## Forecast Performance Charts## Calculation of the AFWA Performance Index## by Robert J. Craig, AFWA## Methodology### Calculation of skill scoresOnce the forecast and persistence rms errors have been determined for a particular combination of parameter/forecast range and for a particular day, the daily skill score may be calculated. This is defined in terms of Reduction of Variance, i.e., where:
*r*is the rms forecast error_{f}*r*is the rms persistence error_{p}
The smaller the ratio between forecast and persistence errors, the closer the skill score will be to 1 (perfection). If the forecast error is greater (worse) than the persistence error, then the skill score will be negative. ### Compilation of the indexThere are several further stages in the compilation of the Index from the skill scores. Firstly a weighted average, S _{j}, of the individual skill scores for day j is calculated, where:
*w*is the weight for the i-th component (see table below)_{i}*SS*is the skill score for the i-th component for day j._{i,j}
The following table defines the weights, w
Once the weighted sum, S N |
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Modified January 6, 2011 8:01 PM
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