Evaluation of ECMWF ensemble forecasts of cyclone structure evolution through ET Using the Cyclone Phase Space

Operational model ensembles provide one approach to exploring – and potentially quantifying – the confidence in the numerical guidance provided to the forecaster. Extratropical transition of tropical cyclones is an especially difficult numerical forecasting challenge as the storm evolution appears to depend critically on phasing of the tropical storm and key systems in the midlatitude environment. Indeed, there is much evidence of problematic operational model forecasts of ET. ECMWF ensemble forecasts of storm structure evolution in the framework of the cyclone phase space (CPS) are evaluated for a number of recent ET events. In particular, we evaluate the spread of the ensemble forecasts of storm structure trajectory through the CPS, as well as the more traditional metrics of track and intensity. We contrast the spread of the ensemble members with the spread of the 12-36h structure changes from the NOGAPS model for a similar set of storm cases.