The SPC Storm-Scale Ensemble of Opportunity: Overview and Results from the 2012 Hazardous Weather Testbed Spring Forecasting Experiment

Given that NOAA may not have the computing resources to generate a formal storm-scale ensemble system in the near future, the Storm Prediction Center (SPC) has developed the storm-scale ensemble of opportunity (SSEO) as a practical alternative. The SSEO is comprised of seven deterministic convection-allowing model runs already available to the SPC. The data are processed as an ensemble to generate ensemble fields such as mean, maximum, and exceedance probabilities. A few select storm-attribute variables are included in the SSEO for severe weather forecasting, including hourly maximum fields (HMFs) of simulated reflectivity, updraft helicity, updraft speed, and 10-m wind speed. In addition, other fields (e.g., precipitation and 2-m temperature) are processed in the SSEO to assist with the fire weather and winter weather forecasting responsibilities of the SPC.

Initial development of the SSEO occurred in 2011 when it was tested for severe weather applications during the 2011 Spring Forecasting Experiment (SFE) held at the NOAA Hazardous Weather Testbed. More extensive evaluations including subjective and objective verification were conducted during the 2012 SFE. The fractions skill score (FSS) was calculated for the SSEO updraft helicity neighborhood probabilities relative to observations of severe weather reports. Similarly, the FSS was computed for the SSEO simulated reflectivity neighborhood probabilities as compared to observed radar reflectivity. The skill scores of the SSEO were compared to those from the University of Oklahoma Center for Analysis and Prediction of Storms (CAPS) storm-scale ensemble forecast (SSEF) system and the Air Force Weather Agency (AFWA) storm-scale ensemble to gain a relative sense of the performance of the SSEO. Subjective and objective results during the 2012 SFE indicate that the SSEO provides useful information for severe weather forecasting and compares favorably to formal storm-scale ensemble systems.