Ideally, a rainstorm warning should be issued when the weather is still clement and well before heavy rain turns up. Progressive verification is designed to account for three factors, namely severity of rainstorm, intensification of rainstorm and timeliness of rainstorm warnings. Compared to simple binary verifications, the operational progressive verification scheme on rainstorm warnings adopted in HKO rewards close calls but penalizes warnings with short lead time or little intensification.

THEMIS (Tools for Harmonised Evaluation of Model products, seamlessly Integrated guidance, and SWIRLS) is a web-based tool for routine verification of nowcasting and numerical model verification. It is capable of computing the following metrics for deterministic forecasts and displaying the results in time series, bar charts, line charts, and heat maps:

• Performance diagram
• ROC curve
• hit, miss, false alarm, correct negative
• Probability of Detection (POD)
• False Alarm Ratio (FAR) and Probability of False Detection (POFD)
• Critical Success Index (CSI), also known as Threat Score (TS)
• Frequency bias
• Fractional Skill Score (FSS)
• Equitable Threat Score (ETS), also known as Gilbert Skill Score (GSS)
• Heidke Skill Score (HSS)
• Accuracy

In addition, THEMIS is also capable of computing the following additional metrics for probabilistic forecasts:

• Brier Skill Score (BSS)
• Reliability diagram