Blanco County News
Weather Partly Cloudy 84.0°F (41%)
Despite Rainfall, Fire Weather Persists
Wednesday, April 13, 2011 • Posted April 13, 2011

As of press time, several huge wildfires are raging throughout the state. The first is a rapidly-moving wildfire in Jeff Davis county that has spread to over 60,000 acres and destroyed about 40 homes in Ft. Davis before raging farther north and east. 

Another fire in west Texas has burned 16,000 acres in Midland County and destroyed about 34 homes, according to Alan Craft, a Texas Forest Service spokesman.

A third fire in north Texas has already claimed over 71,000 acres in a rural area of rolling plains. This fire hasn't destroyed any homes and was ignited by a welder's torch about 175 miles west of Fort Worth. A smaller, 10,500 acre fire in Garza county south of Lubbock had been mostly contained according to fire officials. Finally, a 60,000-acre fire ravaged Potter and Moore counties in the Panhandle. There have been some other small fires throughout the state. 

The conditions were (and are) ideal for fire weather. It has been hot, dry, and windy. Those factors, combined with shriveled and withered vegetation, create dangerous fire weather situations. It doesn't help that the high winds keep air tankers grounded. Those who pay attention to the weather have noticed that we've been in Red Flag Warnings.

To the public, a red flag warning signifies high fire danger with an increased probability of a quickly spreading grass fire within a 24 hour period. A red flag warning is a forecast warning issued by the National Weather Service (NWS) to inform firefighting and land management agencies about conditions that are ideal for wildland fire ignition and propagation. After drought conditions, when humidity is very low, and especially when high or erratic winds (which may include lightning) are a factor, the red flag warning becomes a critical statement for firefighting agencies, which often alter their staffing and equipment resources dramatically to accommodate the forecast risk. Please note that burn bans are not issued by the NWS; they are issued by county commissioners on a county-by-county basis. 

The weather criteria for fire weather watches and red flag warnings varies with each Weather Service Office’s warning area based on the local vegetation type, topography, and distance from major water sources, but usually includes the daily vegetation moisture content calculations, expected afternoon high temperature, afternoon minimum relative humidity and daytime wind speed.

A separate but less imminent forecast may include a Fire Weather Watch, which is issued to alert fire and land management agencies to the possibility that Red Flag conditions may exist 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 NWS agency is reasonably confident of the impending conditions. The term “Fire Weather Watch” is headlined in the routine forecast and issued as a product. That watch then remains in effect until it expires, is canceled, or upgraded to a Red Flag Warning.

The following is a related fire weather product also issued by the local NWS: 

FIRE WEATHER PLANNING FORECAST FOR SOUTH CENTRAL TEXAS

NATIONAL WEATHER SERVICE AUSTIN/SAN ANTONIO TX

338 PM CDT MON APR 11 2011


...RED FLAG WARNING CONTINUES THROUGH 8 PM FOR BREEZY AND DRY

CONDITIONS ACROSS CENTRAL AND EASTERN PORTIONS OF SOUTH CENTRAL

TEXAS...

...ELEVATED TO NEAR CRITICAL FIRE WEATHER CONDITIONS ARE FORECAST

FOR WESTERN PARTS OF SOUTH CENTRAL TEXAS TUESDAY AND THURSDAY

AFTERNOONS...AND FOR MUCH OF SOUTH CENTRAL TEXAS ON FRIDAY...


...DISCUSSION...

CRITICAL FIRE WEATHER CONDITIONS WILL PERSIST THROUGH EARLY THIS

EVENING OVER EASTERN PARTS OF SOUTH CENTRAL TEXAS. GUSTY WINDS

GRADUALLY RELAXING FROM WEST TO EAST...BUT VERY DRY AIR WILL

REMAIN IN PLACE TONIGHT INTO TUESDAY EVENING. ELEVATED TO NEAR

CRITICAL FIRE WEATHER CONDITIONS ARE EXPECTED TO DEVELOP WITH THE

RETURN OF SOUTH WINDS...WITH THE HIGHER 20-FOOT WINDS NEAR 15 MPH

POSSIBLE OVER VAL VERDE COUNTY. GULF MOISTURE IMPROVES HUMIDITY

RECOVERY LATE TUESDAY NIGHT...WITH GOOD MOISTURE OVER THE AREA

THROUGH WEDNESDAY. DRY AIR INTRUDES WESTERN COUNTIES THURSDAY

EVENING...AND ANOTHER DRY FRONT IS EXPECTED FRIDAY. THIS FRONT

IS EXPECTED TO BE SLIGHTLY WEAKER THAN THE ONE THAT MOVED THROUGH

EARLY THIS MORNING.


MOISTURE RECOVERY SHOULD BE GOOD BY SATURDAY NIGHT...WITH A SLIGHT

CHANCE OF RAIN EXPECTED EARLY NEXT WEEK. HOWEVER...NO SIGNIFICANT

RAIN PATTERNS ARE EXPECTED OVER THE NEXT WEEK.


INCLUDING THE CITIES OF...LLANO...BURNET...BLANCO

338 PM CDT MON APR 11 2011


...RED FLAG WARNING IN EFFECT UNTIL 8 PM CDT THIS EVENING...


                      TONIGHT      TUE          TUE NIGHT    WED


CLOUD COVER           CLEAR        CLEAR        MCLEAR       PCLDY

CHANCE PRECIP (%)     0            0            0            10

PRECIP TYPE           NONE         NONE         NONE         NONE

TEMP (24H TREND)      42 (-16)     84 (+2)      52           84

RH % (24H TREND)      65 (-35)     16 (+6)      87           29

20FTWND-AM(MPH)                    E  8                      S  9

20FTWND-PM(MPH)       N 16         E 10         SE 12        S 13

MIXING HGT(FT-AGL)                 4499                      3968

TRANSPORT WND (MPH)                SE 15                     S 17

CWR                   0            0            0            0

LAL                   1            1            1            1

HAINES INDEX          3            4            4            4

There are a lot of factors that contribute to whether or not a Red Flag Warning or Fire Weather Watch is issued. These include clouds, temperature, humidity, precipitation, and some more complicated factors such as 20-foot winds, mixing heights, transport winds, chance of wetting rain (CWR), lightning activity level (LAL), and the Haines Index. 

Mixing heights represent the top of the layer of the atmosphere through which relatively vigorous mixing will take place. Simply, it is the height at which smoke will lose its buoyancy and stop rising. A well-mixed layer is a layer in the atmosphere in which the lapse rate (change of temperature with increasing height) is roughly dry adiabatic (5.5 degrees Fahrenheit per 1,000 ft). Mixing heights commonly go through large daily and seasonal variations.

Transport winds are defined as the average wind speed and direction of all winds within the layer bounded by the surface and the mixing height. Knowledge of transport winds is crucial to smoke management programs. Transport winds provide information about the horizontal dispersion (location and distance downwind form the source) of suspended particulates from prescribed fires and slash burns. The first step in determining transport winds begins with a calculation of the mixing height using a current (or forecast) upper air sounding (usually from a weather balloon) and a surface temperature forecast. Once the mixing height has been determined, the transport wind can be calculated by averaging the reported winds (or forecast winds) from the surface to the mixing height. Since wind is a vector (having both direction and magnitude), the averaging process begins with the calculation of the zonal (U-component) and the meridional (V-component) of the wind at each level. (weather.gov)

The LAL uses information from atmospheric stability indices, CAPE values (a measure of convective energy), and 700-500 mb lapse rates to calculate the potential for thunderstorms. An LAL of 1 means no lightning is forecast. If there is a threat of instability and the ensuing thunderstorms, an LAL of 2 or 3 is issued depending on if lightning is expected to be isolated or scattered (respectively). If the threat increases to very unstable air accompanied by moderate to heavy rainfall and scattered to frequent lightning, an LAL of 4 or 5 is typically issued. If the air mass below 700 mb is expected to stay dry, then there is a threat of dry lightning, which signifies an LAL of 6. 

The 20-Foot Wind is the average 10-minute wind speed (mph) and direction 20-feet above the vegetation. Twenty feet is used because of smoke rising and also, in large fires, the flames can climb to this height, and it is imperative to know where the wind is going to carry them. 

The Haines Index is a lower atmospheric stability index. It exists to predict the potential for the growth of large, plume-dominated fires. The scale ranges from 2 (very low) to 6 (high). 

The CWR is the chance of rainfall exceeding 0.10 inches.

The weather for the rest of the week:

  • Wednesday Night: Mostly cloudy, with a low around 66. South wind between 10 and 15 mph, with gusts as high as 20 mph. 
  • Thursday: Mostly cloudy, with a high near 87. South southwest wind between 5 and 10 mph. 
  • Thursday Night: Mostly cloudy, with a low around 58. West southwest wind around 10 mph. 
  • Friday: Mostly sunny, with a high near 82. North northwest wind between 10 and 15 mph, with gusts as high as 20 mph. 
  • Friday Night: Partly cloudy, with a low around 42. North northeast wind between 5 and 10 mph. 
  • Saturday: Mostly sunny, with a high near 79. 
  • Saturday Night: Mostly cloudy, with a low around 52. 
  • Sunday: A 20 percent chance of showers. Partly sunny, with a high near 79. 
  • Sunday Night: Mostly cloudy, with a low around 64. 
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