Abstract
Severe-convective hailstorms are one of the most frequent weather hazards across the United States. However, studies evaluating the ability of various environmental indices to differentiate lower-end severe hail (≤1.25 in, 32 mm) from significant hail (≥2.0 in, 51 mm) prior to storm formation are limited and typically overlap very little with microphysically based research. To bridge this gap, this study builds a database of 520 hail reports that sort into one of four hail-diameter ranges. For each report, various thermodynamic and wind-related fields are then extracted from Rapid Update Cycle (RUC) model analysis to create a parameter-based hail climatology.
 Analysis of these environmental indices indicates most wind-based parameters display weaker magnitude winds and resultant shear for the smallest hail-size bin compared to the three largest. Further, the three largest hail diameter bins reveal nearly identical parameter values in the lowest 6 km AGL. In contrast, non-traditional shear layers that include winds in the upper portions of a storm (>6 km AGL) display some skill to differentiate larger hail sizes, especially for ≥3.5-in (89-mm) hail. Thermodynamic variables produced mixed results, with variables such as CAPE displaying a slight tendency to increase as binned hail size becomes larger but still with significant overlap. On the other hand, non-traditional parameters such as the hail-growth-zone thickness reveal a relationship toward decreased depth as the binned hail size increases, but with little to no increase in hail-growth-zone CAPE. Finally, evaluation of the significant severe parameter (SSP) and a new index called the large hail parameter (LHP) display mixed results. Skill at delineating ≤1.25-in (32-mm) report from 2.0-3.25-in (51-83-mm) cases for LHP (SSP) is slightly better (worse) than 0-6-km AGL bulk vector shear. However, the LHP displays improved skill over any other parameter to differentiate ≥3.5-in (89 mm) reports from those with less than 2.0-in (51-mm) diameter hail. The LHP formula creates improved skill by including non-traditional environmental parameters typically associated with storm longevity, precipitation efficiency, and hail-growth rates.
Highlights
The United States has one of the highest frequencies of severe-convective weather in the world with large hail accounting for a sizable percentage of this climatological normal (Frisby and Sansom 1967; Laing and Fritsch 1997; Doswell and Bosart 2001; Cintineo et al 2012)
Throughout the operational lifespan of the Rapid Update Cycle (RUC), the model underwent a wealth of changes to improve performance and capability
CAPE is a measurement routinely used by operational forecasters to estimate environmental thermodynamic instability, where larger values correlate with the potential for greater updraft velocity
Summary
The United States has one of the highest frequencies of severe-convective weather in the world with large hail accounting for a sizable percentage of this climatological normal (Frisby and Sansom 1967; Laing and Fritsch 1997; Doswell and Bosart 2001; Cintineo et al 2012). States due to hail damage exceed $1.0 billion (Changnon 1972, 1999). A single event with extremely large hail (diameters ≥3.5 in or 89 mm) has the potential to surpass that. A commonly referenced example of this type of case is the 5 May 1995 Fort Worth, TX hailstorm in that people attending the “Mayfest” event were caught outdoors in a storm producing hail with diameters exceeding 4.0 in (102 mm). Numerous people were treated for injuries, some critical, and storm damage exceeded $2.0 billion (Edwards and Thompson 1998). Despite the impact of these extremely large hail events, the definition of what constitutes
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