Israel L. Jirak, Steven J. Weiss, and Christopher J. Melick Storm Prediction Center , Norman, OK

1. Evaluation of the Storm-Scale Ensemble of Opportunity (SSEO) During the 2011 Hazardous Weather Testbed Spring Experiment Israel L. Jirak, Steven J. Weiss, and Christopher J. Melick Storm Prediction Center, Norman, OK • SE2011 Results • The fractions skill score (FSS) was calculated for the smoothed binary neighborhood probability (ROI=40 km; σ=40 km) of updraft helicity ≥ 25 m2s-2 for the SSEO versus practically perfect hindcasts of severe weather reports (ROI=40 km; σ=120 km) during SE2011. • The FSS was calculated similarly for the OU CAPS 24-, 15-, and 5-member storm-scale ensemble forecast (SSEF) system during SE2011. Examples • Statistics • The FSS is used here as a neighborhood approach in verifying high-resolution models to provide an objective measure of the agreement between probabilistic forecasts and observations. • Although the distributions of FSS for individual 3-h forecasts were similar among the ensembles, the SSEO had the highest cumulative (i.e., weighted) FSS during SE2011. • The number of members included in the SSEF did not seem to have a strong impact on the statistical results for updraft helicity during SE2011 when verified against severe weather reports. • Introduction • The Storm Prediction Center (SPC) has developed a 7-member experimental storm-scale ensemble of opportunity (SSEO). • The SSEO: • consists of deterministic storm-scale models already available operationally to SPC. • provides a practical alternative to a formal storm-scale ensemble, given limited computing resources in NOAA. • contains hourly maximum storm-attribute fields, such as simulated reflectivity, updraft helicity, updraft speed, and 10-m wind speed. • The SSEO was examined during the 2011 HWT Spring Experiment (SE2011) to explore its utility in severe weather forecasting. • SSEO Membership • SSEO Characteristics • NSSL WRF-ARW tends to produce values on the upper end for hourly max fields while the NMMB Nest tends to fall at the lower end. • The ARW models generally produce stronger updrafts, yet the NMM models generate larger updraft helicity on average. • Recent updraft fix implemented into NMMB Nest not included in these statistics. • 2011 Tornado Events • 16 April 2011 (18-21Z) • 27 April 2011 (18-00Z) • 22 May 2011 (21-00Z) – Joplin, MO • Summary • The SSEO generated by SPC is a practical approach to storm-scale ensembles and is currently available operationally to SPC forecasters. • The NSSL WRF-ARW falls at the upper end of the SSEO distribution in terms of magnitude and spatial coverage of the hourly maximum storm-attribute fields while the new NMMB CONUS Nest falls at the lower end. • Subjective and objective evaluations (e.g., FSS) from SE2011 indicate the SSEO often performed as well as or better than the larger-member SSEF in forecasting the occurrence of severe weather.  • The SSEO performed reasonably well in highlighting the threat areas in many of the historic tornado events from this year. Acknowledgments. We would like to thank Ming Xue, Fanyou Kong, and Kevin Thomas from OU CAPS for generating and providing SSEF data and Jack Kain and Patrick Marsh of NSSL for making the data available to SPC. SSEO SSEO 30-h Forecasts of Smoothed Neighborhood 3-hr Probabilities for UH ≥ 25 m2s2 Valid at 06Z on 02 June 2011 (shaded) With Verifying Practically Perfect Hindcast (contoured) and Reports 3-hr spaghetti plot of UH ≥25 m2s2 3-hr nbrhd prob of UH ≥25 m2s2 SSEO SSEF - 24 member SSEO SSEO FSS = 0.84 FSS = 0.68 SSEF - 5 member SSEF - 15 member 6-hr ensemble max of UH 6-hr nbrhd prob of UH ≥25 m2s2 SSEO SSEO FSS = 0.70 FSS = 0.76 SE2011 Domain Maximum Values (18Z – 06Z) Cumulative FSS FSS Distribution of 3-h Forecasts 3-hr spaghetti plot of UH ≥25 m2s2 3-hr nbrhd prob of UH ≥50 m2s2 1-km AGL Reflectivity 10-m Wind Speed Updraft Helicity Updraft Speed * *