High-Resolution Rapid Refresh Model-Based Verification of Snow Squall Prediction in the High Plains and Mountain West

The National Weather Service has started issuing warnings for a new type of hazardous weather, snow squalls. Snow squalls are intense, but limited duration, periods of moderate to heavy snowfall typically accompanied by high surface winds resulting in reduced visibility and dangerous whiteout road conditions. Due to the shallow convective nature of snow squalls, relatively sparse radar network, beam blockage by terrain, and sparse surface observation network, model guidance will be a key supplement to observations in issuing snow squall warnings. Because this type of alert is new, the ability of operational high-resolution models to accurately capture snow squalls needs to be assessed.

A previous study revealed that the I-80 corridor from Salt Lake City, Utah to Cheyenne, Wyoming and many Mountain West ranges, including the Absaroka Range, Medicine Bow Mountains, Uinta Mountains, Wasatch Range, Wind River Range, etc., frequently exhibit elevated occurrences of the Snow Squall Parameter (SNSQ, Banacos et al., 2014). In this study, we analyze four cool seasons (Sept-May, 2016-2017 through 2019-2020) of surface and/or radar-observed snow squalls in these regions. Given the relatively small scale of snow squalls, i.e., not captured by global and even some regional numerical weather prediction models, we measure the forecast skill of the High-Resolution Rapid Refresh (HRRR) model. The HRRR-derived SNSQ, as well as the model's ability to co-locate the component hazards of snow squalls (low visibility, high wind gusts, snowfall, and abrupt surface freezing), is tested. A technique is developed for a more representative characterization of the environment in which snow squalls form, capturing not just shallow surface-based convection (as captured by the Banacos et al. SNSQ parameter), but also squalls driven by potential or symmetric instability and/or frontogenesis. Such squalls typically appear banded, aligned with the jet stream, and the instability typically is not surface-based.