The Fire Weather Planning Forecast (product identifier: WBCFWFAKQ, MND header: FNUS51 KAKQ), also known as a presuppression forecast, is issued at least once daily--usually by 6:00 a.m. The purpose of the product is to provide weather forecasts tailored to support fire control efforts in South Central Virginia, The Lower Eastern shore of Maryland and Virginia, and Northeastern North Carolina. Fire Weather Planning Forecasts may also be updated at any time of day if needed.
Headlines. A headline is required when Red Flag Warnings and/or Fire Weather Watches are in effect. The headline will include the warning type, location,
and effective time period. Significant trends of locally-defined critical weather elements may also be headlined for non-watch or non-warning periods.
Discussion. The discussion should be a brief, clear, non-technical description of weather patterns that influence the weather in the forecast area.
2-Day Tabular Forecasts. The FWF product has three 12-hour time periods in the morning forecast. Each time period includes the following weather elements:
CLOUD COVER - Expressed in terms similar to those used in the zone forecast product.
PRECIP TYPE - Expressed in terms similar to those used in the zone forecast product.
CHANCE PRECIP (PoP) (%) - Rounded to the nearest 10 percent and should be the same as Probability of Precipitation (PoP) used in the zone forecast product.
TEMP - Expressed in terms similar to those used in the zone forecast product. Based on ASOS measurement standards.
RH (%) - Relative Humidity is expressed in terms of maximums and minimums for the day.
20FTWIND/AM(MPH) - Morning forecast portion of the daytime winds. Derived from ASOS measurement standards. The winds will be indicated as 20-foot level, which is estimated by multiplying the 33 foot model wind forecast by a factor of 0.8. Use the 8-point compass for the wind direction.
20FTWIND/PM(MPH) - Either the afternoon portion of the daytime winds or both the evening and early morning portions of the nighttime winds.
LAL - Lightning Activity Level. See glossary for details.
HAINES - Defined as Low Level Haines Index. See glossary for details.
MIXING HGT(FT-AGL) - The forecast maximum mixing height. Nighttime minimums are not included in the forecast.
TRANSPORT WINDS(MPH) - The average direction and speed of the wind within the mixing layer during the time of maximum afternoon mixing. Nighttime transport winds for minimum mixing are not included in the forecast.
SMOKE DISPERSAL - Provides a measure of the atmosphere's ability to rid itself of smoke. See glossary for details.
Extended Forecast (Days 3 through 7). In this section, a forecast period is broken down into 24-hour blocks of time beginning at 12 midnight and ending at 12 midnight the next day. Weather elements in the outlook period will include Sky/Weather, Temperature, and 33-foot winds.
Extended portions of these fire weather forecasts show a slight resemblance to local or zone forecast products. Due to automated formatting procedures, there may be slight discrepancies between the zone forecast and the fire weather forecast. However, the graphical forecast provided on the Wakefield NWS homepage should represent both forecasts.
Note: Routine fire weather forecasts are used to give a general overview of the expected weather conditions for a given area. For specific weather forecasts and data, please contact the NWS at 757-899-4200.
ASOS: The Automated Surface Observing System is the predominant weather observing unit used for verification of NWS forecasts.
Fire Weather Watch: Fire weather watches are issued to alert fire and land management agencies to the possibility of red flag conditions beyond the first forecast period (12 hours). The watch is issued generally 12 to 48 hours in advance of the expected conditions, but can be issued up to 72 hours in advance if the forecaster is reasonably confident. A watch may be issued (or continued) in the first 12-hour time period for dry thunderstorm events.
Haines Index (HI): An atmospheric index used to indicate the potential for wildfire growth by measuring the stability and dryness of the air.
The HI numbers are computed for 3 different elevations using the following parameters:
HI = STABILITY TERM (A) + MOISTURE TERM (B)
NWS Wakefield uses low elevations to compute the moisture and stability terms for HI where:
A = 950-850 MB TEMP
B = 850 MB TEMP-DEW POINT
Moisture and stability terms are categorized as follows:
|A=1 when 3 deg C or less||B=1 when 5 deg C or less|
|A=2 when 4-7 deg C||B=2 when 6-9 deg C|
|A=3 when 8 deg C or more||B=3 when 10 deg C or more|
Haines Index classifications are assigned to values 2 through 6 as shown below:
|Haines Index||Potential for Large Fire Growth|
|2 or 3||Very Low|
Lightning Activity Level (LAL): A number, on a scale from 1 to 6, which reflects frequency and character of cloud-to-ground lightning.
LAL Guide for Fire Weather Observers: Cloud-to-ground lightning discharges (cg) in individual storm cells
|Cloud and Storm
|Areal Coverage||Counts cg / 5 min||Counts cg / 15 min||Average cg / min|
|2||Cumulus clouds are common but only a few reach the towering stage. A single thunderstorm must be confirmed in the rating area. The clouds mostly produce virga but light rain will occasionally reach ground. Lightning is very infrequent.||<15%||1-5||1-8||<1|
|3||Cumulus clouds are common. Swelling and towering cumulus cover less than 2/10 of the sky. Thunderstorms are few, but 2 to 3 occur within the observation area. Light to moderate rain will reach the ground, and lightning is infrequent.||15% to 24%||6-10||9-15||1-2|
|4||Swelling cumulus and towering cumulus cover 2-3/10 of the sky. Thunderstorms are scattered but more than three must occur within the observation area. Moderate rain is commonly produced, and lightning is frequent.||25% to 50%||11-15||16-25||2-3|
|5||Towering cumulus and thunderstorms are numerous. They cover more than 3/10 and occasionally obscure the sky. Rain is moderate to heavy, and lightning is frequent and intense.||>50%||>15||>25||>3|
|6||Dry lightning outbreak. (LAL of 3 or greater with majority of storms producing little or no rainfall.)||>15%||-||-||-|
Mixing Height: Mixing height is defined as the height above the surface through which relatively vigorous mixing will take place due to convection. The mixing height is generally found at the base of a temperature inversion. Units used are above ground level (agl).
Probability of Precipitation (PoP): The probability of measurable precipitation (0.01 or greater) at any given location within an area defined by a forecaster.
Red Flag Criteria: The basic red flag criteria are a combination of weather and fuel conditions met for any 3 hours or more in a 12 hour period. These criteria are defined as the following:
In addition to the basic criteria above, a combination of other elements may result in Red Flag conditions. Haines Index of 5 or 6, wind shifts associated with cold frontal passages, first significant lightning (wet or dry, 15% coverage of thunderstorms or more) event after an extended hot and dry period, and poor relative humidity recovery.
Red Flag Warning: A red flag warning is used to alert fire and land management agencies that red flag conditions exist or are imminent. A red flag warning will be issued immediately when there is high confidence that red flag criteria will occur within the next 24 hours, or if those criteria are already being met.
Smoke Dispersal: Dispersion is a combination of vertical mixing and horizontal transport. These two components are independent of each other. Vertical mixing is a function of atmospheric stability. A stable airmass is characterized by poor vertical mixing; an unstable airmass is characterized by good vertical mixing. Horizontal transport is a function of wind speed: the stronger the wind, the better the horizontal transport.
Smoke Dispersion will be forecast using one of the following terms:
|VERY POOR||very high air pollution potential|
|POOR||moderate to high air pollution potential|
|FAIR||marginal air pollution potential|
|GOOD||moderate to low air pollution potential|
|VERY GOOD||low air pollution potential|
|EXCELLENT||very low air pollution potential|
Dispersion is related and often interchanged with the term "VENTILATION". The ventilation index is a product of mixing height TIMES the transport wind and is measured in knot-feet.
Transport Winds: The mean wind speed and direction of the layer between the surface and mixing height. Miles Per Hour (mph) is used for wind speed and an 8-point compass is used to describe wind direction.