Essential Organizing and Evolving Atmospheric Mechanisms Affecting the East Bay Hills Fire in Oakland, CA (1991)

This study examined atmospheric mechanisms affecting the East Bay Hills Fire (1991) in Oakland, California, using the Advanced Weather Research and Forecasting numerical model (WRF) and North American Regional Reanalysis (NARR) dataset. High-resolution WRF simulations at 16km were downscaled to 4km and 1km to analyze primary and secondary circulations at synoptic and meso-α/meso-β scales before the fire. Findings indicate that a ridge over the Great Basin and a trough off the Pacific coast created a strong pressure gradient over northern California, resulting in favorable meso-α conditions for the hot, dry northeasterly winds, known as "Diablo winds," that initiated the wildfire. Additionally, mountain waves from the jet stream enhanced the sinking air on the Sierra Nevada's western side. The main conclusion is that jet circulations did not directly transport warm, dry air to the fire but established a vertical atmospheric structure conducive to wave amplification and breaking, and downward dry air fluxes, leading to the necessary warm and dry low-level air for the fire. The Hot-Dry-Windy (HDW) fire weather index also indicated how favorable the environment was for this tragic event.