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

2. South Texas Fire Weather
Topics we will cover:
Importance of weather to your prescribed burns
Winds
Relative Humidity
2. Stability
Ventilation basics
Mixing Heights
Transport Winds
4. Critical fire weather patterns to avoid
NWS products and services at your fingertips

3. South Texas Fire Weather
Section 1:
Importance of weather to your prescribed burns

4. 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.

5. Wildland Fire Behavior
Wind
Stability
WEATHER
Wildland Fire
Behavior
TOPOGRAPHY
FUELS
Fuel Moisture
Terrain
Fuel Temperature
Fuel Characteristics

6. It can not be
overemphasized:
A basic knowledge
and awareness of
weather is essential
for making critical
fire management
decisions.

7. Why is weather so important?
Two of the most critical weather elements:
Wind and Relative Humidity

8. Effects of Wind on Wildland Fire Behavior
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

9. Effects of Wind on Wildland Fire Behavior
The direction of the fire spread is determined mostly by direction of the wind
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.

10. Why is weather so important?
Wind
Generally the wind prescription for a successful burn is:
5 to 15 mph
and a
Steady wind direction

11. Why is weather so important?
Wind
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
5 to 15 mph
and a
Steady wind direction

12. Why is weather so important?
Wind
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
5 to 15 mph
and a
Steady wind direction

13. 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

14. 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

15. 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

16. Critical Winds Thunderstorm Winds
Two Characteristics important to fire weather
Lightning
Indraft and downdraft winds are most important

17. 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

18. 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

19. Critical Winds Low-Level Jet Low-Level Jets
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

20. 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 mph

21. Local Winds Sea Breeze
1998 Perry Fire

22. 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

23. Why is weather so important?
Relative Humidity
Generally the RH prescription for a successful burn is:
Above 30%
and
Below 60%

25. South Texas Fire Weather
Section 2:
Stability

26. 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.

27. Promotes the formation and growth of
Unstable atmosphere
Promotes the formation and growth of
vertically developed clouds, thunderstorms and tall smoke columns
Rising
Air

28. An unstable atmosphere
is most often associated with critical or extreme wildland fire behavior.

29. The Effects of Unstable Atmospheric Conditions
on Wildland Fire Behavior
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.

30. 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

31. A stable
atmosphere
will tend to
suppress or reduce
wildland fire
behavior.

32. The Effects of Stable Atmospheric Conditions on Wildland Fire Behavior
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.

33. Weather balloons measure temperature, moisture, wind and atmospheric stability within a column of atmosphere above a point on the earth’s surface.

34. Indicates potential for large plume-dominated
Haines Index
A combination of:
stability and dryness
Indicates potential for
large plume-dominated
fire growth.

35. The Potential for Large Plume
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

36. 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.

38. South Texas Fire Weather
Section 3:
Ventilation Basics

39. Ventilation Basics
Two important weather elements that affect ventilation are:
Mixing Height and Transport Wind

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. Dissipation of the Nighttime Inversion
Late Morning
Mid-Morning
Early Morning
Transport Wind
Transport Wind
Transport Wind
Top of the Inversion Rises as it Weakens
Surface Inversion
Has Dissipated
Top of Surface Based Inversion
20-foot Wind
20-ft wind
20-ft 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.
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.

47. Nighttime Inversions Break
What to Expect When
Nighttime Inversions Break
Winds often increase suddenly and
possibly become gusty and erratic
2. Air temperature increases suddenly
3. Relative humidity decreases suddenly.

48. Ventilation Basics Mixing Height
Inversions behind shallow cold front can result in low mixing heights

49. and prescribed burning should be reconsidered
Ventilation Basics
Mixing Height
Generally stagnant/stable conditions occur with mixing heights less than
1700 feet
and prescribed burning should be reconsidered

50. What are Transport Winds?
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

51. 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

52. Ventilation Basics Mixing Height
Caution: If mixing heights are low and transport winds are low (less than 9 mph), then smoke may not disperse

53. Ventilation Basics Preferred Conditions
Preferred stability for effective burn: neutral or slightly unstable
Preferred Mixing Height: feet
Preferred Transport Winds: mph

54. South Texas Fire Weather
Section 4:
Critical Fire Weather Patterns

55. 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

56. 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.

57. Critical Fire Weather Patterns
Red Flag Criteria for South Texas
Coastal Counties
RH at or below 40%
And
20-Foot winds sustained or frequently gusting at or above 25 mph
Inland Counties
RH 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

58. 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)

59. South Texas Fire Weather
Section 5:
NWS Products and Services

60. NWS Products and Services
Fire Weather Planning Forecasts
Fire Weather Watches
Red Flag Warnings
Spot Forecasts
Digital Service on the internet

61. NWS Products and Services
Fire Weather Planning Forecast

62. NWS Products and Services
Fire Weather Planning Forecast

63. 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 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

64. NWS Products and Services
Red Flag Warning

65. South Texas Fire Weather
Digital Services on the internet

66. South Texas Fire Weather
Digital Services on the internet

68. Spot Weather Forecast

69. Spot Weather Forecast

70. NWS Products and Services
Spot Forecasts

71. South Texas Fire Weather
Digital Services on the internet

72. South Texas Fire Weather
Digital Services on the internet

73. South Texas Fire Weather
Digital Services on the internet

74. South Texas Fire Weather
Digital Services on the internet

75. South Texas Fire Weather
Digital Services on the internet

76. Thank You! Questions? Jason Runyen 361-289-0959 x 1