South Texas Fire Weather Jason Runyen Fire Weather Program Leader National Weather Service Corpus Christi FM 2508 Fire Jan 1, 2011

South Texas Fire Weather Topics we will cover: 1.Importance of weather to fire behavior Winds Relative Humidity 2. Smoke Management Mixing Heights Transport Winds 3. Critical fire weather patterns 4. NWS Fire Weather Decision Support Products and Services

South Texas Fire Weather Section 1: Importance of Weather: Winds and Relative Humidity Section 1: Importance of Weather: Winds and Relative Humidity

Why is weather so important? Success with 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 Fire Behavior WEATHER TOPOGRAPHY FUELS Wind Stability Fuel Moisture Fuel Temperature Fuel Characteristics Terrain Most variable over space and time

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

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. Effects of Wind on Wildland Fire Behavior

Wind Adjustments Midflame Calculations Based On Fuel Type Litter Crowns GrassEye-level ¾ X Crown or 20’ 1X Litter ¼ X ¼ of the 20 foot wind 20 foot wind is the midflame wind ¾ of the 20 foot wind

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 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 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 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 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 Winds Thunderstorm Winds Critical 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 Winds Thunderstorm Winds 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 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 Thunderstorm Winds Critical Winds Thunderstorm 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 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 surface and significantly increase rates of spread Critical Winds Low-Level Jets Critical Winds Low-Level Jets

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 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

Local Winds Sea Breeze

Relative Humidity The Ratio of the amount of moisture (water vapor) in the air to the amount the air could hold when saturated at the same air temperature.

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 Behavior

Relative Humidity Generally the RH prescription for a prescribed burns in South Texas is: Above 30% and Below 60%

South Texas Fire Weather Section 2: Smoke Management: Mixing Heights and Transport Winds Section 2: Smoke Management: Mixing Heights and Transport Winds

Smoke Management Two important weather elements that affect smoke management are: Mixing Height and Transport Wind

Mixing Height Maximum height in which rapid vertical mixing takes place in the atmosphere Typically is the height where a temperature inversion occurs

Top of the Mixed Layer The convective mixing layer can become “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.

Mixing Height Visual indicators of unstable conditions, higher mixing heights and better smoke dispersion: Clouds grow vertical and smoke rises to great heights Cumulus type clouds with large vertical depth Gusty winds Good visibilities Dust devils and firewhirls

Mixing Height Visual indicators of stable conditions, lower mixing heights, and poorer smoke dispersion Low clouds, overcast conditions Stratus type clouds Light/steady winds or calm winds Poor visibilities (fog or haze) Low smoke column

VISUAL CLUES OF STABILITY STABLE UN-STABLE

Four Types of Inversions That Can Effect Mixing Layer Four Types of Inversions That Can Effect Mixing Layer

Evolution of the Daytime Mixed Layer Evolution of the Daytime Mixed Layer 55ºF 58ºF 63ºF75ºF80ºF88ºF 8 AM 10 AM Noon 2 PM 8 AM 1500 Ft AGL 10 AM 2800 FT AGL Noon 7400 Ft AGL 2 PM 12,000 Ft AGL Maximum Mixing Height G r o u n d

Mixing Height – Morning Inversion 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

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

Dissipation 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 Morning Transport Wind Surface Inversion Has 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 suddenly 3. Relative humidity decreases suddenly.

Frontal Inversion A frontal inversion forms when a cooler, more dense airmass moves beneath a warmer, less dense airmass (cold front as shown), or when a warm airmass slides over the top of a cooler airmass (warm front). The frontal inversion is strongest behind the advancing frontal boundary, as much as a hundred miles or more. warmer, lighter air cooler, heavier air

Seasonal Variation in the Height of the Mixing Layer The depth of the mixing layer varies considerably during the year. The top of the mixing layer is lowest in the coldest winter months and highest in the warmest summer months. The height of the mixing layer rises quickly in the spring with strong warming at the ground, and lowers quickly in the autumn as temperatures fall.

Mixing Height Generally stagnant/stable conditions occur with mixing heights Less than 1500 feet These conditions are most common in the WINTER MONTHS and during early morning in the SPRING and FALL

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

Transport Wind Lighter transport winds will allow smoke to achieve higher heights, closer to mixing height Stronger transport winds will spread smoke out horizontally, at lower heights

Smoke Management Caution: If mixing heights are less than 500 feet and transport winds are less than 9 mph, then smoke may not disperse.

Smoke Management What could happen to smoke overnight on days when the transport winds are low and mixing heights are less than 1500 feet?

Inversion does not Completely Erode Early Morning Top of Surface Based Inversion Transport Wind 20-ft wind Late-Afternoon Light Transport Wind Low Inversion 20-ft wind

Development of the Nighttime Inversion Inversion begins to lower after sunset Un-dispersed smoke may also begin lowering Mid-Evening Inversion nears the surface towards sunrise Smoke settles back to the surface Early Morning

Smoke Management Preferred Conditions for Smoke Dispersal Preferred stability for effective burn: neutral or slightly unstable Preferred Mixing Height: Above 1500 feet Preferred Transport Winds: 9-20 mph

South Texas Fire Weather Section 3: 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.

When Fuels are Cured Red Flag Weather 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

When fuels are NOT cured: 1)Red Flag Weather Criteria AND 2)Dry to Extremely Dry Fuel Moisture for NWS to issue a Red Flag Warning

South Texas Fire Weather Section 4: NWS Fire Weather Decision Support Products & Services