Diagnosing Synoptic and Local Scale Processes Leading to the 8 August 2023 Downslope Wind-Driven West Maui Fires

Presented on February 11, 2026, by

Yi-Leng Chen
Professor
Atmospheric Sciences
University of Hawaii

ABSTRACT:

The synoptic and island-scale conditions driving the West Maui downslope wind–driven fires under moderate drought were examined using observations and real-time high-resolution model forecasts. A rare, slowly westward-moving blocking pattern was present. After 1200 UTC 7 August 2023, an expanding eastern high-pressure cell lowered the trade wind inversion below 850 hPa. Subsidence from the anomalous North Pacific Subtropical High (NPSH) produced very warm and dry air aloft (RH < 30%) with low TPW (< 25 mm).

As the NPSH shifted westward, the inversion recovered above the West Maui Mountain tops. By 0000 UTC 9 August (1400 HST 8 August), anomalously strong surface pressure (~1034 hPa) northwest of Hawaiʻi generated rare strong easterly to northeasterly trades (>13 m s⁻¹). With a strong cross-barrier flow (>20 m s⁻¹) above the mountain top, and a critical level aloft, severe downslope windstorms, mountain waves, and a hydraulic jump on the lee side of the West Maui Mountains were produced. Consequently, the lee side coastal urban areas had increased fire risks in the early afternoon due to warm, dry air associated with the downslope wind and the turbulent mixing in the lowest levels related to the diurnal heating cycle under clear skies.