Models / Explained
The GEFS ensemble
GEFS, the Global Ensemble Forecast System, is NOAA’s ensemble. It runs 31 slightly different forecasts, a control and 30 perturbed members, four times a day out to 16 days, with a 35-day run on the 00Z cycle. Its job is to show the range of outcomes and how much the members agree, which a single run cannot do.
By Weather Decision Solutions. Published . Updated .
At a glance
| Item | Detail |
|---|---|
| Produced by | NOAA, through NCEP and its Environmental Modeling Center |
| Type | Global ensemble: 31 members (one control plus 30 perturbed) |
| Grid spacing | About 25 km (version 12, C384 grid) |
| Forecast range | 16 days on every cycle; 35 days on the 00Z cycle |
| Runs | Four a day: 00Z, 06Z, 12Z and 18Z |
| Current version | GEFSv12, implemented September 23, 2020, coupled to a wave model |
| New runs in 4070 | 00Z, 06Z, 12Z and 18Z |
What GEFS is
A single model run can be right or wrong. An ensemble asks a better question: if the starting conditions had been slightly different, how different would the outcome be? GEFS answers by running the model 31 times, each time with small, deliberate differences. The result is a set of forecasts that you read together.
Where the members agree, the atmosphere is in a predictable state. Where they scatter, the forecast is open. NOAA’s Weather Prediction Center notes that the GFS ensemble mean began to beat the single operational GFS run for 500 mb heights at about day 3.5. The spread is as important as the mean, because it tells you what to be wary of.
GEFS has a coarser grid than the GFS, so it does not draw fine detail. Its strength is in showing consistency across days 3 to 16.
Where it does well
- It measures confidence. Spread shows where the forecast is steady and where it is not.
- It gives probabilities: the chance of 0.25, 0.5 or 1 inch of precipitation, or of ice, or of CAPE above set thresholds.
- It runs four times a day, with a range of 16 days and a 35-day run on the 00Z cycle.
- Its members make spaghetti plots, which are the easiest way to see whether a ridge, trough or storm track clusters.
Where it falls short
- Coarser detail. A 25 km grid cannot draw an individual storm or band.
- The mean smooths. If members disagree about a feature, the mean erases it. Read the mean together with the spread.
- It is one model’s ensemble. WPC notes that a multi-model ensemble may verify better than an ensemble from one model.
- Spread is model uncertainty, not a probability of what will happen at your address.
What it is typically used for
- Judging confidence in a storm track or pattern three to ten days out.
- Seeing whether a dramatic run has company among the members.
- Checking the odds of a given rainfall total, an ice event or strong storms.
- Following tropical systems alongside the ECMWF ensemble.
How to see GEFS in 4070
GEFS appears in several 4070 tools, all part of 4070 Pro, which starts with a seven-day free trial.
- Model Explorer has GEFS spaghetti plots, which draw contours such as 500 mb heights and sea-level pressure for each member, and GEFS mean and spread maps for temperature, precipitation, snowfall, thickness and 500 mb height.
- Probability maps include the chance of 0.25, 0.5 and 1 inch of precipitation, ice of 0.25 inch, and CAPE above 250, 500 and 2000 J/kg.
- Plumes show all 31 members at a U.S. point.
- Storm Tracks follows developing lows through the GEFS and ECMWF ensembles, and ensemble storm tracks follow tropical systems. These are model runs, and the National Hurricane Center remains the official source.
To read the spread, see ensembles and spaghetti plots.
How it fits with the other models
GEFS is the ensemble partner of the GFS. The ECMWF EPS is the European ensemble, and two ensembles from different centers that agree are stronger evidence than either alone. The NBM blends many models into one calibrated forecast. Add the AI-GFS for a machine-learning opinion.
Specifications change when agencies upgrade their models. The Sources below link to the producing agency. Official National Weather Service forecasts, watches and warnings remain the authority for what is expected where you live.
Common questions
What does GEFS stand for?
GEFS stands for Global Ensemble Forecast System. It is NOAA’s ensemble, with 31 members, run four times a day.
How many members does the GEFS have?
The current version has 31 members: one control run and 30 perturbed members.
How far out does the GEFS go?
Out to 16 days on every cycle, and to 35 days on the 00Z cycle.
What is the difference between the GEFS and the GFS?
The GFS is one run, called deterministic. GEFS runs 31 slightly different versions, so it shows how much the forecast can change. GEFS uses a coarser grid than the GFS.
What does ensemble spread tell me?
Spread shows how far apart the members are. Narrow spread means higher predictability, and wide spread means the forecast is still open. It is a measure of model uncertainty and not a chance of an outcome at your address.
Where can I see the GEFS in 4070?
In Model Explorer as spaghetti plots and as mean and spread maps, in Plumes with all 31 members, and in Storm Tracks. These tools are part of 4070 Pro, with a seven-day free trial.
Sources
- NCEP EMC, Global Ensemble Forecast System. GEFSv12: 31 members (30 perturbed and one control), about 25 km, four runs a day, 16 days on every cycle and 35 days on the 00Z cycle.
- NOAA WPC, Ensemble training. An ensemble is two or more forecasts verifying at the same time; the high-resolution control is the best member only about 5 to 7 percent of the time; the GFS ensemble mean began to beat the operational GFS for 500 mb heights at about day 3.5; a multi-model ensemble may verify better than one from a single model.
- ECMWF, 30 years of ensemble forecasting. The atmosphere is chaotic, so no forecast system can be perfect; ensemble spread is a measure of predictability and forecast confidence.
See GEFS in 4070.
Model Explorer, Compare Models and Plumes are part of 4070 Pro. Start with seven days free, then $14.95 a month.