Forecasting September Atlantic Basin Tropical Cyclone Activity

Abstract

September is the most active month for Atlantic basin tropical cyclone activity with about 50% of all hurricane activity occurring during this month. Utilizing National Centers for Environmental Prediction?National Center for Atmospheric Research (NCEP?NCAR) reanalysis data, a prediction scheme for forecasting September tropical cyclone activity has been developed. Based on hindcasting results from 1950 to 2000, 30%?75% of the variance for most tropical cyclone parameters can be hindcast by the end of July. This hindcast skill improves to 45%?75% by the end of August. Similarly, cross-validated hindcast skill explains from 20% to 65% for most variables by the end of July, improving to 30%?65% by the end of August. Simple least squared linear regression was utilized to calculate hindcast skill, and variables were selected that explained the largest degree of variance when combined with the other predictors in the scheme. These predictors tend to be global in nature and include zonal and meridional wind at 200 and 1000 mb and sea level pressure measurements at various global locations. Many of the predictors are strongly correlated with global modes such as El Niño?Southern Oscillation (ENSO) or the Pacific decadal oscillation (PDO). Based on this September hurricane prediction scheme, U.S. tropical cyclone landfall probability forecasts can also be issued. In addition, the 1 August forecast of September activity can also be applied to improve the hindcast skill of Gray's 1 August seasonal statistical forecast.