Weather: Critical Fire Weather

1. Hot-Dry-Windy Index
2. Summary
3. Critical Wind Events
   • Breakdown of the Upper Ridge and Cold Frontal Passage
   • Foehn or Downslope Winds
   • Thunderstorm Dynamics, Outflows, and Downbursts
   • Sea Breeze Fronts
   • Tropical Systems
4. Hot, Dry, and Unstable Events
5. Fire Slowing or Stopping Patterns

Hot-Dry-Windy Index

The Hot-Dry-Windy Index (HDW) was designed to help users determine which days are more likely to have adverse atmospheric conditions that make it more difficult to manage a wildland fire. It combines weather data from the surface and low levels of the atmosphere into a first-look product. HDW was designed to be very simple – a multiplication of the maximum wind speed and maximum vapor pressure deficit (VPD) in the lowest 50 or so millibars in the atmosphere. Because HDW is affected by heat, moisture, and wind, seasonal and regional variability can be found when comparing HDW values from different locations and times.

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Summary

• Synthesis of Knowledge of Extreme Fire Behavior: Volume 1 for Fire Managers, Chapter 3
• Critical Fire Weather Patterns; Program for Climate, Ecosystem, and Fire Application (CEFA)

The four critical weather elements that produce extreme fire behavior are low relative humidity, strong surface wind, unstable air, and drought. The critical fire weather patterns that support these conditions can be separated into two primary categories: those that produce strong surface winds, and those that produce atmospheric instability. In both cases, an unusually dry airmass for the region and season must also occur. In brush and timber fuels, drought becomes an important precursor by increasing fuel availability.

Most periods of critical fire weather occur in transition zones between high- and low-pressure systems, both at the surface and in the upper air. The surface pressure patterns of most concern are those associated with cold fronts and terrain-induced foehn winds. Cold front passages are important to firefighters because of strong, shifting winds, and unstable air that can enhance the smoke column or produce thunderstorms. Foehn winds occur on the lee side of mountain ranges and are typically very strong, often occurring suddenly with drastic warming and drying. The area between the upper ridge and upper trough has the most critical upper air pattern because of unstable air and strong winds aloft that descend to ground level.

East of the Rocky Mountains, most critical fire weather patterns are associated with the periphery of high-pressure areas, particularly in the pre-frontal and post-frontal areas.

In the Northern Plains, Great Lakes, and the Northeastern United States (US), pre-frontal high pressure from the Pacific, Northwestern Canada, and Hudson Bay all can produce very dry conditions. Cold fronts produce relatively short-lived periods of high winds and instability that can produce extreme fire behavior.

In the Southeastern US, drought is frequently associated with the La Niña state of the southern oscillation pattern or a blocking ridge aloft near the Atlantic coast. Often critical weather patterns follow the frontal passage that brings extremely dry air due to a strong westerly or northwesterly flow. Look for strong winds that accompany the flow. Beware of advancing tropical storms as well.

In the Southwestern US, the breakdown of the upper ridge, before monsoons develop, is manifest at the surface with breezy, dry, unstable conditions that transition to potentially very windy conditions as it finally breaks down. During transition to the monsoon pattern, shallow monsoons can produce gusty wind, low relative humidity (RH), and lightning without much precipitation.

In the Rocky Mountain and Intermountain Regions, the most significant pattern is the upper ridge surface thermal trough that produces a dry and windy surface cold front.

• Along the eastern slopes of the Rocky Mountains, weather patterns producing Chinook winds bring strong downslope winds that are unusually dry and warm.
• In the intermountain west, critical fire weather is associated with upper troughs and overhead jet streams, or surface dry cold front passages.

Along the Pacific Coast, from Washington to California, weather patterns producing offshore flow or foehn wind are the most important.

In the Pacific Northwest, the east wind produces strong winds and dry air west of the cascades. The upper ridge breakdown is similar to that described for the rocky mountain & interior west.

In California, the most important are the north and mono winds of north & central regions and the Santa Ana and sundowner winds of Southern California. The subtropical high aloft bring heat waves.

In Alaska, the primary pattern is the breakdown of the upper ridge with a southeast flow. It can bring gusty winds and dry lightning to the interior of Alaska after a period of hot, dry weather.

In Hawaii, the dry season is May – September and the wet season is from October – April. Critical fire growth events can occur any time of the year when live fuels are drought stricken and/or when herbaceous green-up has senesced. Critical fire weather patterns are concentrated around trade wind periods whenever strong high pressure develops to the north and/or when a significant tropical cyclone passes to the south of the main Hawaiian Islands. Strong southwest winds from frontal systems have led to critical fire weather, creating down-sloping winds across what are, typically, windward sides.

The above descriptions include key words and catch phrases meteorologists typically use to describe critical fire weather growing patterns. These terms will often be used to explain weather patterns but are not exclusively used. The terminology will often be found in National Weather Service Area Forecast Discussion (AFD) and fire weather planning forecast discussions as well as predictive service seven-day outlook assessments.

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Critical Wind Events

Breakdown of the Upper Ridge and Cold Frontal Passage

Breakdown of the upper ridge involves three main stages:

• First stage represents warmer-drier-breezy and unstable conditions.  
• Second stage wind speeds will increase while conditions remain warm-dry and unstable.
• Third stage is defined by a cold frontal passage.

Significant fire growth across the west and Alaska can be tied to all three stages but occurs most frequently during the second stage. Significant fire growth across the east is primarily related to post-cold frontal conditions or the third stage but can occur during the second stage.

The above graphics are an analysis of locations for fire growth with respect to cold frontal passage and general breakdown of a high-pressure ridge. CFA is after the cold front passage, CFB is before cold front passage, WSL is warm sector of Low, and WS is warm sector of departing High.

Foehn or Downslope Winds

Foehn or downslope wind events have many regional names. You might recall that foehn or downslope winds are caused by air-forced over mountain ranges and through mountain passes in association with stable conditions. Common examples are Santa Ana and Chinook winds.

Thunderstorm Dynamics, Outflows, and Downbursts

Thunderstorms in the vicinity of a fire have the potential to produce outflow gust fronts or downbursts, regardless of whether the updraft is fed by the fire, or not. Any evidence of precipitation means the storm has developed to the point where it can produce these types of winds, as well as lightning. Rain at the ground or virga is a potential warning sign. Outflow gust fronts are winds radiating outward but primarily in the direction of storm motion, from the base of the convection. They are present in all well-developed convection and last tens of minutes to an hour or more. They can travel tens to hundreds of kilometers. Downbursts are much less common, shorter lived, and affect a much smaller area. Either type of wind has the potential to abruptly change the speed and direction of fire spread.

Sea Breeze Fronts

Sea breeze or sea breeze fronts can bring gusty, shifting winds, and changes in humidity and stability that can drive fire behavior along coastal regions. The few hours leading up to the onset of the sea breeze are the warmest and driest and coincide with increasing wind speeds and unstable conditions. Following the passage of the sea breeze front, conditions will become cooler as well as more humid and stable. Sea breezes are more critical than land breezes because they occur during daylight hours.

Tropical Systems

Tropical systems, including tropical storms and hurricanes, produce an area of relatively warm, dry, and windy conditions around their northern and western periphery as they move ashore.

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Hot, Dry, and Unstable Events

Critical fire growth periods are also tied to hot, dry, and unstable weather patterns. These patterns are generally tied to an upper ridge and a strong mid-level dry intrusion. The upper ridge and dry intrusion are ingredients that oftentimes set up the breakdown of the upper ridge pattern. A strong heat bubble combined with unusual mixing will create above normal temperatures, sometimes a heat wave, and very low day and nighttime humidity values. The unusual mixing is caused by an unstable atmosphere related to the heat bubble.

Thermal lows can develop near the surface and help concentrate this instability. The hot, dry, and unstable weather pattern is typically related to the subtropical or Bermuda high. Subtropical highs typically impact the western half of the US and Bermuda highs the eastern half. On rare occasions the highs will coalesce and create a super high which can impact large portions of the country.

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Fire Slowing or Fire-Stopping Patterns

There are three primary reasons why geographical areas experience fire slowing and/or fire-stopping periods; they are related to partial or whole-scale change in the weather and fuel regime. Such change includes partial green-up or fuel moistening promoted by periods of precipitation. Such change also includes temporary changes such as a cool, moist, stable weather regime replacing hot, dry, and unstable conditions during a multi-day period. Understand that the fire slowing and/or fire-stopping period represents a temporary change.

There are four main fire slowing or stopping patterns across the country:

• Closed Low-Deep Trough-Frontal Passages
• Monsoon Bursts
• Tropical Storms
• Smoke Events

The season-ending period is like the fire slowing period with the main differences being the degree of change in fuels and weather conditions and how long they persist. During the season-ending period, there is an overwhelming change to fuel conditions, such as long-lasting green-up or a significant rise in larger sized fuel moisture values. For most geographical areas, there is not just one event that brings the season’s end. It is an accumulative effect of a few or several weather events. For example, the Southwest monsoon develops during a span of several days to weeks and brings about a mosaic change to the live and dead fuels.