What a seasonal forecast is, and how it differs from a weather forecast
Seasonal forecasts predict the odds of a warmer, cooler, wetter or drier season months ahead. Why they are probabilistic, what they can and cannot tell you.
Weather forecasts predict events; seasonal forecasts predict odds
A weather forecast tells you that rain arrives on Thursday afternoon. Beyond roughly two weeks that kind of statement is impossible, because the atmosphere is chaotic: tiny errors in the starting state grow until the exact sequence of weather is unpredictable. A seasonal forecast does not try. It asks a different question: over the coming month or three-month season, is the average more likely to be warmer or cooler, wetter or drier, than usual?
Where the predictability comes from
Seasonal signal comes from parts of the climate system that change slowly. The ocean stores heat for months, so sea-surface temperature anomalies such as El Niño persist and steer the atmosphere above them. Soil moisture, snow cover, sea ice and the stratosphere add smaller, regional contributions. A seasonal model couples an atmosphere to an ocean and land surface so that these slow components can shape the statistics of the weather it simulates.
Ensembles, not single runs
Because the weather itself is unpredictable at this range, a seasonal centre runs the model many times, typically 50 or so, from slightly different starting states. Each member produces a different season. The forecast is the whole set: its average is the expected departure from normal, and the share of members in each category is the probability of that category. See ensemble-mean anomaly and tercile and quintile probabilities.
What a seasonal forecast can tell you
- Whether the season leans warm or cold, wet or dry, relative to the model’s own climate.
- How strong that lean is, from the size of the anomaly and the probability of the outer terciles.
- Whether several independent models and successive runs agree, which is a rough guide to confidence.
What it cannot tell you
- When within the season the weather happens, or how many cold snaps or storms there will be.
- Conditions for a specific day, week or event.
- Anything with certainty. A 60% probability means the other outcomes still happen 40% of the time.
Seasonal forecasts are issued monthly by centres such as ECMWF, the UK Met Office, Météo-France, DWD and CMCC, and collected by the Copernicus Climate Change Service. This site shows those five models on the same maps so that their agreement and disagreement is visible at a glance.