Introduction

“Behind good forecast practises are often hidden good theories; equally, good theories should provide a basis for good forecast practises.” Professor Tor Bergeron, personal communication 1974

The aim of this User Guide is to help meteorologists make optimal use of the forecast products from ECMWF, develop new products and reach new sectors of society and thereby satisfy new demands. This is done by presenting the forecast system and advising on how best to use the output, not least how to build up trust in the forecast information. The emphasis is on the medium-range forecast products, since the way forecasters deal with medium-range NWP output differs in many ways from how they deal with short-range NWP on the one hand and monthly and seasonal NWP on the other. The main outline:

1.      The ECMWF deterministic forecast system, i.e. the dynamical model, the data assimilation and the forecast delivery system, are described in broad and non-technical terms.

2.      The interpretation of the deterministic NWP output is complicated by its often counter-intuitive, non-linear behaviour.  The deterministic output should therefore not be over-interpreted, in particular not in the medium-range or when extreme weather is likely. Then the use of probabilities or other risk assessments are needed.

3.      A good forecast that is not trusted is a worthless forecast. The Ensemble Prediction System (EPS), which is given extensive coverage, provides a basis for formulating the most accurate categorical forecasts and the probabilities of alternative developments. Methods to combine the deterministic and probabilistic outputs are suggested.

4.      In the medium-range the use of statistical know-how counts as much as synoptic experience, since daily operational work is to a large extent a matter of assessing, combining and correcting NWP information. In two appendices statistical concepts for validating and verifying deterministic and probabilistic forecasts and for making the best use of NWP information are presented.

5.      The forecaster is not a computer. Throughout the User Guide forecasters are advised not to try to imitate an NWP system, but to perform quite differently, with fewer details, more uncertainty and no “U-turns”.

This User Guide is the fruit of several years of discussions with scientists, forecasters and meteorologists who are interested in statistics, both from Europe and elsewhere. The interaction between these three specialized groups has been the main driving force and inspiration for this publication.

The User Guide gives only an introduction to the forecast information provided by ECMWF. Users are advised to keep themselves updated about the products through the ECMWF Newsletter and web site.