Fire Weather forecasting in Maine

Mark Bloomer

Fire Weather Focal Point

 

     National Weather Service offices nationwide participate in a specialized branch of forecasting known as “Fire Weather”.  A fire weather forecast is a forecast specifically tailored to outline weather conditions that effect wildfires.  These include the potential for a fire to start, for a fire to grow and spread, and for the smoke produced by a fire to disperse through the atmosphere.  Fire weather forecasts are used by forest officials to help determine the risks of wildfire, and whether or not conditions are safe to conduct controlled burns.  In the event a wildfire is taking place, a fire weather forecast can be used by firefighters to help determine how rapidly the fire may spread and in what direction it will most likely progress.

     Several weather conditions have a direct effect on fire, but the two most important ones are moisture and wind.  Atmospheric moisture will have a direct influence on how wet or dry the “fuels” are.  Fuels refer to the naturally occurring materials available for burning including grasses, leaves, branches and trees.  In fire weather we refer to four general classes of fuels categorized by how quickly they burn. One hour fuels are the fine kindling fuels such as dried leaves and grasses.  Ten hour fuels include small twigs and woody shrubs.  Hundred hour fuels include large branches and small trees.  And thousand hour fuels are large tree trunks.  The moisture of the larger fuels is generally determined by whether we have been in a dry weather pattern or a rainy pattern.  The fine fuels, however, respond much more directly to how moist or dry the air is.  It is the moisture of the fine fuels that often determines how easily a fire can get started and how quickly a fire can spread.  A sunny day with low humidity can allow the fine fuels such as leaves, grasses and forest litter to quickly dry and become flammable.  High humidity, on the other hand, will quickly increase the amount of moisture contained in the thin fuels and make fire less easy to get started.   

     Wind is the other meteorological variable that has a major impact on fire.  It effects how quickly fire can spread and in what direction fire can progress.  Fire will grow in the direction of the wind as the wind pushes flames and embers downwind igniting new materials.  Wind shift can be especially critical for firefighters dealing with a large fire.  For example, a south wind shifting to a west wind behind a cold front may cause a fire to accelerate its advance eastward.  This can put firefighting troops along the eastern flank of the fire in considerable danger.  Advance warning of the wind shift can allow the troops to reposition themselves and their equipment out of danger.  Even subtle wind shifts can have a significant effect on the behavior of fire.  Local variations in winds due to diurnal heating and cooling must often be taken into consideration in a fire weather forecast.  For example, on a sunny day with light winds, localized breezes will flow up the sunlit warm sides of mountains and flow down the shady cool sides of mountains.

    Fire weather forecasting is also concerned with the effects that weather has on the smoke being produced by a fire.  Will the smoke pool close to the ground and cause an area of pollution and low visibility, or will the atmosphere disperse the smoke and carry it away from the blaze?  A fire weather forecast includes a prediction of how deep the mixed layer of the atmosphere will be.  During the day when solar heating is producing thermal currents in the atmosphere, the mixed layer will typically rise several thousand feet in altitude.  At night as the ground cools and the low level air cools, the mixed layer may decrease to less than a hundred feet.  A deeper mixed layer will help dilute the smoke while a shallow layer will cause smoke to stay close to the  ground.  Wind direction and speed within the mixed layer will determine where the smoke is carried.  The combination of mixing depth and transport winds produces a factor known as the ventilation rate which determines how well the atmosphere will disperse the smoke.

    Fire weather is seasonal.  The risk of wildfire is lowest during the winter when the ground is snow covered and highest during the early spring soon after the snow melts.  During early spring the landscape is covered with dead and dried grasses and weeds.  These dried materials can quickly become flammable under the spring sunshine soon after the snow is gone.  During late spring and summer the fire danger decreases a bit as the moist green growth of the new season emerges. Fire danger will increase again in the fall as weeds go to seed and die and the leaves fall and dry.  However, the autumn fire weather peek is not as intense as the springs since many plants in the fall still have some moisture and vitality up until the first snows.