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Sub-seasonal forecast

The output is rather different from the shorter period output of medium range ensemble products.  It provides a more general overview of the forecast up to Day 46.

The sub-seasonal forecast suite consists of an ensemble of 100 members plus one control member and is run every day based only upon 00UTC data.  The output focuses mainly on the week-to-week changes in the weather rather than attempting to give unsupportable detail on individual days.  It is presented as forecast 7-day Monday to Sunday means of the anomalies relative to SUBS-M-climate for the corresponding Monday to Sunday forecast week.   

Specialised products for the sub-seasonal range also include information on potential tropical cyclone activity and the Madden-Julian Oscillation (MJO) during the coming month.

Sub-seasonal forecast output

Starting each Monday, a Monday-Sunday forecast period is maintained by all the sub-seasonal forecasts made each day during the following seven days.   Thus the products of each sub-seasonal forecast may be easily compared with previous forecasts.  The same Monday-Sunday periods are also used to produce re-forecast products.

It is presented as forecast 7-day Monday to Sunday means of the anomalies, relative to SUBS-M-climate, for the corresponding Monday to Sunday forecast week.   

Differing forecast periods are selected from each sub-synoptic run to keep the 7 day forecast period constant at Monday-Sunday to allow easy comparison from run to run (See Table 1)

There are no forecasts for:

• Week 0 on days other than Mondays because the sub-seasonal forecast period is less than a full seven days.
• Week 6 on Mondays, Tuesdays and Wednesdays because the sub-seasonal forecast extends only to Day 46.

Table 1: The Monday - Sunday period is preserved by starting the Monday-Sunday forecast period at a time step 24 hours earlier on each sequential forecast run.  See Table 2 for an illustrative example.

Table 2: Illustrative example of sub-synoptic output based on a week in February 2025.  The Monday - Sunday period is preserved by using an earlier time step to start the Monday-Sunday forecast period on each sequential forecast run.  See Table 1.

Fig8.2-1: Example sequence of sub-seasonal forecasts.  The same Mon-Sun 7 day period is output from each sub-seasonal forecast run for ease of comparison.  In the example (Mon 10 to Sun 17 March 2025), Europe is consistently shown with a warm anomaly against SUBS-M-climate.  However, the runs on Thu, Fri and Sat indicate much warmer in Central Europe but the run on 9 Mar shows less warmth.

Forecast resolution

• 0.4° x 0.4° lat/long grid or any multiple thereof (global or sub-area)
• On model (Octahedral) O320 grid (global or sub-area)
• Spectral components (T_CO319) for upper-air fields (global area only)

Availability

Sub-seasonal range output is available:

• at 20:00 UTC for forecast products each day.
• at 21:00 UTC for re-forecasts on 1/3/5/7/9/11/13/15/17/19/21/23/25/27/29/31 (excluding 29 February) of each month.

Additional sources of information

More information on products available from the atmospheric model ensemble, sub-seasonal forecast (Set VI - ENS sub-seasonal) are available on the ECMWF website.

(FUG Associated with Cy49r1)