Environmental Conditions and Forecast Error Evolution Associated with Synoptic Progressiveness Errors in GFS Forecasts

Cutoff lows are often associated with high-impact weather; therefore, it is critical that operational numerical weather prediction systems accurately represent the evolution of these features. However, medium-range forecasts of upper-level features using current and previous versions of NCEP’s Global Forecast System (GFS) are often characterized by excessive synoptic progressiveness, i.e., a tendency to advance troughs and cutoff lows too quickly downstream. To better understand the processes associated with progressiveness errors in the GFS, this research jointly evaluates the environmental conditions that favor erroneously progressive cutoff lows in conjunction with the evolution of meso- to synoptic-scale forecast errors in the days preceding the diagnosed progressive displacement error. Feature-relative composite analysis of erroneously progressive cutoff lows over North America indicate that these displacement errors are preceded by the onset of negative 500-hPa geopotential height errors in the environment (i.e., heights that are too low) surrounding the composite feature, most notably through an under-amplification of an upstream ridge and heights that are too low poleward of the forecast feature. At the same time, mid-tropospheric wind speeds and cyclonic vorticity advection are too strong on the downstream and equatorward side of the forecast feature, consistent with greater forcing for height falls and faster downstream progression than verifying analyses. This composite behavior is most prominent in forecasts using GFS version 15 (June 2019–March 2021) and is corroborated by examination of individual cases of progressiveness errors. Further research is planned to identify and examine deficiencies in processes represented by the GFS, including parameterized physics, which may influence these errors.