How to interpret the skill map for the Environment Canada temperature and precipitation anomaly forecast ?
The temperature and precipitation anomaly forecasts for each month and season during the period indicated on the forecast charts were compared with the observed anomalies (based on 3 categories: ABOVE, BELOW and NEAR NORMAL). The skill maps show the percent correct. This score is calculated using a 3 by 3 contingency table. The higher the percentage, the better the forecast over the verification period. A purely random “chance” forecast would be, on the average, 33 percent correct. However, the data used to calculate the percent correct cover only 30 years (Derome et al., 2000; Plante and Gagnon, 2000; Servranckx et al., 2000). Thus, the threshold to be statistically significant is close to 40% (with 10% confidence level) and not 33%. With a longer dataset of, say, 10000 years the threshold would have been 33%. The fact that the threshold is 40% ensures that a forecast system is not better than 33% only by pure chance (true 18 times out of 20, 90% of the time). This means that a percen
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- How to interpret the skill map for the Environment Canada temperature and precipitation anomaly forecast ?
