A detailed analysis of SPC “High Risk” outlooks, 2003–2009

For the NOAA/NWS/Storm Prediction Center (SPC), a “High Risk” (HR) represents the greatest level of forecast threat for high impact severe weather. On average, HR outlooks are only issued a few days out of the year and are only issued when confidence is high that widespread very damaging tornadoes and/or severe thunderstorms will occur.

This study focuses on SPC High Risk outlooks issued over the period 2003-2009. While HR outlooks can be issued for the potential of widespread damaging winds, we focus only on tornado-based risk areas. In addition to traditional verification (which uses the tornado reports from Storm Data), we look at additional aspects of forecast performance, including coverage of lightning, convective watches, and tornado warnings in the HR outlooks, regional and temporal trends in HR verification, and convective environments (in terms of CAPE, bulk shear, storm-relative helicity, etc.) associated with these high impact forecasts. 64% of HR outlooks issued during this period verified as HRs based on tornado report coverage, while 73% of HRs met the equivalent high risk threshold when considering tornado warning coverage. Greater instability and less convective inhibition in the convective environment appear to be a determining factor for the successful verification of a HR outlook, while vertical shear profiles are nearly always very favorable in HRs and are less of a discriminator in determining successful verification. HR outlooks issued for the Midwest were most likely to verify, as well as outlooks issued in late spring and early summer and outlooks that were upgraded to a HR earlier in the forecast cycle.