Conditions Conducive to Accumulating Small Hail in Northwest California, its Impacts on Travel, and Methods of Communicating the Hazard (129)

Matthew Kidwell, National Weather Service, Eureka, CA

Jonathan Garner, National Weather Service



Cold core, upper-level troughs moving over the west coast bring significant accumulations of small hail to areas west of the Cascades. When small hail accumulates on roadways and then transitions to black ice it causes numerous accidents. This study has taken another look at an existing ingredients based forecast technique for the purpose of validation. The results of this validation study have been used to adjust the existing parameter values used in the technique.

These cold core upper-level troughs moving over the relatively warm marine environment force moderate to steep low level lapse rates which generate strong convective showers along the coast. This, combined with low freezing levels creates an environment that is favorable for significant amounts of small hail. The showers are generally widespread but are often discrete and short lived. The most dangerous conditions occur when nearly all of the showers contain small hail resulting in significant accumulation on roadways. Travelers can be caught off guard finding themselves suddenly driving on a sheet of ice at high speeds.

This is a challenging weather phenomenon to communicate to the public ahead of time and as the event unfolds. Weather stories, social media posts, and hazardous weather outlooks are all used in advance of the event. During the event it isn't practical to issue nowcasts and special weather statements for individual showers due to the widespread and short-lived nature of the showers. Real-time emails are sent to media partners for broadcast on commercial radio. This serves to alert drivers of the potential hazard when these showers are expected to persist over the next several hours. This is a unique hazard that is challenging to both effectively forecast and communicate to the public.