Climatology and Case Studies of Northeast Severe Convection with Low-Predictive Skill

This study objectively identifies severe weather events with low-predictive skill from 1980 to 2013 over the northeast U.S. Storm Prediction Center (SPC) 0600 UTC Day 1 slight risk convective outlooks and valid storm reports are used to evaluate the predictability of severe weather events during the study period. The slight risk areas, projected on a 40 x 40 km grid over the northeast U.S., and valid storm reports of hail, wind, and tornados aggregated over each 24-hour forecast period are used to calculate probability of detection (POD), false alarm ratio (FAR), and critical success index (CSI) skill scores. Events scoring in the lowest 25th percentile of POD and FAR are separated into two categories to construct a climatology consisting of high-impact severe weather cases with low POD (type one) and low-impact cases with high FARs (type two). Type one events have no discernable trend through the 33-year period, whereas type two events decline over recent years with 1 event occurring from 2006 to 2013. Type one events are subdivided according to Convective Available Potential Energy (CAPE) and 1000-500 hPa vertical wind shear and compared to cases with high-predictive skill, defined as having a CSI in the highest 25th percentile of high-impact cases. We find that type one events occur more often with 1000-500 hPa environmental shear magnitudes below 16 ms-1 (low shear) than cases with high predictive skill. Furthermore, high-impact events under low shear conditions have a median CSI of 0.137, while high-impact events under higher shear conditions have a median CSI of 0.186. Case studies of type one events, segregated by convective mode and severe threat type, are conducted to further analyze mesoscale features such as complex terrain and equivalent potential temperature gradients that impact predictability. In weak synoptically forced environments, surface boundaries due to differential heating and channeled flow within complex terrain, as well as sea breeze boundaries near the Great Lakes, serve as focus areas for convective initiation. Strong synoptically forced environments typically concentrate sharp surface boundaries where synoptic forcing is maximized. In both scenarios, preliminary results indicate type one wind storms propagate into environments with higher boundary layer heights after convective initiation. The dry air aloft and elevated downdraft CAPE values above 1000 J/kg likely contribute to enhanced downbursts, cold pool production, and surface wind speeds, especially in environments with low vertical wind shear. Additionally, type one hail cases exhibited localized midlevel cooling, contributing to severe hail formation in environments with otherwise marginal diurnal heating and destabilization.