Jeff Milne-November 17

Investigating Updraft Helicity Compared to SPC Forecasts and Vertical Profiles of Updraft Helicity

As part of the severe weather forecasting process, forecasters often use updraft helicity as a proxy for storm rotation. Updraft helicity is calculated by integrating the product of vertical velocity and vertical vorticity between two layers in convective allowing models. Updraft helicity is commonly displayed as an hourly maximum field for deterministic or ensemble models and as an ensemble probability of updraft helicity exceeding a threshold during a specified time period. Two commonly used thresholds are 25J/kg and 100J/kg. To determine the utility of the 25J/kg and 100J/kg thresholds when forecasting for tornadoes, ensemble probabilities generated from several thresholds and the Storm Prediction Center tornado probability forecast were compared with a simulated “practically perfect” forecast generated from tornado reports. The resulting fractions skill score shows that 50J/kg, 75J/kg, and the SPC forecast all outperform both 25J/kg and 100J/kg. Additionally, the SPC forecast is most correlated with the 50J/kg ensemble probabilities, though the 50J/kg ensemble probabilities outperform the SPC forecast. The correlation suggests that, though forecasters do not look at 50J/kg ensemble probabilities when making a forecast, forecasters identify features that may also be highlighted in the 50J/kg ensemble probabilities. For most days, the SPC forecast was neither the best forecast nor the worst forecast. To further examine how updraft helicity is generated in the model, vertical profiles of the product of vertical velocity and vertical vorticity were created. These profiles show where, vertically, updraft helicity is generated by the model.