Cool Season Small Hail over the West Coast of the United States: Environments, Hazards, and Decision Support

Abstract

National Weather Service (NWS) forecasters across the west coast of the United States often deal with cool season hail showers that produce hazardous driving conditions. These small hail events, with diameters averaging 5.8 mm, occur with cold upper troughs that support weak instability favorable for low-topped convection and reflectivity values averaging 48 dBZ. The public generally assumes that heavy snow is common across west coast mountains, while heavy rain prevails near sea level. However, motorists can be caught offguard when wet, relatively warm low elevation roadways suddenly transition to icy hail-covered conditions. Thus, west coast small hail events represent an opportunity for the NWS to provide tailored messaging that can modify public perceptions and optimize outcomes. This research examines environments supportive of accumulating small hail over the western United States during the period 2008–2018, and supplements the environmental analysis with a summary of enhanced impact-based decision support techniques used to alert NWS partners and the general public.