Using the Hot-Dry-Windy Index to Analyze the August 18th Wildfire Outbreak in Eastern Washington

Extreme fire weather conditions in Eastern Washington on August 18th 2023 allowed for the rapid growth of 3 major wildfires; the Gray, Oregon, and Winona Fires. 2 deaths, as well as the destruction of over 700 structures occurred as a result of these fires, prompting an emergency declaration by the Governor of Washington. This study examines the meteorological events that led to this extreme event and uses the Hot-Dry-Windy Index (HDWI) to find how the event compares to previous extreme fire danger events. The HDWI was calculated at 3 local Automated Surface Observing System (ASOS) Stations: Fairchild AFB (KSKA), Deer Park Airport (KDEW), and Spokane International Airport (KGEG), and then compared to the entire period of record for summer months (June through September). Our results found that the max reported HDWI on August 18th reached 532.18 at KSKA, 496.72 at KDEW, and 488.58 at KGEG. These values were well above the 99th percentile for all daily observations, with KSKA and KDEW reaching into the 99.9th percentile. These results emphasize the unprecedented nature of this event and offer insight into the usefulness of the Hot-Dry-Windy Index in operational fire weather forecasting.