Evaluation of an Alberta Hail Growth Model Using Severe Hail Proximity Soundings in the United States

A one dimensional hail growth model called “Hailcast” is tested as an operational forecast tool. The model uses observed or forecast soundings as input. Hailcast employs an ensemble method to produces 25 different updraft members,each using a slightly different temperature and dewpoint for the lifted parcel. This method accounts for some of the variability present and uncertainty in choosing a representative parcel temperature and dewpoint.

Hailcast was originally developed to forecast hail in South Africa and Alberta, where hail diameters larger than 2” are rare. In this study, a database of 382 severe (>= 0.75” diameter) hail proximity soundings within the contiguous U.S. was collected from 1997 to 2002. Unlike the Alberta database, this database includes 203 significant (>= 2” diameter) cases. Assuming severe hail of any size is likely, the model shows relatively high skill at delineating between low-end severe (< 2”) and significant (>= 2”) hail.

The correlation between observed and forecast maximum hail diameter ranges from 0.65 to 0.74 depending on how reports are filtered. These numbers are in general agreement with past evaluations by Brimelow. Dividing the database into < 2" (NON-SIG) and > 2" (SIG) cases, the model is able to achieve a very high detection rate near 0.89 while falsely detecting SIG hail only 23% of the time. It is noteworthy that the model produces accurate forecasts despite large variability in thermodynamic environments in the U.S. large CAPE ranges), unlike those commonly observed in Alberta.