An Examination of Tropical Cyclogenesis Using Composites of NCEP GFS Deterministic Model Analyses for the Period of 2011-2015

The National Hurricane Center (NHC) routinely forecasts the potential for tropical cyclone (TC) formation at the lead times of 48 and 120 h. These forecasts are issued every 6 h during hurricane season and are expressed in terms of probabilities. The genesis forecasts are heavily dependent on numerical model guidance, particularly beyond 24 to 48 hours. Since what constitutes TC genesis in global model output is not obvious, operational forecasters are left to subjectively determine the likelihood of tropical cyclone formation from an examination of various model fields, such as mean sea-level pressure and 850-hPa relative vorticity. In this study, we examine cases of TC genesis in the NCEP GFS deterministic model from 2011-2015. Storm-centered composites, which are constructed from analyses leading up to and including the time of TC formation, are used to objectively quantify thresholds that can be used to identify TC genesis in the model fields. Employing thresholds derived from the composite analysis will help forecasters to more objectively assess of the probability for TC genesis. In addition, these thresholds provide a basis to evaluate the ability of the GFS model to reliably forecast TC genesis.