Tropical Cyclone Tracks Using the Interpolated Global Ensemble Model from NCEP

The National Centers for Environmental Prediction’s Global Ensemble Forecast System (GEFS) is currently running as version 12. It became operational in September 2020, and is based on the same finite volume cubed sphere dynamical core as the deterministic Global Forecast System (GFS) model. As shown in Colby (2019), the current version has better spread than previous versions. This ensemble is composed of one control forecast and 30 perturbed members.

To make its official tropical cyclone track forecasts, the National Hurricane Center (NHC) consults many model track forecasts, including the interpolated GEFS ensemble mean forecast (AEMI) and the Track Variable Consensus (TVCN) model, an equally weighted consensus of early model forecasts from at least two of the GFS, the U.S. Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System, the United Kingdom Meteorological Office’s global model, the European Centers for Medium-Range Weather Forecasting’s global model, and NCEP’s Hurricane Weather Research and Forecasting Model (HWRF).

To improve the use of the GEFS member forecasts, the track errors of the 6-hour forecasts of each of the 30 GEFS members was calculated for the 2021 and 2022 hurricane seasons. For each forecast, the seven members with the smallest errors were identified. These member forecasts were then restarted with their 6-hour location relocated to the best track location. The differences in latitude and longitude for that relocation were then applied to the rest of the member track to produce a new interpolated forecast. The track errors from the interpolated forecasts were compared to those of TVCN and the interpolated HWRF. TVCN usually had smaller errors than the new interpolated forecasts, but the track errors for HWRF were larger for more than 50% of the forecasts. The table shows the summary for the two seasons. There were 338 48-hour forecasts and more than 50 168-hour forecasts in 2021, 221 48-hour forecasts and more than 40 168-hour forecasts in 2022.

Given these results the new interpolated forecasts, easily constructed in real-time, provide useful and more accurate forecast information for NHC forecasters.

References:

Colby, F.P., 2019: The Spread of Tropical Storm Tracks in Three Versions of NCEP’s Global Ensemble Model: Focus on Hurricane Edouard (2014). Wea. Forecasting, 34, 577–586, https://doi.org/10.1175/WAF-D-18-0153.1