How three-month seasonal means are built
Three-month means average the monthly forecasts; three-month probabilities are recalculated from member averages. Why the two are not interchangeable.
Why three months
Seasonal outlooks are usually issued for three-month windows because a single month at long lead is noisy while a season averages some of that noise away. When comparing with another service’s seasonal chart, match its valid window, reference period, variable and product definition.
Each month has equal weight
This site gives each month equal weight, with the window named by its first month. Three-month anomalies are calculated from the forecast and hindcast member means, or from the three imported monthly anomalies. Those approaches are equivalent before rounding when the member selections and reference climate match. This is not a day-weighted average or, for precipitation, a seasonal total. A September to November window from a September run covers leads 1 to 3. A cell needs a value in every constituent month to get a three-month value, so masked cells stay masked.
Probabilities are recalculated, not averaged
Three-month probabilities cannot be produced by averaging the three monthly probabilities. Instead, each forecast member and each hindcast member is averaged over the three months first, new tercile and quintile thresholds are computed from the averaged hindcast members, and the averaged forecast members are counted against them. This is the C3S method, and it means a three-month probability can be larger or smaller than any of its monthly parts.
Available windows
With six monthly leads there are four three-month windows per run. When comparing runs a month apart, the shared windows are those that both runs can complete, which is three. The period picker shows only those. A window that crosses the New Year is labelled with both years, and the workspace also shows the conventional season abbreviation made from the month initials: DJF is December, January and February, SON is September, October and November, and so on. A three-month panel is labelled with the leads of all three months, so a window starting at lead 3 reads “lead months 3–5”.
Both monthly and three-month products use the same colour boundaries for a given field, so switching between them does not change the legend. An equal-weight mean lies between its three constituent anomalies. Opposing monthly departures can cancel; persistent departures need not become weaker than each monthly value.
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.