By Zach Butler, Meteorologist Posted 4 months ago July 26, 2023
How Spring Weather Affects Wildfire Season
Did you know that spring weather significantly influences a region’s wildfire activity during the summer? The main reason for this is that spring weather (wet vs dry) helps dictate how much new vegetation grows in the way of grasses and shrubs. Additionally, the spring weather affects how moist the existing dead wood will be during the summer.
The amount of new vegetation growth and the amount of existing dead vegetation during the spring affects what is called the ‘fuel load’ for wildfires in the summer. Another factor is drought, which can affect both the amount of fuel available as well as the moisture characteristics of that fuel.
With wildfire season becoming more active around the Western United States, let’s take a deeper look into the science of spring weather and fuel loads, as well as where the fuels are currently most likely to ignite wildfires.
Surface (left) and satellite imagery (right) of the Bedrock Fire in Oregon.
What is the ‘Fuel Load’ and How Does it Affect Wildfires?
The fuel load is defined as the total amount of combustible material in a defined space. Fuel load is quantified in heat units or in its equivalent weight in wood. A given area with more wood, grass, or shrubs that are drier will have a greater fuel load. The fuel load increases as temperatures warm, humidity decreases, and with less precipitation.
The fuel load is an important variable that meteorologists and wildfire operations monitor because it helps determine when wildfires can occur and spread into large fires. If a given area experiences a wildfire but the vegetation is still moist, the wildfire will not spread as rapidly, given the same conditions but with dry vegetation.
Wildfires are most likely to ‘blow up’ when fuels are plentiful and unusually dry from drought. The availability for wildfires to grow and spread depends on many other factors in the environment and atmosphere such as the micro-climate, soil conditions, atmospheric stability, terrain steepness, and more. We’ll cover these topics in News Articles throughout the summer and fall.
How Spring Weather Affects the Fuel Load
The weather in the spring is a major contributor to the fuel load. A wet spring will mean that more vegetation can grow and thus increase the fuel load (as long as it dries out eventually). A wet spring will also mean that existing dead wood will stay moist longer.
We have seen a wet spring and early summer for many areas across the Western US this year (2023) due in part to the record-setting snow pack for some areas. The above-average snowfall this past winter caused above-average snow melt, and therefore more water availability.
Below is California contrasting May 2022 on the left and May 2023 on the right. The additional snow present in May 2023 can potentially cause more significant wildfire conditions once the ‘green-up’ has dried out in the summer.
This means that due to a wet winter and or wet spring (for example in California), the potential for significant wildfire activity could increase by late in the season (late summer/early fall) if and when vegetation and fuels dry out. Wildfire seasons are not always worse with wetter winters and/or springs, but they can be a significant factor.
Connecting this to Wildfires
Wildfires need fuel, combustion, and favorable hot, dry, and windy weather to be the greatest hazard.
Once the fuel loads have accumulated due to spring and early summer vegetation growth, summer heat, and dry weather can cause fuels to become ready for combustion. The vegetation loses its moisture and becomes dry enough to allow wildfires to start and grow quickly. The summer is when the accumulation of the past few months of weather affects when, where, and how large wildfires can be.
The National Weather Service issues Red Flag Warnings, which means critical fire weather conditions are occurring or will occur shortly. Critical fire weather conditions mean gusty winds with low relative humidity, that allow fires to spread and grow quickly.
Where are the Fuels ready to Ignite?
Wildfires have recently ignited throughout the Western US during the middle of July 2023. Hot, dry, and windy conditions over the past month have allowed fuels to dry out and become ready for combustion. In addition to this, several areas throughout the Western US are still in a drought, despite the wet winter and spring.
This includes areas throughout the Northwest in Oregon, Washington, Idaho, and Montana. These areas have had a warmer-than-average and drier-than-average spring and early summer. This has caused drought conditions and current wildfire activity in part due to the combustion of dry fuel loads.
Areas in drought are more prone to have dry fuels, which means wildfires are more likely. Associated with these areas of drought are many locations where wildfires are currently as well as where the greatest hazard for fires to ignite this summer.
Below is the National Interagency Fire Center’s Significant Wildland Fire Potential map for the month of July. The above normal risk in red correlates with many areas currently in a drought.
Many areas in California are fortunately in the green (below normal fire potential), which is due to the record-setting winter, which is still allowing many forests to have fuel loads that are too moist. This could change in the coming months if warm and dry weather continues.
To see how wildfire smoke could disrupt your summer adventures, use the OpenSnow smoke map which shows the location of forecasted wildfire smoke across North America and up to 60 hours (2.5 days) into the future.