Real-Time Fire Weather Forecasting for the Maui Wildfire on August 8

On August 8, 2023, strong winds damaged power infrastructure that might cause fire ignition of the extensive dry grasslands upstream of the Lahaina. Driven by strong easterly winds, wildfire growth and spread rapidly and then hit the West Maui town of Lahaina. The disaster wildfire resulted in a significant loss of life, extensive economic damage, and a need for improved prediction and preparedness strategies.

While the mechanisms behind the Lahaina wildfire are complex, several critical factors contributed to the fire-conducive weather conditions. On August 6, a cut-off low, originating from a blocking pattern, is positioned to the north of Hawaii. This cut-off low is located between two high-pressure cells, aligned in an east-west orientation. By August 7, the western high-pressure cell moved eastward following the passage of cut-off low. This high-pressure cell then merged with the semipermanent subtropical high, producing an unusual center pressure of approximately 1040 hPa at surface. Anticyclonic circulations that centered at north of Hawaii produced strong ENE trades on their south/southeastern flank. Concurrently, Hurricane Dora presence south of Hawaii, implying strong winds driven by pressure gradient force. Soundings at Lihue show strong trade wind inversion around 1.5-km and a critical level at 500-400 hPa. These large-scale conditions are favorable settings for windy events across the state of Hawaii. Unlike the more frequently occurring high wind events of the winter months, this incident occurred in August, offering a distinct scenario characterized by the potential for fire outbreaks due to the dry season and elevated temperatures. This risk is further compounded when accompanied by below-average annual rainfall.

Given the nature of this event, the localized characteristics of the high wind across the region and its relative predictability are analyzed using an operational real-time forecast. This forecast, generated twice daily and provided to the National Weather Service (NWS) and residents. A downslope windstorm is forecasted on the lee sides of the west Maui Mountain which its top below the trade wind inversion. The windstorm demonstrates features of a hydraulic jump and a wave trapped below the trade wind inversion. A zonal low-level jet that reached 35 m s^-1 is evident over the leeside slope. The top of the trade wind inversion layer is lifted on the windward side and reaches the highest level before reaching the crest. A significant downward tilt is simulated above the leeside slope, accompanied by a decrease in the depth of the trade wind layer. The importance of high-resolution forecasts over complex terrain will be discussed through sensitivity tests. Operational, high-resolution prediction systems are valuable to provide realistic forecasts of winds, especially over the leeside of terrain impacted by downslope windstorms. Based on realistic high-wind forecasts, strategically shutoff power in vulnerable areas could reduce the potential risk of electrical fires.