A Diagnostic Verification of the Precipitation Forecasts Produced by the Canadian Ensemble Prediction System.

Abstract

A comparatively long period of relative stability in the evolution of the Canadian Ensemble Forecast System was exploited to compile a large homogeneous set of precipitation forecasts. The probability of exceedance of a given threshold was computed as the fraction of ensemble member forecasts surpassing that threshold, and verified directly against observations from 36 stations across the country. These forecasts were stratified into warm and cool seasons and assessed against the observations through attributes diagrams, Brier skill scores, and areas under receiver operating characteristic curves. These measures were deemed sufficient to illuminate the salient features of a forecast system. Particular attention was paid to forecasts of 24-h accumulation, especially the exceedance of thresholds in the upper decile of station climates. The ability of the system to forecast extended dry periods was also explored. Warm season forecasts for the 90th percentile threshold were found to be competitive with, even superior to, those for the cool season when verifying across the sample lumping together all of the stations. The relative skill of the forecasts in the two seasons depends strongly on station location, however. Moreover, the skill of the warm season forecasts rapidly drops below cool season values as the thresholds become more extreme. The verification, particularly of the cool season, is sensitive to the calibration of the gauge reports, which is complicated by the inclusion of snow events in the observational record.