The following changes to the Fire Weather Outlooks were implemented on Wed. March 15, 2000.
Critical Fire Weather Areas will replace the Risk Categories in the Outlooks.
Risk Categories (1 - 5) WILL NOT appear on the outlooks. However, the Risk Category Table (shown below) will be used INTERNALLY by the SPC as a guideline in the determination of areas to be outlooked. This table has been updated from the one used in 1999 based upon comments by forecasters and Regions.
A Critical Fire Weather Area will normally be outlooked when, in the judgement of the fire weather meteorologist, a Risk Category 1 or higher is forecast in the central or eastern U.S., or a 3, 4, or 5 is forecast in the West. However, during the hot dry summer months (June through August) in the heavily forested areas of the West, only a 4 or 5 risk category would normally necessitate a Critical Fire Weather Area. In areas where the Fire Danger Class is reported, internal SPC guidelines have been updated to include only areas that have a High, Very High, or Extreme Fire Danger Class. Also, in the western United States, dryness of 100 and 1000 hour dead fuels will be more closely monitored since the 10 hour fuel moisture is nearly always very low throughout the summer.
When, in the judgement of the Fire Weather Outlook meteorologist, the equivalent of a risk category 3 (or higher) exists in the central or eastern U.S., or a category 5 exists in the West (i.e., a major fire weather event is forecast), the "Critical Fire Weather Area" phrase will be updated to read "Extremely Critical Fire Weather Area". Normally a risk category 5 in the West would be in a critical fire weather pattern such as a Santa Ana.
Each Critical Fire Weather Area in the Outlooks will be identified by an area number in the text and identified on the graphic by solid lines and the label "Area 1", "Area 2", etc.. When an Extremely Critical Fire Weather Area is outlooked, it will be identified as such in the text and the graphic area label will read Area # - Extremely Critical Fire Weather.
When no areas are expected, the graphic and text should say: NO SIGNIFICANT LARGE SCALE FIRE WEATHER CONDITIONS EXPECTED. The synopsis will always be issued, even when there are no fire weather outlook areas.
Also, the outlooked areas should focus on the forecast weather conditions and not on the fuels. Using a Fire Danger Class of high (and above) along with our other criteria automatically addresses the fuel issue.
IMPACT OF THE CHANGES
These changes should reduce the size and number of the fire weather areas, especially in the West. There will be many days when no fire weather areas are outlooked, especially during the winter months.
As an example, in 1999, we highlighted the West with Risk Category 4 on less than 25% of the days from June 1 through August 31 and the areas were rather small. On several days during the first week of August, the Risk Category 4 areas correctly centered on the largest wildfire outbreak in Northern Nevada in 35 years. Using our revised guidelines, the "Critical Fire Weather Area" (previously Risk category 4) would be used in this case.
The following Risk Category guidelines have been revised from 1999.
|INTERNAL FIRE WEATHER RISK CATEGORIES (METEOROLOGICAL BASED) Fuels that are highly volatile may contribute to more extreme fire behavior. Forecast above normal maximum temperatures >75 F (May 1 through Sept. 30) or greater than 60 F (Oct. 1 through Apr. 30)|
|SFC WIND 10-<20mph and not heavily forested||SFC WIND 20-<30mph||SFC WIND 30-<40mph||SFC WIND 40mph or greater|
|SFC RH Florida Peninsula 30 - <40%||1||2||3||4|
|ALL US * 20 - <30%||2||3||4||5|
|ALL US <20%||3||4||5||5|
AREAS EXCLUDED: SPC WILL NORMALLY EXCLUDE AGRICULTURE LAND AND BARREN LAND FROM THE FIRE WEATHER OUTLOOK.
MODULATING FACTORS IN THE DETERMINATION OF CRITICAL FIRE WEATHER AREAS:
LONG TERM: High drought indices (KBDI greater than 400 and/or Palmer DI moderate to extreme drought) or an area significantly deficient from normal precipitation for one month or longer. KBDI is designed primarily for the southeast US stations and should not be used in the West. KBDI is reported daily at many forest stations. Palmer data is collected weekly on Saturday and reported on Tuesday, thus any rains on Saturday through Monday could result in inaccurate readings and should be taken into account.
Above normal maximum temperatures, generally greater than 75 degrees F (May 1 through September 30) or greater than 60 degrees F (October 1 through April 30) are TEMPERATURE GUIDELINES and are only approximate.
SHORT TERM: No wetting rains forecast during the period (A wetting rain here is taken to be >=0.10 (one-tenth) inch over approximately 50 percent of the area). Steep afternoon lapse rates should also be considered, especially when higher winds or drier air aloft may be mixed downward to the surface.
THE FOLLOWING CONDITIONS SHOULD EXIST: 1) The majority of the Fire Danger Class indices across the area are High, Very High, or Extreme* (when reported) or 2) Dead or cured vegetation (primarily 1-hour dead fuels) anytime of the year, including annual grasses in the springtime over the central and eastern U.S. and (when reported) 3) Low dead fuel moisture (100-hr<=10%, 1000-hr<=10%), or 4) Dry thunderstorm forecast (less than 0.10 inch liquid precipitation, average relative humidity less approximately less than 30% in the sub-cloud layer and cloud bases around 10,000 ft above ground level or higher). *it is not uncommon to find isolated moderate reports within an area.
DRY THUNDERSTORMS - Although thunderstorms can range from "wet", to a "mix of wet and dry storms", and to finally a "dry thunderstorm", SPC forecasts for dry thunderstorms will be for areas the fire weather meteorologist is reasonably certain will see a significant number of lightning flashes from dry thunderstorms (i.e., a major dry thunderstorm event is forecast with approximately 100 or more flashes expected per 100,000 square miles (an area approximately the size of Colorado) in a 24 hour period). This will likely reduce the number and areal extent of dry thunderstorm forecasts (compared to 1999), but also allow the SPC to cover the major events (such as Montana (July 24, 1999), Northern California (August 22-23, 1999) and northern Nevada (August 1999) . When dry thunderstorms are forecast, they will be shown as hatched areas on the Day 1 and Day 2 graphics.
PLUME DOMINATED FIRES require large quantities of dense, large, dry fuels to produce the large energy release needed for a plume dominated wildfire. Most wildfires will be wind driven fires. However, plume dominated fires may occur (IF sufficient amounts of dry fuels are present) for winds generally at or below 20 mph in the lower levels of the atmosphere (approximately the lowest 6000 ft above ground). The possibility of plume dominated fires would be forecast only for those areas where fuels would be readily available (i.e., forests).
FOSBERG FIRE WEATHER INDEX (FFWI) and Relation to Risk Category Table The Fosberg Fire Weather Index (FFWI) (Fosberg 1978) can be used ("in theory") as a measure of the rate of spread and energy release based ONLY on the temperature, humidity and wind speed. It is most influenced by the humidity and wind speed and very weakly by temperature. Using the humidity and wind speed (assuming TEMP= 80 F), in the SPC Internal Risk Category Table, the following are the approximate ranges of the FFWI:
|Fosberg Fire WX Index||15-25||26-50||51-75||76-85||<85|
The fire danger classes (when reported), the dryness of live and dead fuels and any significant departures from normal precipitation (long term and short term) are first evaluated nationwide to determine if any Risk Categories are warranted.