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.  

The sub-seasonal range forecast ensemble:

    • is run once daily:
      • Day0 to Day46 based on 00UTC data and results are disseminated to Day 46.
    • has horizontal resolution of 36 Km and vertical resolution of 137 model levels.
    • has land-atmosphere and ocean-atmosphere coupling.
    • has 100 ensemble members plus an unperturbed ensemble control member.
    • runs with data analysed from the Short cut off data analysis (SCDA).
    • dissemination schedule:

Section 8.2 Sub-seasonal range output gives a description of sub-seasonal forecast products.

The perturbed members are similar to the unperturbed control forecast member but differ only that initial states and model physics have been perturbed.   This allows exploration of the currently understood range of uncertainty in the observations and the model forecasts. 

A set of sub-seasonal range re-forecasts provide a basis for the sub-seasonal range model climate (SUBS-M-climate).  

Standard output products focus mainly on the week-to-week changes in the weather.  Chart products generally are presented in terms of anomalies relative to the sub-seasonal range model climate (SUBS-M-climate).  The anomalies are mainly shown as 7-day means for calendar weeks Monday to Sunday.   From these one can infer if weather conditions (e.g. temperature, rainfall, etc) are likely to deviate from the average for a location and time of year, and whether such an anomaly could be extreme.

Sub-seasonal range forecasts are coupled to the Wave model (ECWAM) with resolution of 36km and the Dynamic Ocean model (NEMO) run with resolution of ~28km.

Information of available sub-seasonal range ensemble forecast output is given in greater detail.  Some are additional products are only available by arrangement.

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:

  • monsoon evolution.
  • tropical cyclogenesis near the Americas.
  • El Niño/La Nina development (ENSO).
  • aspects of the synoptic pattern over higher northern and southern latitudes including the North Atlantic Oscillation (NAO). 

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)



(FUG associated with Cy50r1)