NWC REU 2012
May 21 - July 31



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Evaluating Likely Hail Impacts from SPC Day One Outlooks

Brittany Recker, Mark Leidner, and Dan Gombos


What is already known:

  • The Storm Prediction Center (SPC) issues probabilistic forecasts of areal hail coverage.
  • This study seeks to discover the relationship between these forecasts and what actually transpired.

What this study adds:

  • Radar data from 8 March – 8 July 2012 was used to evaluate corresponding SPC convective outlooks.
  • The area covered by radar hail probabilities was an order of magnitude smaller than the area covered by SPC outlooks.
  • 23% of radar hail probabilities fell inside SPC 15% outlook.
  • Using the link between what was forecast to what actually occurred established by radar data, users can be better prepared for the impacts of severe hail.


The purpose of this study is to interpret the Storm Prediction Center (SPC) convective outlooks for potential impacts. Radar data was used to provide the link between what has been forecast by SPC outlooks to what actually occurred. Several metrics, such as total area (km2) of SPC outlooks and radar-determined hail, and center of mass positions, were calculated in order to evaluate SPC outlooks. It was found that, over the sample size in this study (n = 108 case days), the percentage of radar hail probability area inside the SPC Slight Risk threshold is approximately 23%, while the percentage of SPC Slight Risk or greater covered by radar data is approximately 12%. A correlation coefficient of 0.595 was also found by looking at the relationship between SPC area-weighted probability and the area of radar data that overlaps the SCP Slight Risk areas. These values, along with the other metrics, will allow convective outlook forecast users to be better prepared for the impacts of severe hail.

Full Paper [PDF]