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Sub-seasonal range forecasts

The sub-seasonal range ensemble forecast suite provides a range of possible future weather states beyond the medium range forecasts.   It bridges the gap between medium-range and seasonal forecasting.  The sub-seasonal range forecast provides an overview of the atmospheric evolution for the 32-day period (Day15 to Day46).   It is not just an extension of the medium-range forecasts but is a completely separate system.  

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Note: The term "Sub-seasonal range" has replaced the term "Extended Range".   The term "Extended Range"is now obsolete.

Specialised products

Specialised products for the sub-seasonal 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.  This has an influence on the forecast skill over the extra-tropics.   The MJO can influence developments elsewhere on the globe:

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Typically the forecast reliability is enhanced when an MJO event is detected in the forecast initial conditions.  MJO predictions are skilful well beyond Day20.    

Differences between Medium range and Sub-synoptic range models

The medium range and sub-seasonal range models have the same structure and physics etc.  But they have different horizontal (but not vertical) resolutions and have differing land-sea masks and orography.   Users should be aware that the output is effectively from different models.  The sub-seasonal range forecasts are not just an extension of the medium-range forecasts but are completely separate systems each starting from their own analyses.  There are two sets of re-forecasts, one for the medium range and one for the sub-seasonal range.  This is especially important if a time-series crosses the boundary between the end of the ensemble 15 day forecast and the start of the sub-seasonal range.  Users need to develop interpolation procedures that take these changes into account.

Additional sources of information

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

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