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Guidance on Interpreting eTRaP Probabilities

The gridded probabilities of exceeding a specific rainfall threshold (e.g. PoP larger than or equal to 200 mm) show the probability that the rainfall accumulation at a specific pixel will exceed the rainfall threshold during the forecast period. Users should keep in mind that these probability values will always be rather low--especially for the heaviest rainfall amounts-- because:

1. Such rainfall amounts are rare when considering only a point or a very small area. When interpreting these probabilities, users should consider how they compare with the climatological probability of a rainfall event of the same magnitude. For example, if a particular location receives 200 mm of rain in 24 hours on average once every 3 years (that is, a daily probability of roughly 1 in 1000), then an eTRaP exceedance probability of 10% means the odds are 100 times higher than normal for this amount to fall at that particular pixel.

2. These probabilities reflect uncertainty in the forecast that spreads the probabilities out over larger areas and reduces their values. For instance, if the deterministic eTRaP forecast shows an area with accumulations exceeding 200 mm in the next 24 hours, the probability of that amount at any given pixel (even within the 200 mm isohyet in the deterministic field) may be significantly lower than 50% if there is a high degree of uncertainty in the forecast (for instance, because of uncertainties in the predicted storm track or differences in the initial rainfall rate fields among the different sensors). So while the probability of 200 mm of rainfall may be low at any given pixel, the probability that such a rainfall accumulation will be observed somewhere in that area of low probabilities is much higher. If rainfall exceeding 200 mm occurs over 20% of the area covered by the 20% probability of rainfall exceeding 200 mm, that forecast is technically correct.