Potential Vorticity Diagnosis of Mesoscale Convective Systems in the Warn-on-Forecast System

Mesoscale convective systems (MCSs) develop as individual thunderstorms merge and grow upscale into a contiguous precipitation area >100 km across. MCSs occur within the U.S. primarily during the warm season and can be associated with severe weather hazards that include tornadoes, damaging winds, hail, and flash floods. Increasing costs of severe and high-impact weather events in the U.S. necessitate improved forecast guidance of MCSs. The experimental Warn-on-Forecast System (WoFS) is a rapidly updating convection-allowing ensemble system used for the short-term (0–6 h) probabilistic prediction of severe weather hazards. Britt et al.’s (2023) systematic verification of WoFS forecasts for 62 MCSs that occurred in 2017–2021 showed that WoFS predicts MCSs particularly well when the system is present in the forecast initial conditions, but that the northern portion of the forecasted MCSs have a northwest position bias compared to observations that increases with lead time. They hypothesized that the position bias may hint that WoFS is misrepresenting the intensity of the northern line-end vortex and/or strength of the rear-inflow jet.

The aim of this presentation is to test the hypothesis that WoFS is producing cyclonic mesoscale vortices that are stronger than observed, and hence, contribute to the northwestward position bias at 3–6 h forecast leads. Potential vorticity (PV) diagnosis is used to examine the development of mesoscale convective vortices and line-end vortices in the stratiform region of observed and forecasted MCSs. One such example is shown in the attached figure for a WoFS ensemble member 6-h forecast of an MCS on 17–18 July 2020 that exhibited a northwest position bias. The 6-h forecast valid at 0600 UTC 18 July shows a well-developed, circular positive PV anomaly at 600 hPa, while the analysis valid at the same time shows a weaker PV anomaly. Also, the 6-h forecast has generally higher reflectivity values in the stratiform region compared to the analysis. This result suggests the possibility that the stronger PV anomaly in the forecast might be linked to increased diabatic heating in the stratiform region compared to the observed system. Analysis of PV in the stratiform region for WoFS forecasts for all 62 cases and a detailed PV budget analysis of select forecasts will be presented.