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 colleagues of 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 (ENSO), and 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-present. In addition, other datasets have been extended back even further (to 1900) which allow 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 1949-1989 and then tested on two independent datasets. One of these datasets is the sixteen year period from 1990-2005, and the other dataset is from 1900-1948. This allows 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 45-60% of the variance in Net Tropical Cyclone (NTC) activity over the following separate time periods: 1900-1948, 1949-1989, 1949-2005, and 1900-2005, respectively. The forecast scheme also shows considerable skill as a potential predictor for giving the probabilities of United States landfall. Large differences in US major hurricane landfall are also observed between forecasts which call for active seasons compared to those which call for inactive seasons.