An Assessment of Environmental Parameters Supportive of Large Hail across the Great Basin

Historically, reliably forecasting hail size has been a significant challenge for meteorologists within the warning environment. Numerous studies and techniques have been developed to improve both the detection and forecasting of large hail, defined as 1 inch in diameter or larger. These new methodologies coupled with improvements in radar data including better volumetric scan strategies, the addition of dual-polarized data, and continued development of radar based hail estimation algorithms have improved operational skill in determining hail size. In recent years, the use of a 50 dBz threshold, known as the Donavon Technique, has shown considerable skill in determining large hail potential in a given thunderstorm, though this technique was originally developed for moist environments more typical to the central and eastern United States.

Across the Intermountain Region, identifying the presence of large hail remains problematic for multiple reasons. Radar coverage across the West exhibits large gaps in coverage, areas of significant beam blockage, and inherent limitations with mountain top radars.  These factors all limit the quality of low level reflectivity data in many locations.  Additionally, given the semi-arid climate of the Interior West, the boundary layer is frequently much drier and deeper than what is observed across the central and eastern United States.  This dry, deep mixed boundary layer tends to reduce melting of hailstones within the sub cloud layer, resulting in large hail reaching the surface at lower thresholds than what the Donavon Technique would suggest. Previously, a study of large hail reported between 1996-2006 across the Great Basin based on the Donavon Technique was performed.  The results of this study adjusted the threshold for large hail across the Great Basin to lover values than previous applications of the Donavon Technique. However, operational experience since the completion of the Great Basin study has also shown values above the established thresholds do not always produce large hail, but often copious amounds of smaller hail. This may imply that pulse type thunderstorms across the Interior West may be too weak to produce large hail given marginal instability values. This study both expands the original Great Basin study to include more recent cases, as well as examines mesoscale parameters such as CAPE and deep layer shear using RUC-derived proximity soundings.