GloFAS Seasonal v3.1 forecast skill is evaluated using the Continuous Ranked Probability Skill Score (CRPSS) for ECMWF SEAS5 36-year reforecasts against a climatology benchmark forecast with respect to GloFAS-ERA5 river discharge reanalysis (also known as forced simulations (sfo))  as proxy observations at n=5997 GloFAS diagnostic river points.  Further details on the evaluation methodology can be found here:  GloFAS forecast skill.

Ensemble forecast skill with lead time

Overall, GloFAS Seasonal is skilful at the global scale for the majority of catchments across all lead times analysed (Figure 1). The CRPSS against climatology decays exponentially as a function of lead time reaching a minimum of 0.01 (interquartile range of -0.09 to 0.09) by week 16. The CRPSS at week 1 = 0.92 (interquartile range of 0.97 to 0.84), week 4 = 0.30 (0.15, 0.48), and at week 8 = 0.10 (-0.03, 0.15).

Figure 1: Skill of GloFAS Seasonal 3.1 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against a climatology benchmark from 1- to 16-week lead time with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. The interquartile range of CRPSS values at each lead time are shown by semi-transparent bands.

Spatial distribution of skill

The spatial distribution of skill is shown in Figure 2. GloFAS Seasonal is skilful compared to the climatology benchmark forecast in all catchments at a 1-week lead time. This reduces to 91%, 74% and 55% by lead times 4-, 8-, and 16-weeks, respectively. The regions of lowest skill against climatology include Northwest Canada and Alaska, North and central Africa, the Northwest coast of South America, and central and southern Asia.

Figure 2: Skill of GloFAS Seasonal 3.1 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against a climatology benchmark for lead times (1- (a), 4- (b), 8- (c), and 16-weeks (d)) with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. The optimum value of CRPSS is 1. Blue (red) dots show catchments with positive (negative) skill.