For seasonal forecast a set of retrospective forecasts (reforecasts or hindcasts) are produced with each version of the forecast system.
Every real-time forecast data needs to be "analyzed" with the help of the relevant set of "hindcast" data, meaning there should be an unequivocal way to know a given forecast and the set of hindcasts come from the same forecast system.
Production schedules
Burst ensembles
Both real-time forecast and hindcast initialize all their members on a given date (e.g. the 1st of the month)
Lagged-start ensembles
Real-time forecasts and hindcasts initialize a number of members over a set of dates. Both the number of members and the dates can be different between real-time forecasts and hindcasts
Examples:
| real-time forecasts | hindcasts | |
|---|---|---|
| NCEP | 4 members every day | 4 members every 5 days (meaning dates are different for each one of the 12 months of the year) |
| JMA | 5 members every day | 5 members on 2 dates 15-days apart (different for each one of the 12 months of the year) |
| UKMO | 2 members every day | 7 members on 4 fixed dates each month (1st, 9th, 17th and 25th) |
| SEAS6 (plans still in discussion) | 101 members on the 1st and on the 16th (+ different lengths: 13 month/24 months for some start dates/members) | 33 members on the 1st and 33 members on the 16th |