Hurricane Erin underwent extratropical transition in September 2001. The hurricane interacted during its transformation stage with a mid to upper level trough and re-intensified rapidly as an extratropical cyclone.

Forecasts of the extratropical transition of Erin have been performed using the global model (GME) of the German Weather service (DWD). The model runs are initialized with ECMWF and DWD analyses. The forecasts are compared against ECMWF and DWD analyses and National Hurricane Centre (NHC) best track data. The dynamics of the extratropical transition of Erin are investigated from a PV-Theta perspective. The aim of this study is to elucidate the physical mechanisms that cause (i) the differences between the forecasts and the analyses and (ii) the differences between the forecasts initialized with ECMWF and DWD analyses data.