Trends in track forecasting for tropical cyclones threatening the United States, 1970-2001

Previous studies have identified statistically significant long-term improvements in forecasts issued by the National Hurricane Center (NHC) for Atlantic basin tropical cyclones. Recently, however, attention has been focused on the forecast accuracy of landfall location and timing, and the long-term improvement trends for this relatively small sample of forecasts were mixed. These results may lead some to conclude that the accuracy of NHC forecasts close to the United States has not improved over time. A statistically robust dataset can be obtained by considering "landfall-threatening" storms, defined as one for which tropical cyclone watches or warnings are in effect for some portion of the continental United States. In this study, long-term trends in accuracy are determined for NHC forecasts issued during these periods of threat and compared to trends for the Atlantic basin overall. A second set of trends are determined for forecasts verifying during the periods of threat. The analysis shows that NHC forecasts for land-threatening tropical cyclones are improving. These improvement trends are statistically significant, although the improvement rates for the land-threatening storms are smaller than those for the basin overall. Over the period 19702001, forecasts issued during the watch/warning stage improved at annual average rates of 0.7%, 1.6%, and 1.9% at 24,48, and 72 h, respectively.