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South Texas Fire Weather
Jason Runyen
Fire Weather Program Leader
National Weather Service
Corpus Christi
South 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 Weather
Section 1:
Importance of weather to
your prescribed burns
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.
• Wind
• Stability
WEATHER
Wildland Fire
Behavior
• Terrain
• Fuel Moisture
• Fuel Temperature
• Fuel Characteristics
It can not be
overemphasized:
A basic knowledge
and awareness of
weather is essential
for making critical
fire management
decisions.
Why is weather so important?
Two of the most critical
weather elements:
Wind and Relative Humidity
Effects 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?
Wind
Generally the wind prescription for a successful
burn is:
5 to 15 mph
and a
Steady wind direction
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?
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 Winds
Cold 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
Critical Winds
Cold 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
Critical Winds
Cold 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
Critical Winds
Thunderstorm Winds
• Two Characteristics important to
fire weather
– Lightning
– Indraft and downdraft winds are
most important
Critical Winds
Thunderstorm 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
Critical Winds
Thunderstorm 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
Critical Winds
Low-Level Jets
• 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
Local Winds
Land 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
Local Winds
Sea Breeze
1998 Perry Fire
Effects of RH on Wildland Fire Behavior
• 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
Why is weather so important?
Relative Humidity
Generally the RH prescription for a successful burn is:
Above 30%
and
Below 60%
South Texas Fire Weather
Section 2:
Stability
Three Types of Stability
Unstable Atmosphere –
Enhances or encourages vertical movement of air
Stable Atmosphere –
Suppresses or resists vertical movement of air
Neutral Atmosphere –
Neither suppresses nor enhances vertical
movement of air. This condition seldom exists
for long periods of time.
Unstable atmosphere
Rising
Air
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.
The Effects of Unstable Atmospheric Conditions
on Wildland Fire Behavior
A) Increased likelihood of fire whirls and dust devils
(both indicators of very unstable conditions)
B) Increased likelihood for gusty and erratic surface
winds
C) The height and strength of convection and smoke
columns often increase significantly, and
D) Increased likelihood of fire brands being lifted to
great heights.
Stable atmosphere
trapped smoke
and haze
Light winds and poor
smoke dispersal from
poor vertical mixing
A stable
atmosphere
will tend to
suppress or
reduce
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.
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.
Weather balloons measure temperature, moisture, wind and
atmospheric stability within a column of atmosphere above a point
on the earth’s surface.
Haines Index
A combination of:
stability and dryness
Indicates potential for
large plume-dominated
fire growth.
Haines Index Numbers
The Potential for Large Plume
Dominated Fire Growth
2 or 3 … Very low potential
4 … Low potential
5 … Moderate potential
6 … High potential
Haines 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.
South Texas Fire Weather
Section 3:
Ventilation Basics
Ventilation Basics
Two important weather
elements that affect
ventilation are:
Mixing Height and
Transport Wind
Ventilation Basics
What is Mixing Height?
Maximum height in which rapid vertical
mixing takes place in the atmosphere
Typically is the height
where a temperature
inversion occurs
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.
Top of the Mixed Layer
Top of the Mixed Layer
Ventilation Basics
Mixing 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 Basics
Mixing 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 Basics
Mixing 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 Basics
Mixing 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 Inversion
Late
Mid-Morning
Early
Morning
Morning
Transport
Wind
Transport
TransportWind
Wind
Top of the Inversion Rises as it Weakens
Surface Inversion
Top of Surface Based Inversion
Has Dissipated
20-foot
Wind
20-ft
20-ft
wind
wind
By mid
When
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morning,
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During
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What to Expect When
Nighttime Inversions Break
1. Winds often increase suddenly and
possibly become gusty and erratic
2. Air temperature increases suddenly
3. Relative humidity decreases suddenly.
Ventilation Basics
Mixing Height
Inversions behind shallow cold front can
result in low mixing heights
Ventilation Basics
Mixing Height
Generally stagnant/stable conditions occur with
mixing heights less than
1700 feet
and prescribed burning should be reconsidered
Ventilation Basics
What are Transport Winds?
Average wind between the surface
and the mixing height
This is the wind that will help
move the smoke out of an
area an help disperse it into
the atmosphere
Ventilation 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 Basics
Mixing Height
Caution: If mixing heights are
low and transport winds are
low (less than 9 mph), then
smoke may not disperse
Ventilation Basics
Preferred 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 Weather
Section 4:
Critical Fire Weather Patterns
Critical 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 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 Patterns
Red Flag Criteria for South Texas
Coastal Counties
Inland Counties
RH at or below 40%
RH at or below 30%
And
And
20-Foot winds sustained or
frequently gusting at or above
25 mph
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 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 Weather
Section 5:
NWS Products and Services
NWS 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 Services
Fire Weather Planning Forecast
NWS Products and Services
Fire Weather Planning Forecast
NWS Products and Services
• 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 Services
Red Flag Warning
South Texas Fire Weather
Digital Services on the internet
www.weather.gov/corpuschristi
South Texas Fire Weather
Digital Services on the internet
Spot Weather Forecast
Spot Weather Forecast
NWS Products and Services
Spot Forecasts
South Texas Fire Weather
Digital Services on the internet
South Texas Fire Weather
Digital Services on the internet
South Texas Fire Weather
Digital Services on the internet
South Texas Fire Weather
Digital Services on the internet
South Texas Fire Weather
Digital Services on the internet
Thank You!
Questions?
Jason Runyen
361-289-0959 x 1
[email protected]
http://www.weather.gov/corpuschristi