Reliability of 3-Hour Probability of Precipitation Forecasts Produced by the National Weather Service Office in Miami Florida

In early 2013, the National Weather Service (NWS) Office in Miami, Florida implemented a new three-hour probability of precipitation (PoP) forecast. The short-term three-hour forecasts provide the general public with more frequently updated PoP values in addition to the twelve-hour and six-hour PoPs. The reliability of the new three-hour PoP forecasts was examined to provide feedback to the NWS with an objective to increase the situational awareness in the areas where the results suggested improvement was necessary. Approximately nine-months of data were analyzed from June 13th, 2013 to March 12th, 2014. Reliability plots and frequency distributions were analyzed for the entire time period, as well as the wet and dry season for the airports of Naples, West Palm Beach, Miami, and Fort Lauderdale. High impact events in which precipitation amounts were above normal were examined to analyze how the precipitation forecasts performed in these extreme events. PoP forecasts for the entire time period were reliable with a small bias depicting a tendency to overestimate. The dry season PoPs were less reliable and showed more of an overestimation compared to the wet season. Certain PoP values, mainly around 50%, 70% and 80%, were underestimated suggesting a possible dry bias. High impact events were well-predicted, shown by trends of increasing PoP values as radar indicated precipitation entering the forecast areas. The high impact events did support the claim for a bias towards underestimating PoP values around 50% and 80%. A detailed analysis of each high impact event is presented in this study.