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Extended-Range Forecast

The ENS runs are extended on Mondays and Thursdays from Day15 to Day46 based on 00UTC data time.

The Extended Range (monthly) forecasts bridge the gap between medium-range and seasonal forecasting.  Twice each week, on Mondays and Thursdays, the ENS is extended to run to 46 days ahead at a reduced resolution (currently 36km) but still with land-atmosphere and ocean-atmosphere coupling.  The extended-range forecast provides an overview of the atmospheric evolution for the 32-day period (Day15 to Day46) and the standard ECMWF output products focus mainly on the week-to-week changes in the weather.  The chart products generally are presented in terms of anomalies relative to the extended-range model climate (ER-M-climate) (e.g. periods potentially warmer or colder than average for the time of year), and are mainly shown as 7-day means for calendar weeks Monday to Sunday.  Extended Range forecasts continue to be coupled to the Wave model (ECWAM) and the Dynamic Ocean model (NEMO) run with resolution of ~55km.

Since the ENS and Extended Range are run with different resolutions, the corresponding forecast fields are generated using effectively a different model having the same structure and physics etc, but which are based on different reduced Gaussian grid in physical space, a different land-sea mask, and a different orography.  Users should be aware of this, especially when generating a time-series of products for a single location that crosses the ENS to Extended Range truncation forecast step (e.g. when generating a time-series of a variable or an meteogram spanning Day15), and develop interpolation procedures that take these changes into account.

Extended Range Products

The extended range output is available on the ECMWF web charts and ecCharts. Guidance on the  Interpretation of the extended range products is discussed elsewhere in the guide.

Interface between ENS and Extended Range products at Day15

The Extended Range does not start from Day0 but uses the results at Day14 of the ENS forecast as a starting point.  However, the resolutions of ENS and Extended Range differ, and the high-resolution spectral representations within the ENS have to be smoothed to the lower resolution of the Extended Range.  This is achieved by using a 24-hour overlap period to reduce the impact of this smoothing on the fields that are most sensitive (e.g. convection and large-scale precipitation) so that the changes in forecast parameters between ENS and Extended Range at Day15 is minimised. 

The change in resolution creates incorrect values for accumulated fields (such as precipitation) that run beyond Day15 (i.e. spanning forecast time steps 360‐366 hours).  To reduce the differences, the accumulated fields are reinitialized at the reduced resolution of the Extended Range for Day15 from interpolated fields obtained from ENS forecast using a conserving interpolation scheme.   These fields are stored in Fields Data Base (FDB) and the Meteorological Archival and Retrieval System (MARS) under:

  • ENFO (ENsemble FOrecast stream):  ENS Day0-15. Also Extended Range ENS from Day16-45 at the lower resolution.
  • EFOV (Ensemble Forecast OVerlap stream):  ENS Day15 forecast fields (steps 336-360 hours) interpolated onto the Extended Range resolution.

Similar fields are stored for wave products from WAM.

A list of the overlap fields is available.

Using EFOV fields to obtain a correct accumulation across Day15  

For time ranges that do not span Day15, the normal method of calculation should be used. For example:

  • Accumulation (TPt+72 to TPt+120)   =   (TPt+120 –TPt+72) for the 72 to 120 hour (start of Day3 to end of Day4) total precipitation.  Here the accumulations from ENS are used.
  • Accumulation (TPt+72 to TPt+360)   =   (TPt+360 –TPt+72) for the 72 to 360 hour (start of Day4 to end of Day15) total precipitation.  Here the accumulations from ENS at T+360 may be used.
  • Accumulation (TPt+384 to TPt+432)   =   (TPt+432 –TPt+384) for the 384 to 432 hour (start of Day16 to end of Day17) total precipitation.  Here the accumulations from Extended Range are used.

For time ranges that do span Day15, the calculation should use ENS accumulations etc up to T+360 and then use the ENS accumulation interpolated to the lower resolution of the Extended Range held in EFOV as the base for the rest of the calculation. In other words such calculations have to be split into two parts.  For example, consider total precipitation (TP) for 48‐hours, from 336 to 384 hours (start of Day14 to end of Day16) for one ENS member:

  • Accumulation  (TPt+336 to TPt+384)  =  (TPt+360 – TPt+336) + (TPt+384 –TPVARt+360)

Where:  TP is the ENS field.  TPVAR is the corresponding EFOV TP field.  The subscript indicates the forecast step (hrs).  The value TPt+384 is the total accumulation from T+0 to T+384 which will have used the interpolated field (TPVARt+360) at the T+360 change of resolution. 

Specialised Products

Specialised products for the Extended Range include information on potential tropical cyclone activity and evolution of the Madden-Julian Oscillation (MJO).   The MJO is characterized by an eastward propagation of an area of more organised convection in the tropics, typically initiated over the Indian Ocean and is important as it can influence developments elsewhere on the globe (eg monsoon evolution, tropical cyclogenesis near the Americas, El Nino/La Nina development (ENSO), and affecting aspects of the synoptic pattern over higher northern and southern latitudes including the North Atlantic Oscillation (NAO).  The MJO predictions are skilful well beyond Day20.  The MJO has an influence on the forecast skill over the extra-tropics.  Typically the forecast reliability is enhanced when an MJO event is detected in the forecast initial conditions.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)


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