Analysis of High-Resolution Model Output for the Yakima Smoke Intrusion Case Study, 28 September 2009

On 28 September 2009 the Naches Ranger District on the Okanogan-Wenatchee National Forest received approval from the Washington Department of Natural Resources (DNR) to ignite an 800-hectare prescribed underburn on Bethel Ridge. Later that afternoon, elevated PM2.5 concentrations and visible smoke were reported in Yakima, WA, about 40 kilometers west of the burn unit. The weather forecast for the day indicated winds would carry the smoke north of Yakima, with good dispersion conditions. We undertook a case study of this event to determine whether the conditions leading to the intrusion of the smoke plume into Yakima could have been predicted, either from forecasts and model output available in real time, and/or from higher resolution model output made available only after the event. We obtained 4-km Weather Research and Forecasting (WRF) model hourly output (available to forecasters on the day of the burn) and at 10-minute intervals, and 4/3-km WRF output every 10 minutes. We found that output from the two model resolutions was similar, with the exception of the planetary boundary layer height over Yakima. We also used the high-resolution WRF output to generate smoke dispersion predictions using the BlueSky Smoke Modeling Framework. The results showed that neither the model output available in real-time, nor the higher-resolution model output generated afterward, would have predicted the wind shift and collapse of the smoke plume that resulted in the smoke intrusion.