**Francisco J. Doblas-Reyes**

**ECMWF, Shinfield Park**

**RG2 9AX, Reading, UK**

**Caio A. S. Coelho**

**Centro de Previsão de Tempo e Estudos Climáticos**

**Rodovia Presidente Dutra, Km 40, SP-RJ 12630-000, Cachoeira Paulista, Brazil**

**David B. Stephenson**

**School of Engineering, Computing and Mathematics, University of Exeter**

**Harrison Building, North Park Road, Exeter EX4 4QF, UK**

Probability forecasts are often discretized into a small set of categories before
being distributed to the users. This study investigates how such simplification can
affect the forecast quality of probabilistic predictions as measured by the Brier
score. An example from the ECMWF operational seasonal ensemble forecast system is
used to show that the simplification of the forecast probabilities reduces the Brier
skill score by as much as 57% with respect to the skill score obtained with the full
set of probabilities issued, especially for a small number of probability categories.
This is mainly due to a decrease in forecast resolution of up to 36%. It is therefore
important that forecast quality is made available for the set of probabilities that
the forecast user has access to as well as for the complete set of probabilities issued
by the forecasting system.