Case Study of a Dry Intrusion (Dry Eye) Event on the Pioneer Fire on the Boise National Forest (ID - USA) on August 12, 2016

Dry intrusions and circulations (dry eyes) are mesoscale weather phenomena visible in satellite water vapor imagery that are often associated with some degree of atmospheric instability and subsidence. Subsidence in the atmosphere is defined as an extensive sinking motion of air. As the air sinks from the free atmosphere downslope along terrain, it is compressed and subject to adiabatic warming, which in turn heats up and severely dries out the air mass and it can and also create locally strong, gusty winds. On a scale of a few hundred miles or less, this process can be potentially threatening within the wildland fire environment as it often exacerbates the dry conditions in place at the surface and accelerates the drying out of smaller size class fuels, possibly leading to blow-up conditions and extreme fire behavior.

A National Weather Service Incident Meteorologist (IMET) assigned to the Pioneer Fire identified a dry eye feature in water vapor satellite imagery on August 12, 2016 and captured an image loop of the phenomenon. The images showed the circulation and path of the intrusion over a 9-hour span of time that began along the Washington/Oregon border earlier in the day and then moved directly over the Pioneer Fire in the Boise National Forest of Idaho that evening. The passage occurred well outside the expected burning period and resulted in dramatic and explosive fire growth on a rather nonthreatening division of the fire, forcing wildland firefighting resources to disengage firelines to safety and tactically suppress spot fires well into the night. InciWeb photos revealed dynamic and intensifying plume dominated fire behavior and smoke columns proximate to the time the dry eye feature passed over the area of the fire.

This study focuses on identifying and forecasting the dry eye phenomenon, and further illustrates how the impacts of even a weak upper level circulation can translate to aggressive, extreme and unexpected fire behavior at the surface. The dry eye acts more or less as a jet streak that induces greater instability in the short term and enhances lift within the convective mixed layer, but it is also a precursor to the strong subsidence and dry air poised to mix down to the surface. The water vapor imagery shows how the dry eye darkened as it passed over the fire area, suggesting that the air mass was considerably dry based on the brightness temperature data values displayed in the imagery. Archived upper air observation maps encompassing the time frame of the dry eye formation and movement clearly reveal a weak upper level disturbance, or wave in the flow pattern that was in place over the region. However, analysis of course resolution graphical weather model output does not clearly capture the evolution of the dry eye feature, or the expected short term changes to atmospheric stability.

It is well recognized that upper atmospheric conditions are important factors in producing more accurate fire weather and fire behavior predictions, particularly for periods of extreme and erratic fire behavior. The atmospheric conditions are, in many cases, predictable hours or even days in advance of the event. A primary goal from this case study and related events is to develop an operational approach between NWS meteorologists and wildland fire managers to identify dry eye circulations and intrusions in water vapor imagery and then implement communication strategies into the operational fire weather and fire behavior forecast environment that will aid the wildland firefighting community to better target when and where the potential exists for extreme fire behavior. Dry intrusions and dry eye circulations should be communicated by NWS meteorologists in fire weather forecasts and briefings, and incorporated into future National Wildfire Coordinating Group (NWCG) courses (i.e., S290 Intermediate Fire Behavior) – all as part of the vital partnership between NOAA and the wildland firefighting community to improve firefighter safety, awareness, and response.