Warm-Conveyor Belt Persistence and Upstream Cyclone Tilt Shape the Predictability of a Rossby Wave Breaking Event

Misrepresentation of warm-conveyor belts (WCBs) in weather forecasting models can impinge on the medium-range predictability of Rossby waves. Yet, the link between WCB errors and Rossby wave evolution remains poorly understood, despite the importance of Rossby wave breaking (RWB) for weather extremes. We investigate this link using a WCB case from 28 April to 4 May 2020, which was associated with anticyclonic RWB over the North Atlantic and substantial ensemble variability in the forecasted Rossby wave evolution. Our analysis uses archived forecasts from the Global Ensemble Forecast System (GFS) and the European Centre for Medium-Range Weather Forecasts (ECMWF), as well as global simulations conducted using the Model for Prediction Across Scales with grid spacing from 60 to 3.75 km. Simulations with more persistent WCBs capture Rossby wave amplification and subsequent anticyclonic wave breaking more accurately, while shorter-lived WCBs yield weaker amplification and cyclonic breaking. A systematic underestimation of the poleward extent of the 500 hPa meridional moisture flux within WCB ascent in GFS and ECMWF forecasts appears to be a key process limiting predictability. Accurate prediction of RWB depends not only on WCB representation but also on the upstream cyclone's propagation and structure, mesoscale dynamics within the WCB, and upstream convection over North America. This case shows that forecasting RWB requires capturing the collective influence of multi-scale processes interacting with the broader Rossby wave packet. Forecast outcomes are strongly shaped by model choice, but initial conditions and horizontal resolution also play discernible roles.