Among the wide variety of performance measures availablefor the assessment of skill of deterministic precipitation forecasts, theequitable threat score (ETS) might well be the one used most frequently. Itis typically used in conjunction with the bias score. However, apart fromits mathematical definition the meaning of the ETS is not clear. It has beenpointed out (Mason, 1989; Hamill, 1999) that forecasts with a larger biastend to have a higher ETS. Even so, the present author has not seen thishaving been accounted for in any of numerous papers that in recent yearshave used the ETS along with bias "as a measure of forecast accuracy".A method to adjust the threat score (TS) or the ETS so as to arrive at theirvalues that correspond to unit bias in order to show the model's orforecaster's accuracy in extit{placing} precipitation has been proposed earlier by thepresent author (Mesinger and Brill, the so-called dH/dF method). A seriousdeficiency however has since been noted with the dH/dF method in that thehypothetical function that it arrives at to interpolate or extrapolate theobserved value of hits to unit bias can have values of hits greater thanforecast when the forecast area tends to zero. Another method is proposedhere based on the assumption that the increase in hits per unit increase infalse alarms is proportional to the yet unhit area. This new method removesthe deficiency of the dH/dF method. Examples of its performance for 12 months offorecasts by three NCEP operational models are given.
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