South Texas Fire Weather Jason Runyen Fire Weather Program Leader National Weather Service Corpus Christi

South Texas Fire WeatherSouth Texas Fire Weather

Jason Runyen

Fire Weather Program Leader

National Weather Service Corpus Christi


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


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.


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.


Wind

Generally the wind prescription for a successful burn is:

5 to 15 mph

and a

Steady wind direction


Wind

5 to 15 mph

and a


• 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


Wind

5 to 15 mph

and a


• 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


• 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



• 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



• 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


• 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



• 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



• 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


Relative Humidity

Generally the RH prescription for a successful burn is:

Above 30%

and

Below 60%


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.


Section 3:

Ventilation Basics

Ventilation BasicsVentilation Basics

Two important weather elements that affect

ventilation are:

Mixing Height and Transport Wind


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


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


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


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.


Inversions behind shallow cold front can result in low mixing heights


Mixing Height

Generally stagnant/stable conditions occur with mixing heights less than

1700 feet

and prescribed burning should be reconsidered


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


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


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


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


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.


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


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)


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


Spot Weather ForecastSpot Weather Forecast

Spot Weather ForecastSpot Weather Forecast

NWS Products and ServicesNWS Products and ServicesSpot Forecasts






Thank You!Questions?Thank You!Questions?

Jason Runyen

361-289-0959 x 1

[email protected]

http://www.weather.gov/corpuschristi

mailto:[email protected]

http://www.weather.gov/corpuschristi

Documents

South Texas Fire Weather Jason Runyen Fire Weather Program Leader National Weather Service Corpus Christi