Release dates:
ECMWF:  on the 6th of each month at 12UTC
All other forecast systems:  on the 10th of each month at 12UTC

Starting from November 2022 a new version of ECMWF SEAS5 output has been introduced in C3S. This new version (v5.1) has been labelled with the keyword system=51 

• What are the changes in this new version of ECMWF SEAS5 data?
  • A different interpolation tool has been used to produce the 1-degree grid, both for hindcasts and real-time forecasts.
  • A new 1-degree grid, now consistent with the grid available from the other participating forecast systems. Specifically, it will have grid-points centered at half-degree latitude/longitude, and hence it has 180 values in the latitude coordinate (from -89.5 to 89.5) instead of the current 181 values (from -90 to 90)
  • A couple of additions to complete the list of available variables: Top solar incoming radiation and 1000hPa level for the 5 variables available at pressure levels. Additionally, on top of the values of runoff, for SEAS5.1 both sub-surface and surface runoff will be also available.

• What are the implications for users of ECMWF SEAS5 data? 
  •  From November 2022, only system=51 real-time forecast data will be available from the CDS
  •  Users will need to use system=51 hindcast data alongside the new real-time forecasts.
    Data from the current version (system=5 ) should not be used, so those users with previously downloaded hindcast data will need to download the new version.

• Why have these changes been introduced?
  • When ECMWF SEAS5 was released in 2017, the C3S 1-degree version was created with the default interpolation tool (emoslib ) available from MARS. With the migration of ECMWF computing infrastructure to its new Data Centre in Bologna, this tool has been discontinued. That means new real-time forecasts produced after the migration can't be produced anymore in a way consistent with the available SEAS5 hindcast in the CDS. After careful evaluation of the impact of the interpolation, it was decided to create a new version of the hindcast data. This has been labelled as SEAS5.1 and encoded with keyword system=51.
  • With this new version of SEAS5 hindcast, a couple of additional improvements were also included:
    • A new 1-degree grid consistent with the one provided for other forecast systems (see details above)
    • A couple of additions to the list of variables (see details above).

*
Currently these data are temporarily only available using CDS API and may appear as (partially or totally) unavailable in the CDS interactive forms.

Detailed information about the dates available as hindcasts or real-time forecasts for each individual forecast system can be found in Start dates available in the CDS per forecast system

For systems with lagged ensembles having multiple start dates, the "nominal" start date is assigned as follows:

Météo-France (all systems):

• Forecasts/Hindcasts: nominal month M includes the members initialised in the last weeks of month M-1 and on the 1st of month M

NOTE: Even though the different members of all Météo-France forecasting system ensembles are initialized using a lagged approach, the real start dates are not revealed to the users, so all members are encoded as if they were initialized on the 1st of the month

UK MetOffice (all systems):

• Forecasts: nominal month M includes the members initialised on the 1st of month M and all available members with start dates back to, and including, the 2nd of month M-1
  • Note that even though just the most recent 50 members are used for C3S products, the complete set of members are available through the CDS
• Hindcasts: nominal month M includes the members initialised on the 1st of month M and on the 25th, 17th and 9th of month M-1

NCEP:

• Forecasts: nominal month M includes the members initialised on the 1st of month M and all available members with start dates back to, and including, the 2nd of month M-1
  • Note that even though just the most recent 52 members are used for C3S products, the complete set of members are available through the CDS
• Hindcasts: nominal month M includes the members initialised on the 1st of month M (if available) and on all start days from month M-1 except the 1st (see table below)

JMA:

• Forecasts: nominal month M includes the members initialised on the 1st of month M (if available) and on all start days from month M-1 except the 1st
  • Note that even though only a subset using the most recent members is used for C3S products, the complete set of members is available through the CDS
    • For CPS2 the subset includes 52 members (4 start dates)
    • For CPS3 the subset includes 55 members (11 start dates)
• Hindcasts: nominal month M includes the members initialised on the 2 available start dates from month M-1

See details about available dates from JMA at section 6 (Other relevant information) of the forecasting system description.

Bureau of Meteorology:

• Forecasts: nominal month M includes the members initialised on the 1st of month M and on the last 10 days of month M-1
  • Note that even though just the most recent 55 members are used for C3S products, the complete set of members are available through the CDS
• Hindcasts: nominal month M includes the members initialised on the 1st of month M and on the last 8 days of month M-1
  • Note that for nominal March, both non-leap years and leap years use the same 8 days of February (i.e. 29th February on leap years is not used)

_{This document has been produced in the context of the Copernicus Climate Change Service (C3S).}

_{The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.}

_{The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.}