Guide to South Texas Fire Weather

Importance of Weather to Fire Behavior

Success in prescribed burns before you ignite a pre- height and transport winds. is directly related to how scribed fire. This guide will also intro- well you understand and duce you to critical fire are able to predict fire be- This guide is designed to weather patterns to be on I N S I D E havior. The safety of all give you a basic under- the look out for before you T H I S G U I D E personnel also depends on standing of many important conduct a prescribed burn. fire behavior knowledge. weather elements to be Winds 1-2 Finally, this guide will Topography, fuels and introduce you to the prod- Relative 2 weather are the elements ucts and services the Na- Humidity that most affect fire behav- tional Weather Service pro- Mixing Height 3 ior. All three elements vides to the fire commu- variables can influence nity. Transport Wind 3 each other. It can not be overempha- Critical Fire 4 Weather is the most vari- sized: A basic knowledge Weather Wildland Fire Behavior Triangle Weather is the most variable in space Patterns able of these elements in and awareness of weather both space and time. and time. is essential for making NWS Products 5-6 Therefore it is critical that critical fire management and Services you have an understanding aware of, including wind, decisions. of what the weather will do relative humidity, mixing

Effects of Wind on Fire Behavior

Predicting wind speed and direction is Bends the flames closer to the un- a must so that fire will burn in a prede- burned fuels, preheating the fuels termined manner. Wind can have sev- ahead of the fire front

eral effects on wildland fire behavior: The direction of the fire spread is de- FOR QUESTIONS PLEASE CONTACT Carries away moisture-laden air termined mostly by direction of the JASON RUNYEN — F I R E and thus hastens the drying of wind WEATHER PROGRAM LEADER AT NWS wildland fuels CORPUS CHRISTI: Influences the amount of fuel con-

[email protected] V Aids combustion by increasing the sumed by affecting the residence time supply of oxygen of the flaming front of the fire. The

stronger the wind, the shorter the resi- Increases fire spread by carrying dence time and the less fuel is con- heat and burning embers to new sumed fuels – Spotting

P A G E 2 More on Wind Wind Tips The general wind subscription for a successful burn is: Light and variable winds create poor burning 5 - 15 mph conditions and an And unpredictable direction of spread A steady wind direction

High wind speeds may Be aware of: Gusty winds reduce fuel consumption, ahead of fronts as well as increase chances of winds shifts behind them. escape, and increase risk of spotting Erratic winds near thunderstorms that can produce Conducting burns when extreme winds and pose winds are forecasted to problems during a burn. change direction is not advised Local winds, such as the sea breeze in South Texas, A stable/consistent wind which can cause wind shifts direction throughout the burn will avoid during the early to mid af- unpredictable fire ternoon during the spring behavior and summer months. Low level jets are streams of fast moving air that develop a few hundred to a couple of thousand feet off the surface ahead of cold fronts in the Spring. These winds can surface mid morning and significantly increase rates of spread.

RH Tips

Fine fuels such as grass will burn with the same Basics on Relative Humidity intensity when the RH is The general relative humidity prescription 45-60% Relative humidity can effect both fuels for a successful burn is: and fire behavior in several ways: Cooler fires result when 25 - 60% the RH is 45-60% As RH increases, fuel moisture increases

Affects rate of spread. (as RH decreases Northwest winds typically produce low RH values rate of spread can increase) across South Texas Some fuels will not burn adequately if RH is too high Gusty winds will promote mixing of drier air aloft, Fire becomes difficult to control if RH is resulting in decreasing too low RH values Fires of different intensity can be achieved by selecting different times of day or night for a certain RH

Above: Minimum RH values are typically achieved during the mid to late afternoon, when max temperatures occur.

GUIDE TO SOUTH TEXAS FIRE WEATHER P A G E 3 What is the Mixing Height?

Mixing height is defined as the maximum Mixing Height Tips height in which rapid vertical mixing takes Preferred mixing heights place in the atmosphere. It is the top of the for prescribed burns are mixing layer, which usually extends from 1700-6000 feet. the surface upward to a layer where the temperature begins to increase with height. Mixing heights will be lowest in the morning There are visual clues you can observe to (sometimes only a few tell whether the mixing height is high and hundred feet), but will an indication of unstable conditions or low typically rise during the mid to late morning of an and an indication of stable conditions. average day.

Mixing heights behind shallow cold fronts in South Texas can be less than 1000 feet.

The image on the top left is an example of higher mixing heights (unstable). Clouds and smoke columns will rise to great heights. Gusty winds may be occurring at the surface as well as good visibilities. The image on the top right is an example of lower mixing heights (stable). Low clouds may be observed during these conditions. Transport Wind Tips Smoke columns will be low (usually less than 1500 feet). Winds at the surface may be light and steady. In addition, poor visibilities in haze or fog may be observed. Lighter transport winds will allow smoke to achieve heights closer to the mixing height, while stronger winds will spread the smoke out at The Transport Wind lower heights.

If mixing heights are low The transport wind is simply and transport winds too the average wind between low then the smoke might the surface and the mixing not disperse from the height. In other words it is surface.

the average wind of the mix- Preferred transport winds ing layer, and the wind that for prescribed burns are 9 will influence where the -20 mph. smoke column will go and how it will disperse.

Above: Smoke column spreads out at the mixing height and the transport wind spreads the smoke downwind

GUIDE TO SOUTH TEXAS FIRE WEATHER P A G E 4 Critical Fire Weather Patterns to Avoid

Critical fire weather patterns are The NWS issues Red Flag Warnings weather conditions which support ex- and Fire Weather Watches for such treme fire danger and/or extreme fire events. Fire Weather Watches will be behavior. These conditions can cause issued up to 72 hours in advance of a unwanted ignitions or control prob- Red Flag Event when reasonably con- lems with existing fires. Critical fire fident. A Red Flag Warning is issued weather patterns can pose a threat to within 24 hours of a Red Flag Event. life and property. The National Most Fire Weather Watches and Red Weather Service (NWS) calls these Flag Events in South Texas are issued weather conditions Red Flag Events. due to low RH with strong winds.

Red Flag Criteria for South Texas Inland Counties Coastal Counties RH at or below 30% RH at or below 40% And And 20-Foot winds 20-Foot winds sustained or frequently sustained or frequently gusting at or above 25 mph gusting at or above 25 mph

Other South TX Fire Weather Patterns to Look Out For

Sudden changes in wind direction, speed and RH after cold front and sea-breeze boundary passages

Fast moving low clouds just after daybreak (once morning inversion breaks these stronger winds can mix down to surface)

Unexpected calm in the wind may indicate an approaching wind shift

Thunderstorms above or close to the burn

Dust devils or whirlwinds developing

Upper level ridges (high pressure) can result in a hot/dry patterns

The far western fringes of tropical systems in the western Gulf of Mexico can produce stable, subsident and dry conditions across

South Texas. Above: A lightning strike combined with a critical fire weather pattern to produce this wildfire on the Padre Island National Seashore in Nov. 2005

GUIDE TO SOUTH TEXAS FIRE WEATHER P A G E 5 National Weather Service Products and Services

Your local NWS graphical and digital The following section Weather Forecast Office products available on our describes in detail these (WFO) issues a suite of website. SPOT forecasts products. All of these products to assist crews are a specialized, site products can be found in their prescribed burns. specific forecast avail- under the Fire Weather These include the Fire able to federal land man- link on the WFO Corpus Weather Planning Fore- agement or those agen- Christi website: cast, Fire Weather cies or officials that rep- www.weather.gov/ Watches, Red Flag resent the forecast is www.weather.gov/corpuschristi Click on Fire Weather Warnings as well as needed to public safety. corpuschristi

Fire Weather Planning Forecast

Fire Weather Planning The Fire Weather Planning Fore- Forecasts are a great tool cast (FWF) is issued daily by for input in decision– WFO Corpus Christi. The heart making related to pre- of the FWF is a tabular forecast suppression and other for Today, Tonight and Tomor- row containing predictions of sev- planning. For specific eral weather elements. The FWF hourly weather informa- also contains a general forecast tion check out our hourly for days 3-7. In addition, at the weather graphs on the top of the FWF is a weather syn- internet! (more info on opsis and any Fire Weather page 6) Watches or Red Flag Warnings that may be in effect (not pictured on the left). Fire Weather Watches and Red Flag Warnings

As described in the Critical Fire Fire Weather Watch and Weather Patterns section, Fire Red Flag Warning Criteria Weather Watches (RFW) and Red for South Texas: Flag Warnings (RFW) are de- 20 ft Winds: ≥ 25 mph signed to provide land manage- And ment personnel with appropriate RH: ≤ 30% (inland counties) notification of the likelihood ≤ 40% (coastal counties) weather conditions that support extreme fire danger and/or fire behavior. The RFW is issued on an as needed basis.

GUIDE TO SOUTH TEXAS FIRE WEATHER P A G E 6 More NWS Products and Services

www.weather.gov/ Digital and Graphical Products corpuschristi Specific hourly weather forecasts for a point are available on our website. On our main page just click the location on the map where you need the forecast. On the next page click the link for Hourly Weather Graph. These hourly forecasts for select weather elements are available out to seven days. Scrolling you cursor over the graph will give a readout at the bottom of the page for the time the cursor is over.

Regional views to fire weather elements for select times out to seven days are also available on our website. On our main page click on the link for Experimental Fore- casts - Graphical. The click on the tab for Fire Weather.

GUIDE TO SOUTH TEXAS FIRE WEATHER