Guide to Plots | Guide to GFS Ensemble Track Plots

Guide to GFS Ensemble Track Plots

This page lists the TECH identifiers of the various intensity forecast aids that may appear in the GFS ensemble track plots on this site. To learn more about TECH identifiers, click here. For general information about the difference between late cycle and early cycle forecast aids, click here. To learn what a vortex tracker is, click here.

About the GFS Ensemble System

The GFS Ensemble System (GEFS) consists of 20 statistically independent perturbations plus one control member which are run 4 times per day (initialized at 00, 06, 12, and 18 UTC). As of the major upgrade in February 2010, each ensemble member is run with 28 vertical levels and a triangular truncation of 190 waves (T190) in the horizontal (this gives an effective grid point spacing of ~80 km). After 180-hr, the ensemble members are truncated to T126, which degrades their performance and makes it difficult for the tracker to follow tropical cyclones. In comparison, the operational GFS is run with 64 vertical levels and T574 in the horizontal (an effective grid point spacing of ~27 km). After 192-hr, the operational GFS is run at T190 through 384 hours (16 days). Initial conditions come from the Gridpoint Statistical Interpolation (GSI) global data assimilation system  (GDAS). This system incorporates a 3D-VAR method to continuously update the background fields used for the initial condition.

The control member uses these initial conditions directly, while the perturbed members use an initial condition which has been obtained using an Ensemble Transform Bred Vector (ETBV) method that is designed to pick out the fastest growing modes in the model (with the hope that these simulate the fastest growing sources of uncertainty in the atmosphere). These perturbations are also subjected to stochastic physics perturbations. Note that this perturbation method is generally best suited for the extra tropics -- no special perturbations are conducted specific to individual tropical cyclones.

The following links provide additional information about the GFS and GEFS:

TECHs of the forecast aids shown in this plot

The TCGP plotting system is configured to plot the tracks of all the GEFS perturbed members as well as the control member and the mean of the ensemble. For comparison, the track of the higher resolution operational GFS is also shown. Since the ensemble perturbations are independent, there is no particular meaning behind the TECHS of the members (e.g. there is no reason to think that the performance of the AP01 member will be different from that of the AP14 member over a long period of time). If one of the forecast aids listed here does not appear in the plot, this means that it was not available in the source a-deck at the time the plot was created. When a system has been newly designated or upgraded to a tropical depression, it is common to only have a few forecast aids available. The tracks of some GEFS members may terminate early if the vortex tracker can not follow the storm. This typically happens if the modeled storm is too weak or ill-defined.

  • AVNO: NWS Global Forecast System (GFS) model (operational run, T574 resolution which is equivalent to a grid spacing of ~35 km)
  • AC00: control member of the GEFS (T190 truncation out to 180-hours, T126 thereafter)
  • AEMN: mean track of the GEFS ensemble (a simple average of members AP01 through AP20)
  • AP01: +01 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP02: +02 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP03: +03 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP04: +04 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP05: +05 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP06: +06 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP07: +07 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP08: +08 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP09: +09 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP10: +10 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP11: +11 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP12: +12 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP13: +13 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP14: +14 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP15: +15 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP16: +16 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP17: +17 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP18: +18 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP19: +19 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)
  • AP20: +20 member of the GEFS ensemble (T190 out to 180-hours, T126 thereafter)



Frequently asked questions

Often users ask why other forecast aids are not included in the GFS ensemble track plots. This section is meant to answer these questions.

Why don't you include the track projections of other ensemble systems such as the MOGREPS or the ECMWF EPS?

The UK Met Office's MOGREPS ensemble, as well as the ECMWF EPS, are very able ensemble systems for predicting the tracks of tropical cyclones. The centers that run these ensembles restrict access to their constituent member states or to commercial users.

TCGP will gladly include the tracker results of any ensemble system from any modeling center so long as they don't mind us putting their data out into the public domain via our open repository.