Fire Weather Science Fire Weather System Project EcoConnect -Fire Implementation, June 2013
South Texas Fire Weather
-
Upload
winter-marsh -
Category
Documents
-
view
36 -
download
2
description
Transcript of South Texas Fire Weather
South Texas Fire WeatherSouth Texas Fire Weather
Jason Runyen
Fire Weather Program Leader
National Weather Service Corpus Christi
South Texas Fire WeatherSouth Texas Fire Weather
Topics we will cover:1. Importance of weather to your prescribed burns
Winds
Relative Humidity
2. Stability
3. Ventilation basics
Mixing Heights
Transport Winds
4. Critical fire weather patterns to avoid
5. NWS products and services at your fingertips
South Texas Fire WeatherSouth Texas Fire Weather
Section 1:
Importance of weather to your prescribed burns
Why is weather so important?Why 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.
Wildland FireBehavior
WEATHER
TOPO
GRAPHY
FUEL
S
• Wind• Stability
• Fuel Moisture• Fuel Temperature
• Fuel Characteristics
• Terrain
It can not be overemphasized:
A basic knowledge and awareness of
weather is essentialfor making criticalfire management
decisions.
Why is weather so important?Why is weather so important?
Two of the most critical weather elements:
Wind and Relative Humidity
Effects of Wind on Wildland Fire BehaviorEffects of Wind on Wildland Fire Behavior
1. Wind carries away moisture-laden air and thus hastens the drying of wildland fuels
2. Once a fire ignites, wind aids combustion by increasing the supply of oxygen
3. Wind increases fire spread by carrying heat and burning embers to new fuels - Spotting
4. Wind bends the flames closer to the unburned fuels, thus preheating the fuels ahead of the fire front
Effects of Wind on Wildland Fire Behavior
5. The direction of the fire spread is determined mostly by direction of the wind
6. Wind 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.
Why is weather so important?Why is weather so important?
Wind
Generally the wind prescription for a successful burn is:
5 to 15 mph
and a
Steady wind direction
Why is weather so important?Why is weather so important?
Wind
5 to 15 mph
and a
Steady wind direction
• Light and variable winds create poor burning conditions and an unpredictable direction of spread
• High wind speeds may reduce fuel consumption, increase chances of escape, and increase risk of spotting
Why is weather so important?Why is weather so important?
Wind
5 to 15 mph
and a
Steady wind direction
• Conducting burns when winds are forecasted to change direction is not advised
• A stable/consistent wind direction throughout the burn will avoid unpredictable fire behavior
Critical WindsCold Front Winds
Critical WindsCold Front Winds
• Fronts
– Boundary between two dissimilar airmasses
– Extend from the center of low pressure
– Move at 20 to 30 mph
• Cold Front
– Boundary separating a cold airmass from a warm airmass
– Can migrate west to east and north to south
• Fronts
– Boundary between two dissimilar airmasses
– Extend from the center of low pressure
– Move at 20 to 30 mph
• Cold Front
– Boundary separating a cold airmass from a warm airmass
– Can migrate west to east and north to south
Critical WindsCold Front Winds
Critical WindsCold Front Winds
• Pre-Frontal Conditions
– Light southeast winds 150 miles ahead of the front
– Winds shifting and increasing from the south as the front approaches
– Winds shifting southwest just ahead of the front and becoming strong
– Winds strongest along the front
– Warm air ahead of the front
– Sometimes but not always a dry airmass
– Unstable airmass
– Favorable burning environment
• Pre-Frontal Conditions
– Light southeast winds 150 miles ahead of the front
– Winds shifting and increasing from the south as the front approaches
– Winds shifting southwest just ahead of the front and becoming strong
– Winds strongest along the front
– Warm air ahead of the front
– Sometimes but not always a dry airmass
– Unstable airmass
– Favorable burning environment
Critical WindsCold Front Winds
Critical WindsCold Front Winds
• Post-Frontal Conditions
– Winds rapidly shift to the northwest as the front passes with speeds remaining strong and gusty
– Temperatures cool rapidly
– RH increases
– Fire behavior typically decreases
• Post-Frontal Conditions
– Winds rapidly shift to the northwest as the front passes with speeds remaining strong and gusty
– Temperatures cool rapidly
– RH increases
– Fire behavior typically decreases
• Two Characteristics important to fire weather
– Lightning
– Indraft and downdraft winds are most important
• Two Characteristics important to fire weather
– Lightning
– Indraft and downdraft winds are most important
Critical WindsThunderstorm Winds
Critical WindsThunderstorm Winds
• Thunderstorm Winds
– Indrafts and downdrafts can change both direction and speed suddenly
– Result in sudden changes in rate and direction of fire as well as intensity
– Indraft speeds range from 10 to 20 mph and gusty
– Downdrafts speeds range from 25 to 35 mph with gusts over 60 mph
• Thunderstorm Winds
– Indrafts and downdrafts can change both direction and speed suddenly
– Result in sudden changes in rate and direction of fire as well as intensity
– Indraft speeds range from 10 to 20 mph and gusty
– Downdrafts speeds range from 25 to 35 mph with gusts over 60 mph
Critical WindsThunderstorm Winds
Critical WindsThunderstorm Winds
• Gust Front
– Leading edge of the downdraft
– Boundary between two dissimilar airmasses, similar to a cold front
– Most of the time, marked by a wind shift, decrease in temperature and increase in RH
• Gust Front
– Leading edge of the downdraft
– Boundary between two dissimilar airmasses, similar to a cold front
– Most of the time, marked by a wind shift, decrease in temperature and increase in RH
Critical WindsThunderstorm Winds
Critical WindsThunderstorm Winds
• Low-Level Jet
– A jet stream 100 feet to several thousand feet above ground
– Develop ahead of cold fronts or troughs
– Wind speeds of 25 to 35 mph
– Can increase lift and plume dominated fire
– Can surface and significantly increase rates of spread
• Low-Level Jet
– A jet stream 100 feet to several thousand feet above ground
– Develop ahead of cold fronts or troughs
– Wind speeds of 25 to 35 mph
– Can increase lift and plume dominated fire
– Can surface and significantly increase rates of spread
Critical WindsLow-Level Jets
Critical WindsLow-Level Jets
Local WindsLand and Sea Breeze
Local WindsLand and Sea Breeze
• Land-Sea Circulation
– Temperature and pressure contrasts between land and sea
– Little if any temperature change over the large body of water
– Large temperature change over land from day to night
– Sea breeze during the day
– Land breeze at night
– Strongest in spring and summer
– Land-Sea breeze 10-20 mph
• Land-Sea Circulation
– Temperature and pressure contrasts between land and sea
– Little if any temperature change over the large body of water
– Large temperature change over land from day to night
– Sea breeze during the day
– Land breeze at night
– Strongest in spring and summer
– Land-Sea breeze 10-20 mph
Local WindsSea Breeze
Local WindsSea Breeze
1998 Perry Fire
• Affects fuel moisture
- as RH increases, fuel moisture increases
• Affects fire intensity
- fires of different intensity can be achieved by selecting different times of day or night as well as different weather conditions
• Affects rate of spread
• Some fuels will not burn adequately if RH is too high
• Fire becomes difficult to control if RH is too low
Effects of RH on Wildland Fire BehaviorEffects of RH on Wildland Fire Behavior
Why is weather so important?Why is weather so important?
Relative Humidity
Generally the RH prescription for a successful burn is:
Above 30%
and
Below 60%
South Texas Fire WeatherSouth Texas Fire Weather
Section 2:
Stability
Stable Atmosphere – Suppresses or resists vertical movement of airStable Atmosphere – Suppresses or resists vertical movement of air
Unstable Atmosphere – Enhances or encourages vertical movement of airUnstable Atmosphere – Enhances or encourages vertical movement of air
Three Types of Stability Three Types of Stability
Neutral Atmosphere – Neither suppresses nor enhances vertical movement of air. This condition seldom exists for long periods of time.
Neutral Atmosphere – Neither suppresses nor enhances vertical movement of air. This condition seldom exists for long periods of time.
RisingAir
Unstable atmosphere
Promotes the formation and growth of
vertically developed clouds, thunderstorms
and tall smoke columns
Unstable atmosphere
Promotes the formation and growth of
vertically developed clouds, thunderstorms
and tall smoke columns
An unstable atmosphere is most often
associated with critical or extreme
wildland fire behavior.
An unstable atmosphere is most often
associated with critical or extreme
wildland fire behavior.
The Effects of Unstable Atmospheric Conditionson Wildland Fire Behavior
The Effects of Unstable Atmospheric Conditionson Wildland Fire Behavior
A) Increased likelihood of fire whirls and dust devils (both indicators of very unstable conditions)A) Increased likelihood of fire whirls and dust devils (both indicators of very unstable conditions)
B) Increased likelihood for gusty and erratic surface windsB) Increased likelihood for gusty and erratic surface winds
C) The height and strength of convection and smoke columns often increase significantly, andC) The height and strength of convection and smoke columns often increase significantly, and
D) Increased likelihood of fire brands being lifted to great heights.D) Increased likelihood of fire brands being lifted to great heights.
Stable atmosphere
Light winds and poor smoke dispersal from poor vertical mixing
Stable atmosphere
Light winds and poor smoke dispersal from poor vertical mixing
trapped smoke and haze
A stable atmospherewill tend to suppress or
reduce wildland fire behavior.
A stable atmospherewill tend to suppress or
reduce wildland fire behavior.
The Effects of Stable Atmospheric Conditions on Wildland Fire Behavior
The Effects of Stable Atmospheric Conditions on Wildland Fire Behavior
A) Limited rise of smoke columns, resulting in poor smoke dispersion and visibility.A) Limited rise of smoke columns, resulting in poor smoke dispersion and visibility.
B) Reduced inflow of fresh air, thereby limiting wildland fire growth and intensity.B) Reduced inflow of fresh air, thereby limiting wildland fire growth and intensity.
C) Lowers surface wind speeds and fire spread rates except in mountainous or hilly terrain.C) Lowers surface wind speeds and fire spread rates except in mountainous or hilly terrain.
Weather balloons measure temperature, moisture, wind and atmospheric stability within a column of atmosphere above a point
on the earth’s surface.
Weather balloons measure temperature, moisture, wind and atmospheric stability within a column of atmosphere above a point
on the earth’s surface.
A combination of:
stability and drynessA combination of:
stability and dryness
Indicates potential for large plume-dominated
fire growth.
Indicates potential for large plume-dominated
fire growth.
Haines Index
Haines Index NumbersHaines Index NumbersThe Potential for Large Plume
Dominated Fire GrowthThe Potential for Large Plume
Dominated Fire Growth
2 or 3 … Very low potential2 or 3 … Very low potential
4 … Low potential4 … Low potential
5 … Moderate potential 5 … Moderate potential
6 … High potential6 … High potential
Haines IndexHaines Index
The drier and more unstable the lower atmosphere,the HIGHER the Haines Index.
The drier and more unstable the lower atmosphere,the HIGHER the Haines Index.
Bottom Line…Bottom Line…
The more humid and stable the lower atmosphere,the LOWER the Haines Index.
The more humid and stable the lower atmosphere,the LOWER the Haines Index.
South Texas Fire WeatherSouth Texas Fire Weather
Section 3:
Ventilation Basics
Ventilation BasicsVentilation Basics
Two important weather elements that affect
ventilation are:
Mixing Height and Transport Wind
Ventilation BasicsVentilation Basics
Maximum height in which rapid vertical mixing takes place in the atmosphere
What is Mixing Height?
Typically is the height where a temperature inversion occurs
Top of the Mixed Layer
Top of the Mixed Layer
The 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.
The 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.
Ventilation BasicsVentilation BasicsMixing Height
Visual indicators of unstable conditions and higher mixing heights
• Clouds grow vertical and smoke rises to great heights
• Cumulus type clouds with large vertical depth
• Gusty winds
• Good visibilities
• Dust devils and firewhirls
Ventilation BasicsVentilation BasicsMixing Height
Visual indicators of stable conditions and lower mixing heights
• Low clouds, overcast conditions
• Stratus type clouds
• Light/steady winds or calm winds
• Poor visibilities (fog or haze)
• Low smoke column
Ventilation BasicsVentilation BasicsMixing Height
Unstable atmosphere and higher mixing heights typically occur during afternoon
• This 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 rise
• The more heating that takes place, the more unstable the atmosphere becomes and the higher the smoke will rise
Ventilation BasicsVentilation BasicsMixing Height
Stable atmosphere and lower mixing heights typically form overnight and in the early morning
• This is because a temperature inversion typically develops just off the surface overnight
• Smoke still in the air will drop back down to the surface
Dissipation of the Nighttime InversionDissipation of the Nighttime Inversion
Early Morning
Top of Surface Based Inversion
Transport Wind
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.
20-ft wind
Mid-Morning
Transport Wind
Top of the Inversion Rises as it Weakens
20-ft wind
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.
Late MorningLate Morning
Transport Wind
Surface InversionHas Dissipated
20-foot Wind
When 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.
What to Expect When Nighttime Inversions Break
What to Expect When Nighttime Inversions Break
1. Winds often increase suddenly and possibly become gusty and erratic
1. Winds often increase suddenly and possibly become gusty and erratic
2. Air temperature increases suddenly2. Air temperature increases suddenly
3. Relative humidity decreases suddenly.3. Relative humidity decreases suddenly.
Ventilation BasicsVentilation BasicsMixing Height
Inversions behind shallow cold front can result in low mixing heights
Ventilation BasicsVentilation Basics
Mixing Height
Generally stagnant/stable conditions occur with mixing heights less than
1700 feet
and prescribed burning should be reconsidered
Ventilation BasicsVentilation Basics
Average wind between the surface
and the mixing height
What are Transport Winds?
This is the wind that will help move the smoke out of an area an help disperse it into the atmosphere
Ventilation BasicsVentilation Basics
Mixing Height
Lighter winds will allow smoke to achieve higher heights, closer to mixing height
Stronger winds will spread smoke out horizontally, at lower heights
Ventilation BasicsVentilation Basics
Mixing Height
Caution: If mixing heights are low and transport winds are low (less than 9 mph), then smoke may not disperse
Ventilation BasicsVentilation BasicsPreferred Conditions
• Preferred stability for effective burn: neutral or slightly unstable
• Preferred Mixing Height: 1700-6000 feet
• Preferred Transport Winds:9-20 mph
South Texas Fire WeatherSouth Texas Fire Weather
Section 4:
Critical Fire Weather Patterns
Critical Fire Weather PatternsCritical Fire Weather Patterns
What is a Critical Fire Weather Pattern?
• Weather conditions which support extreme fire danger and/or fire behavior
• These weather conditions can cause widespread new ignitions or control problems with existing fires
• Pose a threat to life and property
• The NWS calls these conditions Red Flag Events
• Red Flag Warnings and Watches are issued by the NWS in anticipation of such events
Critical Fire Weather PatternsCritical Fire Weather Patterns
Red Flag Events
Two critical fire weather parameters which can cause a Red Flag Event in South Texas are:
Low Relative Humidity
&
Strong/Gusty Winds
These are the conditions NWS Corpus Christi issues Red Flag Watches and Warnings for.
Critical Fire Weather PatternsCritical Fire Weather Patterns
Red Flag Criteria for South Texas
Coastal CountiesRH at or below 40%
And
20-Foot winds sustained or frequently gusting at or above
25 mph
Inland CountiesRH at or below 30%
And
20-Foot winds sustained or frequently gusting at or above
25 mph
These conditions have been determined to be critical to wildfire potential and growth across South Texas
Critical Fire Weather PatternsCritical Fire Weather Patterns
Other Critical Fire Weather Patterns to watch out for
• Sudden changes in wind direction, speed and RH after cold front passages and sea-breeze boundary
• Fast 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 shift
• Thunderstorms above or close to the burn
• Dust devils or whirlwinds developing
• Upper level ridges (high pressure) result in a hot/dry patterns
• Stable/subsident, dry, windy area of a tropical cyclone circulation (typically on west side of systems, such as Rita in 2005)
South Texas Fire WeatherSouth Texas Fire Weather
Section 5:
NWS Products and Services
NWS Products and ServicesNWS Products and Services
1. Fire Weather Planning Forecasts
2. Fire Weather Watches
3. Red Flag Warnings
4. Spot Forecasts
5. Digital Service on the internet
NWS Products and ServicesNWS Products and ServicesFire Weather Planning Forecast
NWS Products and ServicesNWS Products and ServicesFire Weather Planning Forecast
• Fire Weather Watch – alert land management agencies of the potential for a Red Flag event in the near future
Reasonably confident
Generally 12-72 hours in advance of event
Can be issued first 12 hours for dry lightning events
• Red Flag Warning – Red Flag event impending or occurring
High degree of confidence
First 24 hours
NWS Products and ServicesNWS Products and Services
NWS Products and ServicesNWS Products and ServicesRed Flag Warning
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
www.weather.gov/corpuschristi
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
Spot Weather ForecastSpot Weather Forecast
Spot Weather ForecastSpot Weather Forecast
NWS Products and ServicesNWS Products and ServicesSpot Forecasts
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
South Texas Fire WeatherSouth Texas Fire WeatherDigital Services on the internet
Thank You!Questions?Thank You!Questions?
Jason Runyen
361-289-0959 x 1
http://www.weather.gov/corpuschristi