Northeast Winds in Northern California: Identifying Subtypes of Diablo Winds

Diablo winds in Northern California are known for being the driving force behind some of the most deadly wildfires in California history. The Camp Fire of 2018, which had at least 85 civilian casualties is just one of many destructive fires that was driven by these rapidly developing downslope windstorms. These wind events historically occur from fall through spring, overlapping with the end of wildfire season in the fall. Forecasters and decision-makers rely on understanding intricacies of these events, but much about the vertical structure and evolution of Diablo wind events is still unknown. Previous academic studies have assumed that each Diablo Wind event is mechanistically similar, and that the evolution and structure of these events are indistinguishable aside from event-to-event variation. Here we show that two structurally different types of Diablo Winds appear in an analysis of 34 years of 2-km resolution WRF reanalysis data. These subtypes can be described as pressure-gradient driven and upper-level driven winds, and were previously theorized in a 1976 NOAA technical memorandum (Ruscha 1976). We identify subtypes via cross sections, 500mb geopotential height maps, and surface wind maps. Early results suggest that the pressure gradient driven subtype of winds (Type I) show the highest wind speeds closest to the surface, a significant nocturnal uptick in downslope winds, and frequently show extreme but often more geographically localized impacts. Conversely, upper-level driven Diablo Winds (Type II) show elevated wind speeds aloft, higher winds during daytime hours associated with increased mixing, and more geographically widespread impacts than Type I events. These findings represent a significant advancement in the study of Diablo Winds: where these events are viewed more dynamically, potentially leading to more accurate forecasting of impacts in future years.