Determining Optimal Thresholds for Inland Locations of Tropical Cyclone Incremental Wind Speed Probabilities to Support the Provision of Expressions of Uncertainty within Text Forecast Products

As tropical cyclone events unfold, decision-makers require a meteorologist's best wind speed forecast, along with an accompanying expression of uncertainty. Both are necessary in order to effectively manage preparations for life-threatening weather events. Given the inherent error regarding tropical cyclone forecasts, this necessity reveals the shortcoming of deterministic-only wind speed forecasts such as those found within the traditional Zone Forecast Product and Coastal Waters Forecast as issued by the NWS Weather Forecast Offices. To address this shortcoming, the offices in Miami and Melbourne developed a means by which expressions of uncertainty can be consistently and coherently included within these products through the creative use of the National Hurricane Center's incremental wind speed probabilities. This includes the zone-based versions which are used to generate the legacy products, as well as the dynamic point-and-click (point-based) versions found on Weather Forecast Office Web sites. The logic depends, in part, on the exceedance of incremental wind speed probability thresholds as a function of time (e.g., forecast period) for uncertainty involving tropical storm and hurricane force winds. Therefore, it is prudent to identify and adjust (as needed) those thresholds to maximize the responsible detection of a potential event while minimizing false alarms which can lead to the overuse of the enhanced expressions. Such optimal thresholds have been previously determined as a function of time for coastal verification points only using Relative Operating Characteristics diagrams and Threat Scores. However these critical thresholds are not only a function of time but also of space. This paper expands previous verification efforts of the incremental wind speed probabilities to include inland verification and additional validation metrics such as Pierce Skill Scores. The study includes locations stretching across the U.S. Gulf of Mexico and Atlantic coasts using coastal locations and also inland locations suitably determined by using an inland decay model.