A Case Study of High-Impact Wet Snowfall in Northwest Germany (25–27 November 2005) : (Observations, Dynamics, and Forecast Performance)

Accurate numerical weather prediction of intense snowfall events requires the correct representation of dynamical and physical processes on various scales. In this study, a specific event of high-impact wet snowfall is examined that occurred in the northwestern part of Germany in November 2005. First, the synoptic evolution is presented, together with observations of precipitation type and vertical temperature profiles, which reveal the existence of a so-called potential melting layer during the early period of wet snowfall. During the main part, the performance of the operational forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) is investigated. It is shown that only the short-term predictions captured the snowfall event, whereas earlier forecasts were in error concerning the phase and/or amount of precipitation. However, even the short-term forecasts produced the onset of surface snowfall too late (i.e., during the dry snowfall period). Reasons for the misforecasts are errors on various scales. For the early forecasts, they include an inaccurate representation of the upper-level trough and a misplacement of the surface cyclone. For the later forecasts, a slight overestimation of the depth of the potential melting layer and a potentially too fast snow melting process in the model lead to the erroneous prediction of surface rainfall during the wet snowfall period. Hindcast experiments with the high-resolution Consortium for Small-Scale Modeling (COSMO) model also point to the necessity of improving its snow melting parameterization in order to provide useful predictions of potentially high-impact wet snowfall events.