It is well known that forecast skill can vary dramatically from case to case. An example for such variations can be seen when the MRF high resolution control QPF forecasts verifying for the 24-hour period 02031112-02031212 are compared to the verifying raingage-based QPE. While at 1-day lead time the QPF forecast for both major forecast events of the day (one over the NW, the other over the SE US) verifies well, the 2-day and longer lead time forecasts are of high quality only for the NW, but not for the SE event. How a forecaster is to know which forecast to trust and which not? An ensemble of forecasts can provide such information. The 2-day and longer lead time calibrated PQPF forecasts reveal a striking difference in terms of probability values for the half inch accumulated precipitation amount for the NW event (90% probability out to 6 days lead time) vs. the SE event (around 5% probability), providing far greater forecast information for the NW event. Correspondignly, the Relative Measure of Predictability (RMOP) charts at 96- and 24-hour lead time also reveal a striking difference in terms of the predictability of the upper level waves associated with the two precipitation events. The wave in the NW/SE is associated with a high/low level of predictability. Even at 1-day lead time, probabilistic geopotential height forecasts around the low pressure wave in the SE do not reach the 50% value, while even at 4-day lead time they remain as high as 75% in the area of the NW wave, indicating that the latter system had a much higher level of predictability.
 

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