Models / Explained
The ECMWF EPS ensemble
The ECMWF EPS, also called ENS, is the European global ensemble. It runs 51 forecasts, a control and 50 perturbed members, at about 9 kilometers out to 15 days. Its job is to show how far the outcomes can spread, which is why forecasters read it beside the Euro and the GEFS.
By Weather Decision Solutions. Published . Updated .
At a glance
| Item | Detail |
|---|---|
| Produced by | ECMWF, the European Centre for Medium-Range Weather Forecasts |
| Type | Global ensemble: 51 members (one control plus 50 perturbed) |
| Resolution | About 9 km; the open data 4070 uses is on a 0.25 degree grid |
| Forecast range | 15 days (360 hours) |
| Runs | 00Z and 12Z are the main runs; 06Z and 18Z are shorter supplementary runs |
| Extended range | A separate extended-range ensemble runs to 46 days |
| New runs in 4070 | 00Z and 12Z |
What ECMWF EPS is
ECMWF has run ensembles for three decades. The idea is the same as any ensemble: run the model many times from slightly different starting points, and read the range of results. ECMWF explains that errors in the starting state and in the model both grow with time, and that spread among the members is a measure of predictability and confidence.
The EPS has 51 members and keeps the fine 9 km grid of ECMWF’s deterministic forecast. Since IFS cycle 50r1, in May 2026, the control member is the single deterministic forecast, so the Euro and the ensemble control are the same run.
When you read the EPS in 4070, you are reading a range and not an answer. If the members cluster, the pattern is steady. If they split, the forecast depends on something the model cannot yet decide.
Where it does well
- It measures confidence, and it does so from a different system than the GEFS. Two ensembles agreeing is a stronger claim than one.
- It has 51 members, which gives a fine-grained picture of the spread.
- It keeps a fine grid, so its members carry more detail than a coarser ensemble.
- It supports tools such as plumes and storm tracks, where you can see each member.
Where it falls short
- It runs twice a day in 4070, at 00Z and 12Z, so it updates less often than the GEFS.
- The open data version in 4070 is on a 0.25 degree grid and carries a subset of fields.
- The mean smooths away a feature that members disagree about.
- Spread is model uncertainty, not a probability of what happens at your address.
What it is typically used for
- Judging confidence in the pattern and storm track three to ten days out.
- Comparing the EPS with the GEFS to see whether two ensembles agree.
- Checking a plume of rain, snow or temperature at a location.
- Following tropical systems alongside the GEFS and the AIFS ensemble.
How to see ECMWF EPS in 4070
The EPS appears in Model Explorer and in the ensemble tools, all part of 4070 Pro, which starts with a seven-day free trial.
- Model Explorer has ECMWF EPS Mean/Spread maps for 2 meter temperature, total precipitation, 24-hour precipitation and total snowfall, each as a mean and as a spread, for North America.
- Plumes show the 50 perturbed members at a U.S. point on a 0.25 degree grid.
- Storm Tracks follows developing lows through the GEFS and Euro ensembles, and ensemble storm tracks show ECMWF ENS tracks for tropical systems. These are model runs, and the National Hurricane Center remains the official source.
To read plumes and spread, see ensembles and spaghetti plots.
How it fits with the other models
The EPS is the ensemble partner of the ECMWF IFS, just as GEFS is for the GFS. The AIFS also has an ensemble version, which 4070 shows in Plumes and Storm Tracks. Reading the EPS and the GEFS together is the most useful habit for confidence three to ten days ahead.
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 is the ECMWF EPS?
The EPS, also called ENS, is ECMWF’s global ensemble. It has 51 members, one control and 50 perturbed members, at about 9 kilometers out to 15 days.
How many members does the ECMWF ensemble have?
The ECMWF ensemble has 51 members: a control forecast and 50 perturbed members.
How far out does the ECMWF EPS go?
The ensemble forecasts out to 15 days. ECMWF also runs an extended-range ensemble to 46 days.
What is the difference between the ECMWF EPS and the GEFS?
They are ensembles from different agencies. The EPS has 51 members at a finer grid and runs twice a day in 4070. The GEFS has 31 members and runs four times a day. When both agree, the agreement is stronger than either alone.
What does the spread on the EPS map mean?
It shows how far apart the members are. Narrow spread means higher predictability and wide spread means the forecast is open. It is model uncertainty and not a probability at your address.
Where can I see the ECMWF EPS in 4070?
In Model Explorer as mean and spread maps, in Plumes with 50 members and in Storm Tracks. These tools are part of 4070 Pro, with a seven-day free trial.
Sources
- ECMWF, Medium-range forecasts. ENS has 51 forecasts (control plus 50 perturbed members) to 15 days, about 9 km, with main runs at 00 and 12 UTC.
- 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.
- ECMWF, Quantifying forecast uncertainty. Every forecast carries uncertainty from the starting state and from model approximations, both growing with time; ensembles estimate it.
- ECMWF, Open data. The open data set is 0.25 degree, GRIB2, CC-BY-4.0 licensed, and includes IFS and AIFS output; the deterministic (oper) stream runs to 240 hours at 00 and 12 UTC.
- ECMWF, Implementation of IFS cycle 50r1. Cycle 50r1 became operational May 12, 2026, with fully coupled ocean-atmosphere forecasts and convection changes; HRES and the ensemble control, made identical in cycle 49r1, are no longer run separately.
See ECMWF EPS in 4070.
Model Explorer, Compare Models and Plumes are part of 4070 Pro. Start with seven days free, then $14.95 a month.