Seasonal forecast glossary
Short definitions of the terms used on seasonal forecast maps: anomaly, ensemble, hindcast, tercile, lead time, initialization, valid period, teleconnection and more.
- Anomaly
- The departure of a forecast from a reference average, here the model’s own 1993–2016 hindcast climatology for the same start month and lead. Positive means above that average.
- Centre
- An institution that runs a seasonal forecasting system: ECMWF, the UK Met Office, Météo-France, DWD and CMCC on this site.
- Climatology
- The long-term average state used as the reference for anomalies. On this site it is always the model’s hindcast climatology, not observations.
- Copernicus C3S
- The Copernicus Climate Change Service, implemented by ECMWF, which collects seasonal forecasts from several centres into one catalogue with common conventions.
- Ensemble
- A set of model runs, typically around 50, started from slightly different initial states. The spread of the ensemble represents forecast uncertainty.
- Ensemble mean
- The average of all ensemble members. See ensemble-mean anomaly.
- ENSO, El Niño, La Niña
- The El Niño–Southern Oscillation, the tropical Pacific’s swing between warm and cold states and the largest source of seasonal predictability. See ENSO and seasonal predictability.
- Hindcast (reforecast)
- Forecasts made with the current model for past years, used to define the model climatology, tercile thresholds and skill.
- Initialization
- The date a forecast run starts. Seasonal centres issue one nominal initialization per month; some assemble it from daily starts.
- Lead time
- Months from initialization to the valid period. Lead 1 is the initialization month itself. See lead time, initialization and valid period.
- Member
- One run within an ensemble.
- NAO
- The North Atlantic Oscillation, the see-saw of pressure between the Azores and Iceland that dominates European winter weather.
- Probability
- The share of ensemble members in a category, such as the upper tercile. It describes the ensemble, not how often the model is right. See probabilities are not forecast skill.
- Quintile
- One of five equally likely bins of the hindcast distribution. The lowest and highest quintiles mark unusually low and high outcomes.
- Reference period
- The years whose hindcasts define the climatology and thresholds, 1993–2016 for all C3S systems. See hindcast climatology and reference period.
- Skill
- A measure of how well past forecasts matched observations, relative to a simple reference such as climatology.
- SST
- Sea-surface temperature. The most predictable seasonal field.
- System version
- The numbered configuration of a centre’s model in the C3S catalogue, such as ECMWF system 51. It changes when the model is upgraded.
- Teleconnection
- A statistical link between weather in distant regions, usually carried by atmospheric waves from tropical heating.
- Tercile
- One of three equally likely bins: below normal, near normal, above normal. See tercile and quintile probabilities.
- Tercile summary
- A single map showing which outer tercile a model favours where its probability exceeds 40%, and neutral elsewhere.
- Valid period
- The month or three-month window a forecast is about. Three-month windows are named by their first month.