Verification of Quasi-Linear Convective Systems Predicted by the Warn-on-Forecast System (WoFS)

Quasi-linear convective systems (QLCSs) can produce multiple hazards (i.e., straight-line winds, flash-flooding, and mesovortex tornadoes) that pose a significant threat to life and property, and are often difficult to accurately forecast. The NSSL Warn-on-Forecast System (WoFS) is a convective-allowing ensemble system developed to provide short-term, probabilistic forecasting guidance for severe convective events. However, examination of the WoFS’s capability to predict QLCSs has yet to be systematically assessed across a large number of cases for 0--6-hr forecast times. The quality of WoFS QLCS forecasts for 62 QLCS days occurring between 2017--2021 was assessed using object-based verification techniques. First, a storm mode identification and classification algorithm was developed and tuned to identify high-reflectivity, linear convective structures. The algorithm was then used to identify convective line objects in WoFS forecasts and MRMS gridded observations. WoFS QLCS objects were matched with MRMS observed objects to generate bulk verification statistics. Results suggest WoFS's QLCS forecasts are skillful with the 3- and 6-hr forecasts having similar POD (FAR) values near 0.61 (0.31). Additionally, a novel centerline analysis was performed to evaluate orientation, length, and tortuosity (a measure of curvature) differences, and spatial displacements between observed and predicted convective lines. While no systematic propagation biases were found, WoFS centerlines were typically longer, more tortuous, and had larger orientation angles compared to MRMS centerlines. This combination of differences is consistent with the overprediction of the QLCS’s northern bookend vortex and intensity of the rear-inflow jet.