Abstract
Abstract
The 7–8 January 2025 Santa Ana event was unusual in that strong surface winds extended beyond typical gap and pass regions into the San Gabriel Valley (SGV). In contrast to “canonical” Santa Ana conditions—where the San Gabriel Mountains largely block the southward progression of Mojave Desert air—this event featured flow that overtopped the range and generated a downslope windstorm, contributing to the rapid spread of the Eaton Fire into urban areas. Santa Ana days affecting the northern Los Angeles basin are identified using station observations, showing that only about 5% produce downslope winds in the SGV exceeding 10 kt (5.1 m s
−1
), underscoring the rarity of such events. Composite and case analyses indicate that these SGV windstorms are not associated with the canonical coastal ridge/inland trough pattern, but instead occur when a positively tilted upper-level trough is positioned near Southern California, promoting stronger cross-barrier flow and reduced static stability near crest level. Idealized simulations demonstrate that the wind component perpendicular to the range is the primary control on lee-side wind strength, with additional amplification possible in the presence of a crest-level inversion. Application of these diagnostics to a longer record identifies several historical SGV windstorms, some exceeding the January 2025 event in intensity and associated with major wildfires. These results define a distinct, high-impact Santa Ana subtype and provide a physically-based framework for anticipating rare but hazardous downslope wind events in the SGV.