Improvements in tropical cyclone track forecasting in the Atlantic basin, 1970-98

Tropical cyclone track forecasts issued by the Tropical Prediction Center/N ational Hurricane Center for the Atlantic basin have improved over the peri od 1970-98. Improvement is shown at 24, 48, and 72 h. Although this improve ment can be shown without any preconditioning of the data, the question of accounting for forecast difficulty is addressed, building upon the work of Neumann. A decrease in the initial position errors over the same period is also shown. Track forecast errors generated by the Atlantic climatology and persistence (CLIPER) model (run in "best-track" mode) are used as a measure of forecas t difficulty. Using the annual average CLIPER errors in a regression agains t the official forecast errors yields an equation giving an expected error for each year under consideration. The expected error (representing forecas t difficulty) is then subtracted from the observed official errors. The res ulting set of differences can then be examined for long-term trends, diffic ulty having been accounted for. Fitting a straight line to these differences (1970-98) yields the result th at official forecast errors have decreased by an average of 1.0% per year a t 24 h, by 1.7% per year at 48 h, and by 1.9% per year at 72 h. A second-or der fit, however, suggests that the rate of improvement has increased durin g the latter half of the period.