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• ROC value 1.0 implies consistent Hit Rate and no False Alarms.
• ROC value 0.5 implies no skill with as many Hits as False Alarms. 
• ROC value 0.0 implies consistent False Alarm rate and no Hits.

An explanation of ROC diagrams is given in the annex to this guide.

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Fig8.3.3-1: ROC diagram for 850hPa temperature for the verifying period Sep, Oct, Nov derived from SEAS5 runs with data time of early Aug.  Points above the lower left/upper right diagonal signify skill.  The ROC score or value is the area beneath the red curve.

Reliability diagrams

The Reliability diagrams give a measure of the tendency of the forecast system to over- or under-forecast and event.  The diagram plots the frequency of a forecast probability of an event against the frequency that the event occurs.   Ideally these should match and is shown by the diagonal line. Where the plot lies:

• below the diagonal implies over-forecasting.  The forecast probability is greater than the observed frequency.
• above the diagonal implies under-forecasting.  The forecast probability is less than the observed frequency.

An explanation of reliability diagrams is given in the annex to this guide.

Fig8.3.3-2: reliability diagram for precipitation for the verifying period Sep, Oct, Nov derived from SEAS5 runs with data time of early Aug.  Points above the diagonal signify under-forecasting, points below the diagonal signify over-forecasting.

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