Short-range probabilistic forecasts from the Norwegian limited-area EPS: long-term validation and a polar low study

NORLAMEPS is a 42 member short-range EPS run operationally at met.no since February 2005. It combines TEPS, a 21 member version of ECMWF’s EPS with perturbations targeted to Northern Europe, and a HIRLAM-based 21 member LAMEPS, and includes two control forecasts. NORLAMEPS has been upgraded extensively since 2005, including extensions for the IPY-THORPEX project. For a range of investigated weather parameters up to 60 h lead times, NORLAMEPS provides better probabilistic forecasts than ECMWF’s 51 member EPS. Combining LAMEPS with TEPS is valuable when high spatial resolution is not crucial, that is, for precipitation except in summer and wind except during autumn and early winter. The IPY-THORPEX field campaign produced additional observations for two polar lows in March 2008. The impact of those observations is studied with the 21 member LAMEPS. For the first polar low a significant positive impact is found, and for long lead times in particular. For the more complex and the operationally poorer forecasted second polar low, the impact of extra observations was positive only in the first stage of the development. Later, for the more intense part of this polar low, slightly better results were actually achieved without the extra observations.