Presentation on theme: "South Texas Fire Weather"— Presentation transcript:
1South Texas Fire Weather Jason RunyenFire Weather Program LeaderNational Weather Service Corpus Christi
2South Texas Fire Weather Topics we will cover:Importance of weather to your prescribed burnsWindsRelative Humidity2. StabilityVentilation basicsMixing HeightsTransport Winds4. Critical fire weather patterns to avoidNWS products and services at your fingertips
3South Texas Fire Weather Section 1:Importance of weather to your prescribed burns
4Why is weather so important? Success in prescribed burns is directly related to how well you understand and are able to predict fire behavior.The safety of all personnel also depends on fire behavior knowledge.
5Wildland Fire Behavior WindStabilityWEATHERWildland FireBehaviorTOPOGRAPHYFUELSFuel MoistureTerrainFuel TemperatureFuel Characteristics
6It can not beoveremphasized:A basic knowledgeand awareness ofweather is essentialfor making criticalfire managementdecisions.
7Why is weather so important? Two of the most critical weather elements:Wind and Relative Humidity
8Effects of Wind on Wildland Fire Behavior Wind carries away moisture-laden air and thus hastens the drying of wildland fuels2. Once a fire ignites, wind aids combustion byincreasing the supply of oxygen3. Wind increases fire spread by carrying heatand burning embers to new fuels - Spotting4. Wind bends the flames closer to the unburnedfuels, thus preheating the fuels ahead of thefire front
9Effects of Wind on Wildland Fire Behavior The direction of the fire spread is determined mostly by direction of the windWind influences the amount of fuel consumed by affecting the residence time of the flaming front of the fire. The stronger the wind, the shorter the residence time and the less fuel is consumed.
10Why is weather so important? WindGenerally the wind prescription for a successful burn is:5 to 15 mphand aSteady wind direction
11Why is weather so important? WindLight and variable winds create poor burning conditions and an unpredictable direction of spreadHigh wind speeds may reduce fuel consumption, increase chances of escape, and increase risk of spotting5 to 15 mphand aSteady wind direction
12Why is weather so important? WindConducting burns when winds are forecasted to change direction is not advisedA stable/consistent wind direction throughout the burn will avoid unpredictable fire behavior5 to 15 mphand aSteady wind direction
13Critical Winds Cold Front Winds FrontsBoundary between two dissimilar airmassesExtend from the center of low pressureMove at 20 to 30 mphCold FrontBoundary separating a cold airmass from a warm airmassCan migrate west to east and north to south
14Critical Winds Cold Front Winds Pre-Frontal ConditionsLight southeast winds 150 miles ahead of the frontWinds shifting and increasing from the south as the front approachesWinds shifting southwest just ahead of the front and becoming strongWinds strongest along the frontWarm air ahead of the frontSometimes but not always a dry airmassUnstable airmassFavorable burning environment
15Critical Winds Cold Front Winds Post-Frontal ConditionsWinds rapidly shift to the northwest as the front passes with speeds remaining strong and gustyTemperatures cool rapidlyRH increasesFire behavior typically decreases
16Critical Winds Thunderstorm Winds Two Characteristics important to fire weatherLightningIndraft and downdraft winds are most important
17Critical Winds Thunderstorm Winds Thunderstorm Winds Indrafts and downdrafts can change both direction and speed suddenlyResult in sudden changes in rate and direction of fire as well as intensityIndraft speeds range from 10 to 20 mph and gustyDowndrafts speeds range from 25 to 35 mph with gusts over 60 mph
18Critical Winds Thunderstorm Winds Gust Front Leading edge of the downdraftBoundary between two dissimilar airmasses, similar to a cold frontMost of the time, marked by a wind shift, decrease in temperature and increase in RH
19Critical Winds Low-Level Jet Low-Level Jets A jet stream 100 feet to several thousand feet above groundDevelop ahead of cold fronts or troughsWind speeds of 25 to 35 mphCan increase lift and plume dominated fireCan surface and significantly increase rates of spread
20Local Winds Land and Sea Breeze Land-Sea CirculationTemperature and pressure contrasts between land and seaLittle if any temperature change over the large body of waterLarge temperature change over land from day to nightSea breeze during the dayLand breeze at nightStrongest in spring and summerLand-Sea breeze mph
22Effects of RH on Wildland Fire Behavior Affects fuel moisture- as RH increases, fuel moisture increasesAffects fire intensity- fires of different intensity can be achieved by selecting different times of day or night as well as different weather conditionsAffects rate of spreadSome fuels will not burn adequately if RH is too highFire becomes difficult to control if RH is too low
23Why is weather so important? Relative HumidityGenerally the RH prescription for a successful burn is:Above 30%andBelow 60%
26Three Types of Stability Unstable Atmosphere –Enhances or encourages vertical movement of airStable Atmosphere –Suppresses or resists vertical movement of airNeutral Atmosphere –Neither suppresses nor enhances vertical movement of air. This condition seldom exists for long periods of time.
27Promotes the formation and growth of Unstable atmospherePromotes the formation and growth ofvertically developed clouds, thunderstorms and tall smoke columnsRisingAir
28An unstable atmosphere is most often associated with critical or extreme wildland fire behavior.
29The Effects of Unstable Atmospheric Conditions on Wildland Fire BehaviorIncreased likelihood of fire whirls and dust devils(both indicators of very unstable conditions)B) Increased likelihood for gusty and erratic surfacewindsC) The height and strength of convection and smokecolumns often increase significantly, andD) Increased likelihood of fire brands being lifted togreat heights.
30Light winds and poor smoke dispersal from poor vertical mixing Stable atmosphereLight winds and poor smoke dispersal from poor vertical mixingtrapped smokeand haze
31A stableatmospherewill tend tosuppress or reducewildland firebehavior.
32The Effects of Stable Atmospheric Conditions on Wildland Fire Behavior Limited rise of smoke columns, resulting inpoor smoke dispersion and visibility.B) Reduced inflow of fresh air, thereby limitingwildland fire growth and intensity.C) Lowers surface wind speeds and fire spreadrates except in mountainous or hilly terrain.
33Weather balloons measure temperature, moisture, wind and atmospheric stability within a column of atmosphere above a point on the earth’s surface.
34Indicates potential for large plume-dominated Haines IndexA combination of:stability and drynessIndicates potential forlarge plume-dominatedfire growth.
35The Potential for Large Plume Haines Index NumbersThe Potential for Large PlumeDominated Fire Growth2 or 3 … Very low potential4 … Low potential5 … Moderate potential6 … High potential
36Haines Index Bottom Line… The drier and more unstable the lower atmosphere,the HIGHER the Haines Index.The more humid and stable the lower atmosphere,the LOWER the Haines Index.
38South Texas Fire Weather Section 3:Ventilation Basics
39Ventilation BasicsTwo important weather elements that affect ventilation are:Mixing Height and Transport Wind
40Ventilation Basics What is Mixing Height? Maximum height in which rapid vertical mixing takes place in the atmosphereTypically is the height where a temperature inversion occurs
41The convective mixing layer is normally “capped” by a layer of very stable air, which limits the rise of vertically developed clouds and smoke columns. The tops of tall smoke columns and cumulonimbus clouds are often seen spreading out at the top of the mixed layer.Top of the Mixed LayerTop of the Mixed Layer
42Ventilation Basics Mixing Height Visual indicators of unstable conditions and higher mixing heightsClouds grow vertical and smoke rises to great heightsCumulus type clouds with large vertical depthGusty windsGood visibilitiesDust devils and firewhirls
43Ventilation Basics Mixing Height Visual indicators of stable conditions and lower mixing heightsLow clouds, overcast conditionsStratus type cloudsLight/steady winds or calm windsPoor visibilities (fog or haze)Low smoke column
44Ventilation Basics Mixing Height Unstable atmosphere and higher mixing heights typically occur during afternoonThis is because the sun heats the earth’s surface during the day, which in turn heats the air just above the surface and causes it to riseThe more heating that takes place, the more unstable the atmosphere becomes and the higher the smoke will rise
45Ventilation Basics Mixing Height Stable atmosphere and lower mixing heights typically form overnight and in the early morningThis is because a temperature inversion typically develops just off the surface overnightSmoke still in the air will drop back down to the surface
46Dissipation of the Nighttime Inversion Late MorningMid-MorningEarly MorningTransport WindTransport WindTransport WindTop of the Inversion Rises as it WeakensSurface InversionHas DissipatedTop of Surface Based Inversion20-foot Wind20-ft wind20-ft windWhen the surface inversion breaks, fire intensity may suddenly increase with a rush of fresh oxygen. The smoke plume may also rise suddenly and become well formed. After rising high enough, the plume will begin to tilt in the direction of the transport wind, potentially carrying small embers and fire brands down wind from the fire.By mid to late morning, the inversion has weakened considerably with the air next to the ground becoming nearly as warm as the air above the inversion. Fire intensity slowly increases as light drainage winds add more oxygen to the fire. The smoke column also begins to tilt upward and smoke dispersal improves as the top of the inversion rises.During the early morning hours, fire intensity remains low with more smoke than open flame visible. Smoke dispersal also remains poor with the smoke column hanging low and spreading out in several directions as surfaces winds remain very light or calm.
47Nighttime Inversions Break What to Expect WhenNighttime Inversions BreakWinds often increase suddenly andpossibly become gusty and erratic2. Air temperature increases suddenly3. Relative humidity decreases suddenly.
48Ventilation Basics Mixing Height Inversions behind shallow cold front can result in low mixing heights
49and prescribed burning should be reconsidered Ventilation BasicsMixing HeightGenerally stagnant/stable conditions occur with mixing heights less than1700 feetand prescribed burning should be reconsidered
50What are Transport Winds? Ventilation BasicsWhat are Transport Winds?Average wind between the surfaceand the mixing heightThis is the wind that will help move the smoke out of an area an help disperse it into the atmosphere
51Ventilation Basics Mixing Height Lighter winds will allow smoke to achieve higher heights, closer to mixing heightStronger winds will spread smoke out horizontally, at lower heights
52Ventilation Basics Mixing Height Caution: If mixing heights are low and transport winds are low (less than 9 mph), then smoke may not disperse
53Ventilation Basics Preferred Conditions Preferred stability for effective burn: neutral or slightly unstablePreferred Mixing Height: feetPreferred Transport Winds: mph
54South Texas Fire Weather Section 4:Critical Fire Weather Patterns
55Critical Fire Weather Patterns What is a Critical Fire Weather Pattern?Weather conditions which support extreme fire danger and/or fire behaviorThese weather conditions can cause widespread new ignitions or control problems with existing firesPose a threat to life and propertyThe NWS calls these conditions Red Flag EventsRed Flag Warnings and Watches are issued by the NWS in anticipation of such events
56Critical Fire Weather Patterns Red Flag EventsTwo critical fire weather parameters which can cause a Red Flag Event in South Texas are:Low Relative Humidity&Strong/Gusty WindsThese are the conditions NWS Corpus Christi issues Red Flag Watches and Warnings for.
57Critical Fire Weather Patterns Red Flag Criteria for South TexasCoastal CountiesRH at or below 40%And20-Foot winds sustained or frequently gusting at or above 25 mphInland CountiesRH at or below 30%And20-Foot winds sustained or frequently gusting at or above 25 mphThese conditions have been determined to be critical to wildfire potential and growth across South Texas
58Critical Fire Weather Patterns Other Critical Fire Weather Patterns to watch out forSudden changes in wind direction, speed and RH after cold front passages and sea-breeze boundaryFast moving low clouds just after daybreak (once morning inversion breaks those stronger winds will mix down to surface)Unexpected calm may indicate an approaching wind shiftThunderstorms above or close to the burnDust devils or whirlwinds developingUpper level ridges (high pressure) result in a hot/dry patternsStable/subsident, dry, windy area of a tropical cyclone circulation (typically on west side of systems, such as Rita in 2005)
59South Texas Fire Weather Section 5:NWS Products and Services
60NWS Products and Services Fire Weather Planning ForecastsFire Weather WatchesRed Flag WarningsSpot ForecastsDigital Service on the internet
61NWS Products and Services Fire Weather Planning Forecast
62NWS Products and Services Fire Weather Planning Forecast
63NWS Products and Services Fire Weather Watch – alert land management agencies of the potential for a Red Flag event in the near futureReasonably confidentGenerally hours in advance of eventCan be issued first 12 hours for dry lightning eventsRed Flag Warning – Red Flag event impending or occurringHigh degree of confidenceFirst 24 hours