layer will become.

 

During the daytime, mixing height will be lowest at sunrise and should reach maximum thickness by mid‑day. The thickness of this layer will normally decrease fairly quickly as sunset approaches.  Daytime values below 1700 feet are generally associated with poor smoke dispersion. 

 

A simplistic analogy would be the afternoon and evening thunderstorms that commonly occur in the south during the summer. As the maximum heating of the day approaches, the clouds start to boil upwards and eventually may form thunderstorms. Assuming no other potential influences (fronts, etc.), as sunset approaches, the heating that drives these thunderstorms is shut off. Often times,  these thunderstorms will begin to dissipate soon after sunset. 

 

TRANSPORT WIND SPEED: This is the averaged of all wind speeds between the surface and the mixing height.  Spotting and suspended particles from burning are typically transported at this speed. It is available in the forecast as both metric and non‑metric values. Most of EPA’s models use metric values.

 

TURNER STABILITY AND ATMOSPHERIC TENDENCY -Turner stability (Turner 1961) is a measure of atmospheric stability with values ranging between 1 and 7.

 

By definition, Turner values of 1, 2 or 3 are possible only during daytime periods and values of 5, 6 or 7 are only possible during nighttime periods. A Turner value of 4 is possible both during the day and night.  Table 6 summarizes the relationship between Turner stability class and expected fire behavior.

 

Table 6.  Turner Stability Class (Pasquill Stability Class): It is computed as a function of

ceiling height, solar radiation, day/night period and wind speed.  Turner (1961)

Turner Stability Class

Stability of the Atmosphere

Expected Fire Behavior

1

Extremely unstable

Erratic

2

Moderately unstable

Intense

3

Slightly unstable

Normal

4

Neutral

Normal

5

Slightly stable

Normal

6

Moderately stable

Subdued

7

Extremely stable

Subdued

 

 

PLUME TRAJECTORY - This is the direction that a smoke plume is expected to travel. If the plume trajectory were north, the smoke would be moving toward the north, implying southerly winds.  Trajectory as used by the Georgia Forestry Commission system will always imply the direction in which something is going. Wind directions on the other hand are the direction from which the winds are coming from.

 

DRYING POTENTIAL: Drying potential is a function of estimated pan evaporation.  Saturation vapor pressure, vapor pressure, and evaporation are computed by equations ported from the NWS Environmental Study Service Center at Auburn University.  Evaporation is a function of temperature, dew point, cloud cover and wind speed.  Table 7 is a description of evaporation and the associated adjective.

 

Table 7.   Drying Potential adjective and associated evaporation.

Drying Potential

Evaporation (inch /day)

Low

0.00-0.40

Moderate

0.41-0.80

High

0.81-1.20

Very High

1.20+

 

AVAILABLE SUNSHINE: The number of hours of sunshine