GEFS- Global Ensemble Forecast System- A statistical “ensemble” model based on 30 computed variations (called “perturbations”) of the GFS and a control version. (The 30 perturbations and a control comprise the group of models or whats called an “ensemble”.)

How the perturbations are created is a complex science in itself, but they’re generated based on running a simplified GFS version and seeing which direction errors show the greatest growth.

This ensemble version of the GFS attempts to compensate and average the unknowns and chaotic aspects of input data and other inherent limitations of numeric weather modeling. It provides the statistical mean (average) and spread (uncertainty) of numerous parameters such as temperature, rain and pressure based on these 30 GFS variations.

It’s run four times daily. The GEFS forecasts are available in 3 hour increments and its forecasts extend out 384 hours into the future.

Due to the complexity of the GEFS, it takes 6-7 hours to compute each model. A GEFS model run based on the 12z (7 AM EST) GFS won’t be available until after 1 PM EST in the afternoon.