Revised Prediction of Seasonal Atlantic Basin Tropical Cyclone Activity from 1 August

Predictions of the remainder of the season’s Atlantic basin tropical cyclone activity from 1 August have been issued by Gray and his colleagues at the Tropical Meteorology Project at Colorado State University since 1984. The original 1 August prediction scheme utilized several predictors, including measures of the stratospheric quasi-biennial oscillation (QBO), West African rainfall, El Niño–Southern Oscillation, and the sea level pressure anomaly and upper-tropospheric zonal wind anomalies in the Caribbean basin. The recent failure of the West African rainfall and QBO relationships with Atlantic hurricanes has led to a general degradation of the original 1 August forecast scheme in recent years. It was decided to revise the scheme using only surface data. The development of the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis has provided a vast wealth of globally gridded meteorological and oceanic data from 1948 to the present. In addition, other datasets have been extended back even further (to 1900), which allows for a large independent dataset. These longer-period datasets allow for an extended period of testing of the new statistical forecast scheme. A new prediction scheme has been developed on data from 1949 to 1989 and then tested on two independent datasets. One of these datasets is the 16-yr period from 1990 to 2005, and the other dataset is from 1900 to 1948. This allows for an investigation of the statistical significance over various time periods. The statistical scheme shows remarkable stability over an entire century. The combination of these four predictors explains between 45% and 60% of the variance in net tropical cyclone activity over the following separate time periods: 1900–48, 1949–89, 1949–2005, and 1900–2005. The forecast scheme also shows considerable skill as a potential predictor for giving the probabilities of United States landfall. Large differences in U.S. major hurricane landfall are also observed between forecasts that call for active seasons compared with those that call for inactive seasons.