The Fire weather planning forecast (product identifier SATFWFEWX, MND header FNUS54 KEWX), also known as a presuppression forecast, is issued at least twice daily to provide weather forecasts tailored to support fire control efforts in South Central Texas--the first routine issuance by 7:00 a.m. and the second at around 3:30 pm. Fire weather planning forecasts may be updated at any time of day if needed.
Primary components:
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 a minimum of three 12-hour time periods in the morning forecast, and four 12-hour time periods in the afternoon
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 (%) - rounded to the nearest 10 percent and should be the same as Probability of Precipitation (PoP) used in the zone foreast 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.
CWR - chance of wetting rain. Similar to PoP, but a wetting rain is defined as 0.10 for South Central Texas.
HAINES - defined as Low Level Haines Index for South Central Texas. See glossary for details.
MIXING HGT(FT-AGL) - the forecast maximum mixing height for an afternoon in South Central Texas. Nighttime minimums are not included in the forecast.
TRANSPORT WINDS(KTS) - the average direction and speed of the wind during the time of maximum afternoon mixing. Nighttime transport winds for minimum mixing are not included in the forecast.
Extended Forecast. 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. Eventually, these winds will be also be provided as 20-ft 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 experimental graphical wind and humidity forecasts provided on the Austin/San Antonio 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 (830) 606-3617.
Glossary
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.
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)
The Austin/San Antonio NWS Office 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 |
| 4 |
Low |
| 5 |
Moderate |
| 6 |
High |
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 in individual storm cells (cg)
| |
Cloud & Storm
LAL Development |
Areal Coverage |
Counts cg / 5 min |
Counts cg / 15 min |
Average cg / min |
| 1 |
No thunderstorms |
- |
- |
- |
- |
| 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.) |
- |
- |
- |
- |
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 by the Austin/San Antonio NWS Office 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:
| 1. Winds and humidity |
Wind and humidity criteria should be used in combination. In addition to the criteria listed below, forecasters should also take into account that extremely low humidities/extremely hot temperatures, and very strong winds (~ 30 to 50 mph) may also contribute to extreme wildfire behavior. It is important to explain in the FWF synopsis which weather parameters will primary contributors in producing red flag conditions. |
| 1(a). winds |
Expected 20-ft wind speeds of 15 mph or greater OR 10 mph or greater if humidity is below criteria prior to a wind shift of 45 degrees or more. The wind speed criteria for fire weather watches and red flag warnings should be adjusted to 20 feet by multiplying ASOS winds speeds by 0.7. As a rule of thumb, the speed criteria (not accounting for wind shifts) would be met if forecast 33 ft wind speeds in the body of the forecast adapted from the zone forecast product (ZFP) reads as " 15 to 20 mph and gusty" or higher. |
| 1(b). humidity |
Daytime minimum of 25%; Nighttime maximum of 60% |
2. Presence of dry lightning
(LAL=6) |
Thunderstorms containing dry lightning expected in areas that have experienced prolonged dry weather. |
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.
Transport Winds: The mean wind speed and direction of the layer between the surface and mixing height. The Austin/San Antonio NWS Office uses the unit of miles per hour (mph) for speed and a 8 point compass to describe wind direction.
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