GEFS vs ECMWF ENS
GEFS or ECMWF ENS: which ensemble should I use?
Both run many slightly different forecasts to show how uncertain the weather is. GEFS (NOAA) has 31 members and is fully open. ECMWF ENS has 51 members on a finer grid and is generally regarded as the more skilful ensemble, especially beyond a few days.
| GEFS | ECMWF ENS | |
|---|---|---|
| Provider | NOAA / NCEP | ECMWF |
| Type | Ensemble | Ensemble |
| Area | Global | Global |
| Grid size | about 25 km | about 9 km |
| Updates | 4 times a day | 4 times a day |
| Looks ahead | 16 days | 15 days |
| Data licence | Public domain (US Government) | CC-BY-4.0 (open data) |
Choose GEFS when
- You want a free ensemble with simple access.
- You want to compare against the ECMWF ensemble.
- You are looking at US-centred probabilities.
Choose ECMWF ENS when
- You want the ensemble with the best reputation.
- You are planning 4 to 10 days ahead.
- You want to see how wide the range of outcomes is.
On the water
An ensemble tells you how much to trust the single forecast. A tight spread means confidence, a wide spread means keep options open.
Figures come from the Ilma catalog and the providers' own pages. Forecast skill changes as models are upgraded, so this page describes what each model is built for, not a ranking.