Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 138: 34 – 55, January 2012 A Performance of different forecast systems in an exceptional storm in the Western Mediterranean Sea L. Bertotti, a J.-R. Bidlot, b C. Bunney, c† L. Cavaleri, a * L. Delli Passeri, d M. Gomez, e J.-M. Lef` evre, f T. Paccagnella, g L. Torrisi, h A. Valentini g and A. Vocino h a Institute of Marine Sciences, CNR, Venice, Italy b European Centre for Medium-Range Weather Forecasts, Reading, UK c Met Office, Exeter, UK d Department for National Civil Protection, Rome, Italy e Puertos del Estado, Madrid, Spain f M´ et´ eo-France, Toulouse, France g ARPA-SIMC, Hydro-Meteo-Climate Service of Emilia-Romagna, Bologna, Italy h CNMCA, National Meteorological Service, Rome, Italy *Correspondence to: L. Cavaleri, ISMAR, Castello 1364/A, Venice 30122, Italy. E-mail: luigi.cavaleri@ismar.cnr.it † The contribution of these authors was written in the course of their employment at the Met Office, UK, and is published with the permission of the Controller of HMSO and the Queen’s Printer for Scotland. We consider an exceptional storm – ‘Klaus’ (January 2009) – its evolution on the Western Mediterranean Sea, and how the associated wind and wave conditions were modelled by seven of the major systems presently operational in this area. We intercompare the model results and then verify them and the related model ensemble versus the available measured data. Working with short-term forecasts (24 h) only, as expected, each model correctly anticipates the incoming of an exceptional storm. However, even at such limited range, we have found substantial differences among the results of the different models. The differences concern the time the storm should have entered the West- ern Mediterranean Sea, the peak values of wind speed and significant wave height, the general distribution of the fields, and the locations where the maxima were achieved. We have compared the model results versus the available measured data, wind from scatterometer, waves from altimeter, plus a few buoy data. We have found some inconsistencies in the results, model wind data being on average larger than the measured one, while the opposite was true for wave heights. However, the limited amount of data available and its different times and positions, at and off the centre of the storm, impede the drawing of any definite conclusion in this respect. On the whole we feel that our results, although related to a single storm, cast doubts on the reliability of a single forecast system to provide sufficiently reliable and accurate forecasts in case of an incoming exceptional storm. The results, both for wind and waves, have improved using an ensemble of the seven considered models. This suggests that there is no relevant systematic error in the used models except, as possibly suggested by our results, in the case of wave generation under very strong wind and very young sea conditions. Copyright c  2011 Royal Meteorological Society and British Crown Copyright, the Met Office Key Words: forecast; wind; wind waves; extreme conditions; meteorological models; wave models Received 1 October 2010; Revised 27 June 2011; Accepted 30 June 2011; Published online in Wiley Online Library 12 August 2011 Copyright c  2011 Royal Meteorological Society and British Crown Copyright, the Met Office