Exploring the Utility of New Diagnostics for Low-Level Rotation in High-Resolution Rapid Refresh version 3 (HRRRv3) Forecasts

Motivated by a desire to bolster the capabilities of the High Resolution Ensemble Forecast system, output of diagnostics relevant to severe convective storms is being unified across convection-allowing models (CAMs) run at NCEP. As part of this effort, new parameters were included in version 3 of the High-Resolution Rapid Refresh (HRRRv3) model, which was operationally implemented in July 2018 and was run experimentally at NCEP/Environmental Modeling Center (EMC) beginning in late 2017. The new HRRRv3 diagnostics of particular interest to this work are maximum 0–3-km updraft helicity (UH), maximum 0–2-km UH, maximum 0–2-km vertical vorticity, and maximum 0–1-km vertical vorticity. While the traditional midlevel (maximum 2–5-km) UH has proven useful in diagnosing the potential for severe convection in CAM forecasts, low-level UH and low-level vertical vorticity may have utility in providing additional guidance signaling the potential for tornado formation in both classic severe weather environments and high-shear/low-CAPE environments. Maximum 0–3-km UH is currently diagnosed in the High-Resolution Window forecasts, and its addition to HRRRv3 will allow for comparison with other low-level UH and low-level vertical vorticity diagnostics. The utility of these new diagnostics will be explored for several severe weather events reviewed by EMC’s Model Evaluation Group in spring 2018. These events include tornadoes that formed in both high-shear/low-CAPE environments in the southeastern United States and in more classic tornadic environments in the central Great Plains. Subjective evaluation results will be shared, and statistics on each parameter’s climatology will be discussed if time allows.