Abstract
The monthly forecasting system of the Institute of Atmospheric Sciences and Climate of the National Research Council (CNR-ISAC) of Italy is operationally run on a weekly basis in the framework of the Subseasonal-to-Seasonal (S2S) project to produce 41-member ensemble forecasts. The first two years of forecasts, covering 106 weeks from April 2015, are verified against ERA-Interim as weekly averages starting from the first forecast day. Nonprobabilistic scores of 500-hPa geopotential height and 850-hPa temperature anomalies are computed for the extratropical hemispheres. The anomaly correlation coefficient shows enhanced predictive skill during the cold months, when favorable values are occasionally obtained beyond week 2. The root-mean-square error saturates toward the climatological value between weeks 2 and 3. Reliability diagrams are used to evaluate the probabilistic forecast skill of 2-m temperature over Northern Hemisphere extratropical land points, in terms of above- and below-normal events. The forecasting system loses reliability and resolution beyond week 2, but well reproduces the observed 2-yr mean frequency up to week 4, proving to be unbiased. The reliability of the forecasting system systematically outperforms that obtained by persisting the previous week forecast. Beyond week 2, the forecast distribution of below-normal events shows low confidence. However, a reliability diagram based on equally populated bins of forecast probabilities highlights residual resolution up to week 4 at low probabilities. ROC diagrams confirm that the modeling system has greater discrimination capability for below-normal events. The reliability analysis of accumulated precipitation shows minor differences between below- and above-normal events, with lower skill than 2-m temperature.
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