Forecasting Quasi-Linear Convective System Tornado Potential Using the Warn-on-Forecast System (WoFS)

Abstract Accurately forecasting mesovortex tornado potential within quasi-linear convective systems (QLCSs) remains a major challenge. One technique used to support QLCS tornado warning operations is the three ingredients method (3IM), which is based on system balance modulated by line-normal vertical wind shear. However, observational studies examining prolific mesovortex- and tornado-producing QLCSs have identified a substantial line-parallel component of the low-level environmental shear field, which is unaccounted for in the 3IM. Recent numerical studies suggest that line-parallel shear promotes the development of strong, cyclonic mesovortices, thereby increasing the overall likelihood of QLCS tornadoes. Motivated by these recent findings, this proof-of-concept study represents the first attempt to quantify the physical factors comprising the 3IM using a prototype composite forecast parameter QTor. QTor integrates near-storm environmental vertical wind shear components with leading-line geometry to generate system-relative probabilistic forecasts for mesovortex tornado potential. QTor is evaluated using three tornadic QLCS cases and one null case within the Warn-on-Forecast System (WoFS), leveraging object-based methods to identify QLCS structures and isolate their leading lines. Results indicate that QTor forecasts show promise in effectively identifying areas conducive to mesovortex tornado development, providing qualitatively accurate probabilistic forecasts of tornadic portions of the QLCSs with up to 3 h of lead time. Significance Statement Squall-line tornadoes are difficult to forecast due to their small size and transient nature, leading to lower predictability and shorter warning lead times compared to supercell tornadoes. This study introduces QTor, a new forecasting parameter designed to predict tornado potential within squall lines. Results indicate that QTor may effectively identify areas of increased tornado likelihood, predicting tornadic portions of a squall line over 3 h in advance. While still in early development, QTor shows great promise and has the potential to enhance squall-line tornado prediction in the future.