Explicit forecasts of low-level rotation from convection-allowing models for next-day tornado prediction

Three diagnostic fields were examined to assess their ability to act as surrogates for tornadoes in a convection-allowing ensemble system run during the spring of 2015. The diagnostics included midlevel (2–5 km AGL) updraft helicity (UH25), low-level (0–3 km AGL) updraft helicity (UH03), and low-level (1 km AGL) vertical relative vorticity (RVORT1). RVORT1 was used as a direct measure of low-level rotation strength. Each storm’s RVORT1 magnitude and near-storm environment properties were extracted from each hour’s forecasts using an object-based approach. The near-storm environments of storm objects with large magnitudes of RVORT1 were very similar to the environments identified as conducive for the development of tornadic supercells in previous proximity sounding-based studies (e.g., low lifted condensation levels and strong low-level shear). This motivated the use of RVORT1 as a direct surrogate for tornadoes, without the need to filter forecasts with environmental information. The relationship between UH25 and UH03 was also explored among the simulated storms; UH03 only exceeded UH25 in storms occurring within low-CAPE/high-shear environments, while UH03 rarely exceeded UH25 in traditional supercell environments. Next-day ensemble surrogate severe probability forecasts (E-SSPFs) for tornadoes were generated using these diagnostics for 92 forecasts, with thresholds based on the number of observed tornado reports. E-SSPFs for tornadoes using RVORT1 and UH03 were more skillful than E-SSPFs using UH25. The UH25 E-SSPFs possessed little skill, regardless of threshold or smoothing length scale. All E-SSPFs suffered from poor sharpness at skillful scales, with few forecast probabilities greater than 40%.