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 statistical climate of a reference period, including its mean and distribution. Here the hindcast mean defines anomalies and the hindcast distribution defines probability thresholds.
- 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 sampling uncertainties through initial-state differences, start dates or model perturbations. Its spread represents some sources of uncertainty and can be miscalibrated.
- 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, cold and neutral states, involving both ocean and atmosphere. A major source of seasonal predictability. See ENSO and seasonal predictability.
- Hindcast (reforecast)
- Retrospective forecasts using a specified model version for past years. They define model climatology and category thresholds, and can be verified against observations to assess skill.
- Initialization
- The date a forecast run starts. Seasonal centres issue one nominal initialization per month; some assemble it from daily starts.
- Lead time
- The interval from initialization to the valid period. Lead month 1 is the initialization month itself; a three-month window is labelled with the leads of all three months, such as lead months 3–5. 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 subtropical and subpolar North Atlantic, often represented by the Azores–Iceland pressure difference. A major influence on European winters.
- Probability
- A measure of the chance of an event. Here it is estimated from the share of ensemble members in a category, such as the upper tercile. This raw estimate does not establish observed event frequency. See probabilities are not forecast skill.
- Quintile
- One of five nominal 20% categories of a reference distribution. Finite samples and tied thresholds can give unequal frequencies. This site displays the lowest and highest categories.
- Reference period
- The years whose hindcasts define the climatology and thresholds, 1993–2016 for the products shown here. See hindcast climatology and reference period.
- Reliability
- Agreement between forecast probabilities and observed event frequencies across many comparable cases; one aspect of forecast quality.
- Skill
- A measure of how well past forecasts matched observations, relative to a simple reference such as climatology.
- SST
- Sea-surface temperature. Ocean memory can support seasonal predictability, with skill varying by region, season and lead.
- System version
- A C3S identifier matching forecasts to their hindcasts. A new value can reflect a model upgrade, hindcast refresh or processing change; ECMWF system 51 is an interpolation update to SEAS5.
- Teleconnection
- A relationship between climate variations in distant regions. Atmospheric wave responses to tropical heating are one mechanism.
- Tercile
- One of three nominal thirds of a reference distribution: below normal, near normal and above normal. Tied thresholds can make their achieved frequencies unequal. See tercile and quintile probabilities.
- Tercile summary
- A single map showing which outer tercile a model favours where its probability exceeds 40% and beats both other categories. All winning ties and middle-dominant cells remain neutral.
- Valid period
- The month or three-month window a forecast is about. Three-month windows are named by their first month, labelled with both years when they cross the New Year and abbreviated by month initials, as in DJF for December, January and February.
Sources and method
These references explain the underlying science and provider conventions. Descriptions of this site’s calculations, colours and coverage were checked against its processing code and imported data.
- C3S seasonal product definitions
- C3S systems, ensembles and identifiers
- Met Office: reliability and sharpness
- NOAA: ENSO and its ocean–atmosphere components