Verification of National Hurricane Center Tropical Cyclone Size Forecasts

The National Hurricane Center (NHC) analyzes and forecasts tropical cyclone (TC) size based on the maximum extent of 34-, 50-, and 64-kt (1 kt = 0.514 m/s) winds in four quadrants (northeast, southeast, southwest, and northwest) from the center of a cyclone. These predictions attempt to quantify the size variability of the wind field during the life cycle of a TC. Storm size plays an important role in the warning-decision-making process over land and at sea and influences the messaging of TC hazards and preparedness actions. While the NHC routinely verifies its track and intensity forecasts by comparison with the cyclone’s “best track” database, little recent verification work has been performed on wind radii forecasts. The lack of regularly available surface wind data over the oceans makes the storm size analysis uncertainty greater than for track or intensity, although this uncertainty is reduced when wind data are collected by reconnaissance aircraft.

This presentation examines the skill and accuracy of official NHC wind radii forecasts and whether these forecasts are improving, with a focus on cases in the Atlantic basin when aircraft reconnaissance wind data are available. The various global, regional, and consensus models available to the NHC forecasters are evaluated to assess which models provide the best guidance for forecasting the extent of tropical cyclone wind fields. It is shown that the NHC wind radii average errors have decreased over the past couple of decades, and the NHC forecasts are skillful when compared against climatology and persistence. A homogeneous comparison with the models reveals that the NHC mean errors are lower than the errors for any individual model and are comparable to the multi-model wind radii consensus. Some recent developments that could aid in TC size analysis and future plans for NHC wind radii forecasts are also presented.