Wildland Fire Weather Associated with the 14-15 August 2015 Large Fire Growth Events in the Pacific Northwest (USA)

The accurately forecast epic 2015 Pacific Northwest (PNW) Wildland Fire season was the worst on record, burning more than 1.8 million acres and hundreds of structures. These wildfires resulted from a significant number of small lightning-ignited wildfires as early as late-June and continued throughout mid-August, with these wildfires being the most troublesome.

The intent of this presentation is to analyze fire weather conditions between 14-15 August 2015, and the common climatological and atmospheric weather phenomena that contributed to the aggressive and extreme fire behavior observed.

Sea surface temperature (SST) anomalies in the Nino 3.4 region of the central Pacific Ocean were slightly above normal during the 2015 winter which indicated a weak El Nino, record low Winter and Spring snowpack and SWE values, anomalously low soil moisture values, abnormally high regional drought indices, as well as low soil and fuel. Also considered are the causal factors and indicators of dynamic dry intrusions and dry slots; the Red Flag conditions for abundant lightning that ignited most of the fires, as well as dry cold fronts, thermal troughs, subtle and dynamic subsidence inversions and their associated thermal belts, low-level jets, and mesoscale and microscale downslope winds.

Each of the large wildland fires and/or complexes examined, also shared several common climatological and meteorological conditions during the examined period. There were strong associations between falling 500mb heights and large fire growth, consistent with a ‘Breakdown of the Upper Ridge,' a recognized Critical Fire Weather pattern among operational wildland fire weather meteorologists. Most of the fires and complexes, examined in more detail, revealed that there were wide-ranging mesoscale, synoptic, as well as microscale regions of relatively warm to hot daytime and nighttime temperatures throughout the PNW, with the nighttime temperatures being the most indicative of aggressive to explosive fire behavior and large fire growth. Consistent with the high temperatures were anomalously low relative humidity and dew point values and high dew point depressions, which cumulatively contributed to the large fire growth.