Preliminary Reporting on the Case Evaluation of Forecasters' Needs for the Global Forecast System (GFS)

The Global Forecast System (GFS) and its ensemble version (GEFS) are National Centers for Environmental Prediction (NCEP) weather forecast systems that generate data for dozens of atmospheric and land-soil variables, including temperatures, winds, precipitation, soil moisture, and atmospheric ozone concentration. The system couples five separate key models (for atmosphere, ocean, land/soil, sea ice, and aerosol) that work together to accurately depict weather conditions. Just as with any modeling system, there are imperfections in the balance between model analyses, dynamics, and physics that can lead to various biases and errors in certain situations.

In FY23, the Analysis and Nowcast Branch (named AFS11) within the Analyze, Forecast, and Support Office Office (AFSO) of the National Weather Service Headquarters evaluated the GFS using a multi-strategy approach in order to determine what the more stand-out biases and errors are produced by the model. The AFS11 GFS Evaluation Project Team contacted the field offices, performed a real-time MetWatch, and reviewed significant weather events over the past couple of years and collected cases that fell into common categories. The Model Evaluation Group (MEG) at the Environmental Modeling Center (EMC) was also an excellent resource in coming up with error categories and providing cases for the GFS and GEFS.

During this initial evaluation phase, there were 10 error or bias categories revealed, with several subcategories. These included tropical cyclone genesis, tropical and extratropical cyclone location (progressiveness) errors, low Convective Available Potential Energy (CAPE), various surface temperature biases in specific setups, precipitation type, over- or under-forecasting of precipitation amounts in certain scenarios, over-forecasting of snow amount, inconsistencies between snow amount and precipitation type, and under-forecasting of wind speeds in synoptic-scale high wind events. Between the efforts of AFS11 and the previous work compiled by the MEG, over 50 cases fell into these categories. After these cases were all initially evaluated, AFS11 composed an evaluation report to go over details of selected cases from each category. In the future, after a field survey and confirmation by the field, requirements will be developed in an attempt to remove some of these systematic and representative errors and biases and thus improve the GFS and GEFS for use by the field.