2. Fire Weather Forecast Variables

All fire weather forecasts contain three sections: the header, mandatory variables, and optional variables.

Header

Forecast Type, Time/Date, and Location Information

The forecast header documents 1) the type of fire weather forecast (described further in Types of Fire Weather Forecasts), 2) the time and place the forecast was made (forecast office and/or forecaster), and 3) the time and place for when and where the forecast is valid. The header will also note what type of fire weather forecast is being issued (fire weather planning [zone] forecast, spot weather forecast, or incident weather forecast). An annotated forecast header is shown in Figure 1.

Headlines

Headlines are any types of watches, warnings, or advisories (e.g. Red Flag Warning, High Wind Warning, Severe Thunderstorm Watch) that are in effect for the area and time the forecast is valid for. The headline is likely the first part of the forecast that you read and may very well be the most important weather condition that you encounter during your shift.

The National Weather Service (NWS) has a very good webpage describing the differences between fire weather headlines. Fire Weather Watches and Red Flag Warnings have region-specific criteria that are outlined in the local Fire Weather Annual Operating Plan (FWAOP) or by your local NWS, Weather Forecast Office (WFO). An example of the criteria for Red Flag Warnings for the Rocky Mountain Geographic Area is given in Figure 2. Be sure to know what the local criteria are for your area.

Discussion

The forecast discussion outlines the forecast in plain English, provides information on forecaster reasoning, highlights the challenges of the forecast, and provides background on forecast timing, evolution, and confidence. The discussion should explain the pertinent forecast variables and note if any sudden changes in the variables may be seen. For example, this is the place to find information about when thunderstorms may form, when a frontal passage may occur, or movement of cloud cover. In short, the discussion tells the story of the forecast with the important variable of time included. The discussion section is also the place where the forecaster can introduce information regarding uncertainty or confidence in the forecast if variables of the forecast prove especially difficult to predict.

Mandatory Variables

Sky/Weather

The sky/weather section of a forecast describes the expected sky condition and the probability of precipitation (PoP), if applicable. Sky condition refers to expected cloud coverage, as shown in Table 1, but can be omitted if there is a high PoP. The PoP, given in percent, is the likelihood that the forecast area will be impacted by measurable precipitation (greater than 0.01 inch). It does not refer to a specific amount of precipitation, only to the PoP occurring. Further information on what defines the forecast area will be given in Section 3 because it varies with the type of forecast being produced. The PoP will also state the expected precipitation type and if thunderstorms are anticipated.

Table 1:  Sky/weather Adjective Conditions and Associated Cloud Coverage

Sunshine and cloud cover significantly impact fire behavior. Clear skies and ample sunshine lead to warmer fuel temperatures through increased radiational heating which then leads to decreased fuel moisture content. Cloud cover has the opposite effect: it leads to relatively cooler fuels that may retain moisture for longer periods of time. These concepts are directly incorporated into the computation of Fine Dead Fuel Moisture (FDFM) and Probability of Ignition (POI). It may not be the first thing on your mind, but direct sunlight can also lead to dehydration, sunburn, and increased chances of heat-related illnesses.

Thunderstorms bring their own unique set of hazards including lightning, hail, and gusty, or erratic winds that may bring unexpected changes in fire behavior. Recently, there have even been reports of tornadoes impacting wildfires. Thunderstorms are often associated with precipitation, but dry thunderstorms produce very little, if any, rainfall. Key in on the timing and duration of potential thunderstorm activity and likely precipitation amounts either in the sky/weather section of the forecast or within the forecast discussion.

Temperature

The temperature, also known as the dry-bulb temperature, in a fire weather forecast represents the highest temperature expected during a day operational period or the lowest temperature expected for a night operational period. If working in complex terrain, you may also see a forecast for ridges and valleys when the temperature is dependent upon elevation.

Most often, the highest temperature during the daytime will occur during the mid- to late-afternoon hours. Generally, the coldest time of the night is near or just after sunrise. The common daily increase and decrease of temperature is called the diurnal temperature variation, as seen in the top half of Figure 4.

Occasionally, the temperature does not follow the diurnal temperature variation. If this common pattern is not expected, it should be outlined in the forecast discussion. For example, a cold front might move through your area in the middle of the morning (top half of Figure 5). In this case, the temperature would peak just before the front and would likely decrease after the front passes. It is critical that the forecast discussion include these variables of timing, including frontal passages, when applicable.

Some temperature forecasts also include a 24-hour change. This indicates if today is expected to be warmer or cooler than yesterday which can be helpful in determining what type of fire behavior to expect.

Temperature can easily be measured with a thermometer, a sling psychrometer, or an electronic weather meter. It is also good to have a general idea about an area’s average temperature for the time of year. Temperature varies substantially across the US but knowing how far above or below average the temperature is may give you an idea of the potential for extreme wildfire behavior.

Relative Humidity (RH)

The relative humidity (RH) is expressed as a percent and is defined as the ratio of the amount of moisture in the air to the amount of moisture in the air if the air were saturated at the same temperature and pressure. Low values of RH are well-correlated to large fire growth as dry air pulls moisture from the fuels allowing those fuels to ignite and burn more easily. RH is relative to temperature and is not directly related to the absolute water vapor content in the air.

Like temperature, RH often follows a diurnal variation but in the opposite direction: the hottest times of the day are often the times of lowest RH and the coldest times of the day are often the times with the highest RH. This is known as an inverse relationship and is valid for when the absolute amount of water vapor in the air does not change. For the common diurnal temperature variation, the inverse relationship holds as seen in Figure 4. But as with temperature, frontal passages, or other events can lead to unexpected increases or decreases in RH. For example, if a cold front passes early in the day, the minimum RH might be reached just before the front passes as seen in Figure 5.

It is important to remember that RH changes not only with temperature but also with the amount of water vapor in the atmosphere. There are times when the absolute amount of moisture in the air may change but the temperature does not. In these situations, the RH may increase or decrease without a corresponding change in temperature. These are times when the inverse relationship might not hold true.  

There are several mechanisms that can change the absolute amount of water vapor in the air. Often, the water vapor content of the atmosphere decreases throughout the day due to convection and mixing. Additionally, frontal passages may change the moisture content of the air as dissimilar air masses have dissimilar amounts of water vapor present. A dryline is one such front where the humidity can drastically change: humidity may be moderate ahead of a dryline but after a dryline passes, the humidity can fall to exceedingly low values. By examining the dew point temperature, you can get a much better idea of the absolute amount of water vapor in the air.

Red Flag Warnings are, in part, dependent upon local thresholds of minimum RH. Be sure to know what that value is for your area. RH can be measured with a sling psychrometer combined with a psychrometric table or, more accurately, with an electronic weather meter. Temperature and RH data can also be combined to calculate the FDFM and POI.

Temperature and RH also have a large impact on human comfort and health, not just on fire behavior. Heat-related illnesses, ranging from heat cramps to the potentially fatal heat stroke, regularly occur in firefighters both in physical training and on the fireline. It is imperative to know when you might be most susceptible to and what the symptoms are of a heat-related illness.

Wind

The wind forecast as given in the fire weather forecast is the 20-foot wind, unless otherwise specified. It may also be referred to as the surface wind or just wind. Remember that the 20-foot wind is the average wind that will be found 20 feet above the vegetative cover (Figure 6). Wind speed will be given in miles per hour and wind direction is always reported as the direction from where the wind is blowing. For example, a northwest wind is blowing from the northwest. Wind direction is most often given in one of eight compass directions: N (north), NE (northeast), E (east), SE (southeast), S (south), SW (southwest), W (west), and NW (northwest).

It is unreasonable to imagine a situation where the wind would be maintained at a certain direction and speed for an entire operational period over the expansive geographical extent of a large wildland fire. But due to the constraints of the fire weather forecast, a single wind direction and speed value may be the only wind value presented in the forecast. In reality, the observed 20-foot wind is a combination of local wind and general wind effects. Turbulence, friction, terrain, and even the fire itself can also strongly influence the wind speed and direction at your specific location on a fire. And for these reasons, wind speed, and direction are often the most difficult variables to forecast for.

The wind is likely the most important factor in determining fire spread direction and spread rates. Wind direction and speed can obviously change throughout the day, so it is imperative that you know the timing of any potential wind shifts. Oftentimes when thunderstorms are nearby, the winds can change directions suddenly and/or increase in speed very quickly. These changes will impact the fire behavior and potentially your safety. The wind speed and direction often changes in the vicinity of fronts as well. Read and understand the forecast discussion to know if the forecast calls for a wind shift in speed or direction or for thunderstorm winds. You may have to alter your escape route or your safety zone accordingly.

The training provided in S-190, Introduction to Wildland Fre Behavior, and S-290, Intermediate Wildland Fire Behavior and associated content can help you better understand how to apply the forecasted 20-foot wind to the area on a wildfire where you are working. Think about the wind will flow through or around the local terrain and if you have questions, you can always ask to speak with the forecasting meteorologist.

Wind is a key variable to the Red Flag Warning criteria, but the wind threshold is location dependent, as shown in Figure 2. Wind speed can easily be measured by an electronic weather measuring device or by the wind meter contained within the Belt Weather Kit. Recall though that these hand-measured observations are made at eye-level and winds speeds will be less than the measured 20-foot wind. Wind direction can be measured by using a compass and facing into the wind to see which direction the wind is coming from. If the measured wind speed or wind direction occasionally from the forecast, it may just be random turbulence or from local effects. However, if the measured wind speed and/or direction deviate significantly from the forecast, an updated forecast may need to be requested.

Optional Variables

Chance of Wetting Rain

The chance of wetting rain (CWR) describes the probability of getting at least 0.10 inch (one-tenth of an inch) of rain during the forecast period. This amount of rain will typically wet fuels enough to substantially prevent new ignitions while reducing spotting and slowing rates of spread. However, in areas with significant canopy cover, the precipitation may get captured in the canopy before reaching the surface fuels. If the CWR is low, but there is also a chance of thunderstorms, it is likely that dry thunderstorms may occur. The CWR can be, and often is, different than the PoP as given in the sky/weather section of the forecast depending on how much precipitation is expected.

Wetting rains on a fire may or may not bring a positive outcome. Outside of needing to use a rainfly for your tent, wetting rains can negatively impact proposed burnout operations, lead to localized flash flooding, inhibit prescribed fire projects, or create dangerous driving conditions. It is important to know how much rain is to be expected and what resulting actions you need to take to complete your daily tasks. 

Rainfall amounts are measured by automated weather stations or by manual rain gauges. The minimum reporting unit of rain is 0.01 inch (one-hundredth of an inch). Precipitation, especially during thunderstorms, is often not widespread and may vary considerably over short distances. A single Remote Automated Weather Station (RAWS) near your fire could have reported a wetting rain but that does not mean that wetting rains were received across the entire fire area.

Mixing Height

The mixing height, given in feet, is defined as the height to which pollutants are transported vertically due to turbulent or mechanical mixing. In other terms, it is the height that wildfire smoke will readily reach through convection (Figure 7). The layer of air between the surface and the mixing height is known as the mixing layer or well-mixed layer.

Days with high mixing heights allow the smoke plume to be transported high vertically. Nights typically have very low mixing heights and tend to trap smoke in the near-surface layer. This situation may bring hazardous air quality. Like temperature, the mixing height will vary significantly across regions. And like temperature, you may want to know what the average mixing height is for your location and time of year. Days with above-average mixing heights tend to be warmer, drier, and gustier than days with below average mixing height. These are the types of conditions can lead to an increase in fire behavior.

Mixing heights can be measured by weather balloons or estimated by numerical weather prediction systems. Portable weather balloon systems are becoming more commonplace and Incident Meteorologists (IMETs) may use them on assignment to help guide their forecasts

Transport Wind

The transport wind, as shown in Figure 8, is defined as the average wind speed (given in knots) and wind direction through the depth of the mixing layer. This is the wind that will directly impact the smoke plume and has serious implications to smoke management. It is used in calculations for smoke dispersion. The transport wind may also give you an indication of the direction of potential long-range spotting.

Like mixing height, the transport wind can be measured with a weather balloon or estimated from weather models.

Smoke Dispersal

There are a variety of indices describing smoke dispersion but focus here will be given to the Ventilation Index (VI) and the Atmospheric Dispersion Index (ADI). To calculate the VI, the transport wind is multiplied by the mixing height. The resulting VI value has a unit of knots-feet and ranges of values are given adjective smoke condition ratings. Common range-adjective ratings are shown in Table 2. Some states use different numerical thresholds for the adjective ratings so be sure to know what they are for the are you are working in. Higher values equate to better smoke dispersion. The VI is often used in prescribed fire plans to determine if the smoke dispersion will be acceptable. Use caution, however, as excellent smoke dispersion can also imply conditions favorable for large fire growth.

Table 2:  The Ventilation Index and Associated Adjective Ratings

The ADI combines a smoke dispersal model with stability information and the VI to output a value with corresponding descriptors. Table 3 shows the ADI values along with their interpretations.

Table 3:  The Atmosphere Dispersion Index (ADI) With Descriptions

Inversion

An inversion is a layer of stable air where temperature increases with height. There are several different types of inversions (marine, frontal, nocturnal, and subsidence) with the most common being the nocturnal inversion. Nocturnal inversions form at night when cold air settles down to the surface below the warmer air above it, creating very stable conditions. As the ground warms in the morning, the air above the ground warms as well. Eventually the cold, stable air that formed just above the surface during the night will have completely warmed and the inversion breaks. Generally, this occurs from 1000 to 1300. When the inversion breaks, air mixes well vertically and the mixing height can grow substantially in a short period of time. This increases the surface temperature, decreases the RH, and may increase the gustiness of the wind. Knowing when an inversion may break can help you to understand when conditions will quickly become more favorable for large fire growth.

Inversion height and strength can be measured by a weather balloon or inferred from weather models.

Haines Index

The Haines Index, also known as the Lower Atmosphere Severity Index (LASI), gives a numeric value from 2 – 6 that relates to the potential for large, plume-dominated fires; the higher the number, the higher the potential. The Haines Index is comprised of two components: a lapse rate value and the temperature-dew point temperature spread from layers several thousand feet above the surface. Each component gets a value from 1 – 3, and then these are added to get the final Haines Index.

The Haines Index is often (and incorrectly) referred to as a stability index but it does not directly incorporate information on atmospheric stability. It is merely an index that was developed by examining two meteorological variables and correlating those variables to a small database of fires that were perceived to experience large fire growth.

The Haines Index, created in 1988, represented an initial effort towards creating an index that would describe the potential for large fire growth. Recent research suggests that the value of using the Haines Index is minimal at best but deceiving at worst. It is recommended that the fire personnel and fire meteorologists stop using this index altogether.

Lightning Activity Level (LAL)

The Lightning Activity Level (LAL) is a metric that describes the relative frequency of lightning strikes and the associated rainfall intensity. The index runs from 1 to 6 as shown in Table 4: 1 implies no lightning, 2 through 5 are increasing amounts of lightning with corresponding increasing rainfall intensity, and six is dry lightning with a frequency of an LAL value of three. The LAL has historic roots in the development of a fire risk parameter for the National Fire Danger Rating System (NFDRS). However, this risk parameter was phased out decades ago and the LAL remains a lingering legacy of a lightning-derived fire risk factor.

Today, the LAL is used in forecasts to describe the potential for wet or dry lightning strikes. The LAL value is directly computed from the NWS thunderstorm coverage forecast as shown in the crosswalk in Table 4. Because it is duplicate information from what is already included in the sky/weather section of the forecast, it is recommended that both fire managers and firefighters cease using the index.

Table 4:  The Lightning Activity Level (LAL)

Outlook/Extended Forecast

Fire weather forecasts include an extended forecast or an outlook as shown in Figure 9. This section may contain forecasts of fire weather variables for the next 1 – 2 operational periods. These should only be used for broad planning purposes and situational awareness. New forecasts should be obtained at the start of each operational period, if possible, to get the most up-to-date weather information. If additional long-term information is needed, it is recommended that you speak directly with the meteorologist or use a point forecast from the NWS.

Local Weather Variables

The United States has an incredibly diverse geography. This, in part, leads to very diverse climate and weather patterns across the nation. As such, many areas of the US include other forecast variables not contained within the document. It is recommended that you examine your local fire weather forecasts and your local FWAOP to learn more about the potential for additional fire weather variables that are not outlined in this document.

Meteorologists at the NWS WFO can help you better understand local weather conditions and critical fire weather patterns. Building a working relationship with the meteorologists within the NWS will be of great value to you and to your organization by learning more about the forecast process, how forecasts are developed, and their relative accuracy for your area. Additionally, these relationships will have the added benefit of allowing the meteorologists to see how their forecast products are used. This can undoubtedly lead to improved forecasts that can better serve your needs. And finally, ask the meteorologists if they would like to tour your station or join you for a prescribed burn. Relationships are best made in person.