Weather: Fire Season Climatology

1. Drought Assessments
   • Meteorological Indicators
   • Assessments Resources
2. Local Climatology and Current Season Trends
   • Evaluate Fire Occurrence Patterns
   • Identify Normal Seasonal Trends
   • Fire Season Severity
   • Current Trends and Historic Norms and Extremes
   • Evaluate Historic Trends for Weather that Slows or Stops Fire Growth
   • Event Frequency and Duration
   • TERM Waiting Time Distribution
3. Regional Fire Seasonality
   • Alaska
   • Northwest
   • Northern California
   • Southern California
   • Southwest
   • Great Basin
   • Northern/Central Rockies and Great Plains
   • Great Lakes and Northeast
   • Southeast

Drought Assessments

Meteorological Indicators

A wide range of weather based indices are available, based on accumulated precipitation alone, as well as precipitation combined with modeled evaporation and/or transpiration rates. These include spatial representations of soil moisture, vegetative stress, agriculture, and water supply. Many can be found using the assessment resources listed below.

National Fire Danger Rating System (NFDRS) includes the 1000-hr time lag fuel moisture, Energy Release Component (ERC), Growing Season Index (GSI), and Keetch-Byram Drought Index (KBDI) among its outputs. See Fire Danger Section.

Canadian Forest Fire Danger Rating System (CFFDRS) includes the Duff Moisture Code (DMC), Drought Code (DC), and Buildup Index (BUI) among its codes and indices. See CFFDRS Section.

Evaporative Demand Drought Index (EDDI) is an experimental drought monitoring and early warning guidance tool. It examines how anomalous the atmospheric evaporative demand (E0; also known as "the thirst of the atmosphere") is for a given location and across a time period of interest.

Standardized Precipitation Evapotranspiration Index (SPEI)  can measure drought severity according to its intensity and duration, and can identify the onset and end of drought episodes and allows comparison of drought severity through time and space, since it can be calculated over a wide range of climates

Evaporative Stress Index (ESI) describes temporal anomalies in evapotranspiration (ET), highlighting areas with anomalously high or low rates of water use across the land surface.

Standardized Precipitation Index (SPI) is the number of standard deviations that the observed value would deviate from the long-term mean. Since precipitation is not normally distributed, a transformation is first applied so that the transformed precipitation values follow a normal distribution.

Quantitative Precipitation Estimate (QPE) show spatial distribution of precipitation are multi-model estimates. Using a multi-sensor approach, it is one of the best sources of timely, high resolution precipitation information available. 

Assessments Resources

The US Global Climate Change Research Program (USGCRP) is mandated by Congress in the Global Change Research Act (GCRA) of 1990 to assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change.

NOAA Climate.gov is a source of timely and authoritative scientific data and information about climate. It provides news items, maps and data, and teaching resources.

Western Water Assessment NOAA Integrated Sciences and Assessments

Drought Monitor National Drought Mitigation Center

National Integrated Drought Information System National Centers for Environmental Information

National Drought Mitigation Center University of Nebraska – Lincoln

River Forecast Centers NWS Advanced Hydrological Prediction Services provides depictions of river flows and flooding; rain and snow fall in graphic and digital formats

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Local Climatology and Current Season Trends

Evaluate Fire Occurrence Patterns

FireFamily Plus depictions of fire occurrence patterns are good for evaluating ignition patterns, but may provide little insight to climatology and fire growth in regions where incidents commonly exhibit multiple growth events during extended periods of active fire behavior.

Identify Normal Seasonal Trends

Local Winds Climatology

Wind roses are available in FireFamily Plus, Western Regional Climate Center, and in other tools. Ensure it includes only relevant winds, (e.g., exclude other seasons, night-time winds, etc.)

Fire Season Severity

Use Appropriate Fire Potential Indicators (ERC, BUI) and determine season start, peak season, shoulder seasons, and season end.

Current Trends and Historic Norms/Extremes

General Trends

Evaluate and depict current conditions spatially, using a combination of drought assessment resources appropriate for the area of interest.

Current Local Conditions

Graphical Time Series depictions can highlight seasonal departures from norms and exceptional events. This FireFamily Plus Climatology graphic includes historic range (average, max, min), current year, and analog year.

Consult with Local Managers and Experts.

Get their help evaluating the objective analysis and to identify any local anomalies that may not be apparent.

Evaluate historic trends for weather that slows or stops fire growth

Fire Stopping Events

Originally reported by Latham and Rothermel (1993) from opinions of persons familiar with fire and fire weather in the Northern Rockies, example criteria were suggested as 0.5 inches of rain accumulated in 5 days or less. However, other fire potential indicators, such as cloud cover and relative humidity, can help identify periods of continuous low- or no-spread days during a fire season in a locality. These discrete events may not signal the end of the fire or the season. As such, it may be just as important to identify the frequency with which they occur and the duration of their influence as it is to predict a waiting time for the next event.

Fire Season-Ending Date

It is possible to identify the date in a fire season when fire growth is no longer likely or possible. This is generally understood to be the season end. If a fire’s threat to values of concern is more imminent or it is early in the fire season, a prediction of the season end may be less important than suggesting if and when a weather event will put the fire out. In bimodal seasons, such as the spring seasons in the southwest and the lake states, weather criteria can suggest fire ending dates in the early “season”, even though fire potential is expected to rise again in subsequent periods. Anticipating this date may be critical to strategic decisions as the season end approaches.

Event Frequency and Duration

As suggested above and depicted here, it may be valuable to identify the frequency of fire slowing or stopping events, especially if they are more common, such as in the eastern U.S. FireFamily Plus Event Locator can be used to evaluate data from a local RAWS station.

TERM Waiting Time Distribution

FireFamily Plus (version 4.1) includes a TERM tool, in the Weather menu, to produce a waiting time distribution of historic fire- or season-ending dates. For each year evaluated, a single date is selected as the ending date based on established criteria. These dates are recorded and the distribution plotted to estimate the probability that the fire or season will end by a specific date.

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Regional Fire Seasonality

The basic climate/weather factors temperature (hot vs. cold), atmospheric moisture (dry vs. moist), and wind patterns affect the fuel conditions and the tendency for fire start and spread. Fire season characteristics are driven by seasonal and continental-scale weather patterns, their movement, and variation. Seasonal air mass and jet stream changes affect various regions at different times and in different ways.

The following will be key words and catch phrases meteorologists typically use to describe critical fire weather growing and slowing patterns. These terms will often be used to explain weather patterns but not exclusively used. The terminology will often be found in National Weather Service Area Forecast (AFD) and fire weather planning forecast discussions as well as Predictive Service 7-day outlook assessments.

Alaska

Climatology

• Winter – generally very cold and dry.
• Spring – cold and dry early, then warming with increasing downslope wind event potential.
• Summer – warm and dry with lightning in June, then gradually moistening. Occasional wind events.
• Fall – moist initially, then back towards winter conditions.

Fire Activity

• Peak fire activity late spring and summer coincident with warmest/driest conditions, and wind event and dry lightning potential
• Great majority of activity in interior between Alaska and Brooks ranges
• Season ramps up quickly after melt-off and strongly relates to length of day
• Season ends quickly with late summer/early fall moisture increase

Critical Weather Events

• Foehn and Downslope wind - Glacial Katabatic downstream from glaciers, e.g. Juneau fjords, Alaska Range Chinook in the western Tanana Valley, Hillside Katabatic Winds, e.g. Anchorage Bowl, and Yukon Chinook winds in the eastern interior and through the Brooks Range.
• Breakdown of the Upper Ridge - Boreal interior area from Galena to Fort Yukon, warming-drying ahead of the low-pressure system originating from the Sea of Okhotsk with gusty W-SW winds, and Low Pressure moving inland and loses wetting characteristics but keeps enough moisture for drier storms.

Fire Slowing or Stopping Events

• Closed low/occluded low is a low-pressure system that originate from the Sea of Okhotsk and Bering Sea moves inland and stays for a multi-day period or a persistent moist southwest flow impacting the coastal areas.
• Marine Inversion: coastal areas of Alaska especially during night.

Fire Growth Potential Indicators

• FFMC and BUI, ISI and FWI

Northwest

Climatology

• Winter/Spring – cool and moist with frequent and abundant precipitation, especially western portion of area. Relatively dry east.
• Summer – some windy and dry potential early, then becoming generally warm and dry with infrequent wind events due to dry cold fronts.
• Fall – return of cooler, more moist conditions ushered in by a period of windy, dry conditions with cold frontal passages. Potential for dry offshore wind events behind storm systems.

Fire Activity

• Fire activity peaks in summer due to increasingly warm and dry conditions and potential for wind and lightning with dry cold frontal passages.
• Rapid decrease in activity in fall with Pacific moisture on the increase, though this the peak period for dry offshore wind events and a few dry cold front passages are still possible.
• Little to no activity late fall through spring.

Critical Weather Events

• Thermal Low/Subtropical Ridge
• Breakdown of the Upper Ridge and Passage of a dry cold front
• Foehn or Downslope Wind (East Wind west slopes of Cascades and Chinook Wind east slopes of the Cascades)

Fire Slowing or Stopping Events

• Closed Lows/Wet Cold Front
• Marine Layer/Onshore flow
• Smoke Events

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Fire Growth Potential Indicators

• Energy Release Component
• 100-hr fuel moisture
• AVHRR satellite NDVI DA and RG
• NWS QPE (30-60 days)
• Drought Monitor

Northern California

Climatology

• Winter/Early Spring – cool and moist with regular precipitation events, especially in the mountains.
• Late Spring/Summer – some windy/dry potential early, then generally warm and dry with infrequent wind events due to dry cold front influences.
• Fall – return of cooler, more moist conditions ushered in by a period of windy, dry conditions with cold frontal passages. Potential for dry, north through east wind events behind storm systems.

Fire Activity

• Fire activity peaks in summer due to increasingly warm and dry conditions and potential for wind and lightning with infrequent dry cold frontal passages.
• Rapid decrease in activity by late fall with Pacific moisture on the increase, though peak period for dry northeast wind events.
• Little to no activity late fall through early spring.

Critical Weather Events

• Foehn or Downslope Wind (Mono, North Winds)
• Breakdown of the Upper Ridge in the Interior
• Subtropical Ridge/Thermal low

Fire Slowing or Stopping Events

• Closed Low/Pacific Trough
• Marine Layer/Onshore flow
• Smoke Events

Fire Growth Potential Indicators

• Spread Component (SC)
• Burning Index (BI)
• Energy Release Component (ERC)
• Live Fuel Index (LFI)/Growing Season Index (GSI)
• AVHRR satellite NDVI DA and RG
• NASA SPoRT GVF
• NWS QPE (30-60 days)
• Drought Monitor

Southern California

Climatology

• Winter – occasional storm systems with mainly mountain precipitation. Inland intrusions of cool, moist Pacific air. Relatively dry inland lower elevations.
• Spring – less frequent precipitation events and substantial inland intrusions of marine air.
• Summer - hot and dry inland and maritime influence along coast. Occasional influx of monsoon moisture from southeast.
• Fall – end of any monsoon moisture influence and begin of gradual inland shift in marine air mass. Period of greatest potential for dry offshore wind events.

Fire Activity

• Fire activity peaks late spring through fall, when influence of maritime air is diminished.
• Greatest potential for offshore wind events in the fall, when fuels are often driest.
• Little activity winter-early spring due to maritime influence.
• Fires possible at any time with offshore wind events.

Critical Weather Events

• Foehn or Downslope wind (Santa Ana and Sundowners)
• Breakdown of the Upper Ridge away from the coasts
• Subtropical Ridge

Fire Slowing or Stopping Events

• Marine Layer/Onshore Flow
• Closed Low/Pacific Trough

Fire Growth Potential Indicators

• Spread Component (SC)
• Burning Index (BI)
• Energy Release Component (ERC)
• National Fuel Moisture Database
• Live Fuel Index (LFI)/Growing Season Index (GSI)
• AVHRR satellite NDVI DA and RG
• NASA SPoRT GVF
• NWS QPE (30-60 days)
• Drought Monitor

Southwest

Climatology

• Winter – cool to cold with occasional precipitation. Dry, downslope winds possible in lee of Rockies.
• Spring – warming, windy and dry transitioning to hot and dry.
• Summer – hot and dry gives way to warm and moist abruptly with monsoon.
• Fall – turning much drier and mild. Potential for few wind events followed by dropping temperatures.

Fire Activity

• Fire activity increases in spring as it transitions from windy and dry to hot and dry.
• Peak from May – mid-July, with monsoon thereafter.
• Rare secondary fall season as moisture exits and jet drops south and wind event potential returns.
• Little activity late fall - early winter.

Critical Weather Events

• Breakdown of the Upper Ridge
• Subtropical Ridge
• Monsoon transition (Edge of a Monsoon Burst and Edge of Back Door Cold Front)
• Foehn or Downslope wind
• Low Level Jet on Rangeland of the Front Range
• Surface Dryline Passage on Rangeland of the Front Range

Fire Slowing or Stopping Events

• Closed Low-Cold Frontal Passage
• Back Door Cold Front
• Monsoon Burst

Fire Growth Potential Indicators

• ERC and BI
• NFMD
• LFI/GSI
• AVHRR satellite NDVI RG and DA
• NASA SPoRT GVF
• NASA SPoRT RSM (0 to 10 cm)
• NWS QPE (30 day)
• Drought Monitor

Great Basin

Climatology

• Winter – periodic precipitation, mainly over mountains.
• Spring – becoming windy, dry, and warmer.
• Summer – hot and dry. Periodic wind events north and moisture surges south.
• Fall – period of windy and dry conditions often followed by period of fair and dry weather before cooler temperatures and increased precipitation potential.

Fire Activity

• Generally fine fuel types, fire season dependent on cured fuels and windy/dry conditions.
• These conditions occur almost exclusively in the summer.
• Little to no activity outside of summer.

Critical Weather Events

• Breakdown of the Upper Ridge
• Subtropical Ridge
• Edge of a Monsoon Burst (Hybrid)
• Foehn or Downslope Wind (Chinooks down east slopes of the Sierras and west slopes of the Wasatch Mountains)

Fire Slowing or Stopping Events

• Closed Low/Pacific trough
• Monsoon Burst-duration of 3 days or more

Fire Growth Potential Indicators

• ERC and BI
• National Fuel Moisture Database
• LFI/GSI
• AVHRR satellite NDVI DA and RG
• NWS QPE (30-60 days)
• NASA SPoRT RSM 0-10 cm/GVF
• Drought Monitor

Northern/Central Rockies and Great Plains

Climatology

• Winter – regular storm systems and precipitation, especially over mountains. Cold overall with potential for artic air intrusions.
• Spring – period of heaviest precipitation in the mountains, but greatest Chinook wind potential in lee of Rockies and adjacent plains.
• Summer – warm and dry over most mountain areas with occasional wind events north. Increasingly moist across the plains and far south.
• Fall – period of windy/dry potential, then fairly dry and mild until temperatures drop and moisture increases.

Fire Activity

• Fire activity on the plains peaks in spring and fall when windy/dry periods are coincident with dormant or cured fine fuels.
• Fire activity in the mountains peaks in the summer, when it’s warmest and driest and some dry cold frontal passages are possible.
• Little to no activity late fall through early spring.

Critical Weather Events

• Breakdown of the Upper Ridge (Dynamic dry slot and Dry Cold Front)
• Subtropical Ridge (Mid-level dry intrusion)
• Edge of a Monsoon Burst (Hybrid)
• Foehn or Downslope Wind (Chinook)
• Low Level Jet and Surface Dryline on the Great Plains

Fire Slowing or Stopping Events

• Closed Low-Pacific Trough-Cold Frontal Passage
• Monsoon Burst
• Smoke Event

Fire Growth Potential Indicators

• ERC and BI
• National Fuel Moisture Database
• Live Fuel Index (LFI)/Growing Season Index (GSI)
• AVHRR satellite NDVI DA and RG
• NASA SPoRT GVF
• NWS QPE (30-60 days)
• Drought Monitor

Great Lakes and Northeast

Climatology

• Winter – generally cold with dry periods between widespread periodic precipitation.
• Spring – warmer, windier, and drier. Driest immediately behind storm systems.
• Summer – generally warm and humid under Bermuda High influence. Occasional windy/dry events far north.
• Fall – turning much drier, then generally mild and dry with potential for windy and dry periods before temperatures drop.

Fire Activity

• Fire activity maxima in spring and fall, coincident with windy periods near jet stream
• Building warmth and dormant fine fuels in spring, leaf-off in fall
• Season can extend well into summer far north if jet remains active and brings windy/dry events that are preceded by dry conditions of two weeks or more
• Little or no activity winter months

Critical Weather Events

• Post Cold Frontal
• Pre-Cold Frontal Southwest Wind cases
• Bermuda High

Fire Slowing or Stopping Events

• Cold Frontal Passage
• Stationary Front
• Closed Low

Fire Growth Potential Indicators

Canadian Forest Fire Danger Rating System

• Build-Up Index (BUI)
• Fire Weather Index (FWI)
• Initial Spread Index (ISI)

National Fire Danger Rating System

• Ignition Component (IC)
• Spread Component (SC)
• Energy Release Component (ERC)
• AVHRR NDVI RG and DA

Southeast

Climatology

• Winter – generally driest time of year with greatest wind event potential behind passing storms, though widespread precipitation can also occur.
• Spring – windy/dry potential retreats north, and warm, moist conditions become increasingly dominant.
• Summer – warm to hot and humid with light winds. Occasional dry spells. Tropical cyclone activity increases late in the season.
• Fall – very moist initially, then gradual infiltration of dry air. Moist conditions often persist along Gulf Coast.

Fire Activity

• Fire activity maxima in late winter / early spring and fall, coincident with greatest potential for windy/dry conditions behind passing storm systems.
• Dormant fine fuels with low live fuel moisture in winter and spring, leaf-off in fall in northern portion of region.
• Season can extend year-round anytime warm/moist air becomes suppressed south and east.
• Usually little activity summer months, though significant fire activity has historically occurred during unusual dry spells.

Critical Weather Events

• Post Cold Frontal
• Westerly Downslope Wind in the Appalachians and Ozarks
• Sea Breeze
• Tropical Storms
• Bermuda High

Fire Slowing or Stopping Events

• Closed Low/cold frontal passage
• Stationary Front
• Tropical Storm
• Sea Breeze

Fire Growth Potential Indicators

• ERC
• 100 hr Fuel Moisture
• Keetch-Byram Drought Index (KBDI) is sometimes misused
• Standardized Precipitation Index (SPI)
• Crop Moisture Stress Index
• NWS QPE (30 to 60 day)