Since EFAS version 5.4 (XXX 2025) and GloFAS v4.3 (YYY 2025), the sub-seasonal and seasonal products have been redesigned, based on the same methodology and styling across both EFAS and GloFAS and both sub-seasonal and seasonal.

Sub-seasonal product configuration design

The new sub-seasonal products rely on few major contributing components:

1. Real time forecasts: This part is the hydrological forecasts produced in real time. This will give the actual conditions for the sub-seasonal products. In the following we describe the characteristics of these forecast simulations. Where appropriate, the difference between EFAS and GloFAS is specified. If there is no EFAS/GloFAS mentioned, then the method is identical between the two systems:

• • Hydrological model: LISFLOOD (LINK!)
  • Meteorological forcing: the combination of the 9km (horizontal resolution) ECMWF ensemble forecasts (LINK!) and the 36km ECMWF sub-seasonal forecasts (LINK!) from the previous run, both with with 51 ensemble members (one unperturbed and 50 perturbed) to represent the equally likely forecast scenarios of the future.
    • The low-resolution sub-seasonal forcing is taken from the precious available forecast run simply because of timing, as the sub-seasonal meteo forecasts for the same day are only available with several hours delay after the high-resolution forecasts. 
    • The high- and low-resolution forcing is currently combined by a simple mechanical blending by using first the high resolution meteorological forcing for days 1-15, while the low-resolution meteorological forcing for days 16-46, for each of the 51 ensemble members. 
    • The blending can create inconsistencies locally over very complex orograpical areas, due to the resolution change at day15, and also there could be some smaller inconsistencies between the ensemble in the 1-15 vs 16-46 days, due to the mechanical blending which will mix different weather conditions from the low- and high-res meteo data, but overall as an ensemble of 51 scenarios, this will not expected to lead to any larger discontinuities. 
  • River resolution: 3arcmin (~5km) in GloFAS and 1arcmin (~1.5 km) in EFAS
  • Run frequency: Forecasts are generated daily for every 00 UTC
  • Lead time: 46 days (1104 hours)
  • Forecast steps: 6-hourly in EFAS and 24-hourly in GloFAS
  • Forecast hydrological initialisation: From a fillup simulation forced with the shortest-range ENS-Control (unperturbed member of the ECMWF ENS) meteorological conditions

1. Reforecasts: The sub-seasonal products rely on the This part is the hydrological forecasts produced in real time. This will give the actual conditions for the sub-seasonal products. In the following we describe the characteristics of these forecast simulations. Where appropriate, the difference between EFAS and GloFAS is specified. If there is no EFAS/GloFAS mentioned, then the method is identical between the two systems:

• • Hydrolo

Sub-seasonal system

The sub-seasonal system forecasts are use the 

• • General: It should be based on the ECMWF ENS forecasts, primarily the 46-day, 101-member ENS-ext, produced for every 00 UTC, and potentially also the 15-day, 51-member higher resolution ENS (when blending).
  • Time frequency: We should use weekly means. We will have 6 lead times for the Monday, Thursday, Friday, Saturday and Sunday sub-seasonal runs, but only 5 for the Tuesday and Wednesday runs (as the 46-day lead time is not enough to cover 6 weeks).
  • Blending: Do we want to blend the low-res and high-res ensembles, or only use the low-res? We do blending in GloFAS currently. The blending can create inconsistencies locally due to the resolution change, and also there could be some smaller inconsistencies due to the mechanical 1-1 ensemble merging after day15, but these were deemed acceptable in a recent study (https://confluence.smhi.tds.tieto.com/display/ECC/48r1+impact+study+on+river+discharge). With the blending we can only have 51 ensemble members though, which again should be OK. In addition, the reforecasts seem to have the blending configuration for both EFAS and GloFAS, which makes it rather easy or maybe the only feasibly option for now to go for the blending.

• Seasonal:
  • General: It should be based on the SEAS5 data that runs out every month to 7 month with 51 ensemble members in the real time forecasts. So, we should really utilise the whole 7-month period in both EFAS and GloFAS.
  • Time frequency: We should use the (calendar) months as periods (not weeks as currently). It is really something that in every other external example is like that. We need to switch to monthly for sure. For each seasonal run, we would have 7 monthly lead times. 
  • Blending: Again, the question of blending. As, currently there is no blending considered in either reforecasts of real time seasonal. So, as the reforecasts are not using blending, so we should not consider thm for real time anyway. In the long term there will be this question though.