  Seasonal climate forecasts are being used increasingly across a range of
application sectors. A recent UK governmental report asked: How good are
seasonal climate forecasts on a scale of 1-5 (where 5 is very good), and how
good can we expect them to be in 30 years time? Seasonal climate forecasts are
made from ensembles of integrations of numerical models of climate. We argue
that goodness should be assessed primarily in terms of the probabilistic
reliability of these ensemble-based forecasts and that a 5 should be reserved
for systems which are not only reliable overall, but where, in particular,
small ensemble spread is a reliable indicator of low ensemble forecast error.
We study the reliability of regional temperature and precipitation forecasts of
the current operational seasonal forecast system of the European Centre for
Medium-Range Weather Forecasts, universally regarded as a world leading
operational institute producing seasonal climate forecasts. A wide range of
goodness rankings, depending on region and variable (with summer forecasts of
rainfall over Northern Europe performing exceptionally poorly) is found.
Finally, we discuss the prospects of reaching 5 across all regions and
variables in 30 years time.
