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Table of Contents

Extended-Range Forecast

The extended range (monthly) forecasts provide a range of possible future weather states in the extended range.  It bridges the gap between medium-range and seasonal forecasting. 

The extended range forecast ensemble:

  • is run once daily, base time 00 UTC, producing a 32 day forecast (Day15 to Day46). 
  • has horizontal resolution of 36 Km and vertical resolution of 137 model levels.  These values are the same as the IFS version before Cy48r1.
  • has land-atmosphere and ocean-atmosphere coupling.
  • has 100 ensemble members plus an unperturbed ensemble member (CONTROL).

The ensemble consists of one unperturbed control member (the control) and 100 perturbed members.  These are similar to the control 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.   A set of extended range re-forecasts provide a basis for the extended-range model climate (ER-M-climate).  The extended range forecast suite is not an extension of the medium-range forecasts but is a completely separate system.  

The extended-range forecast provides an overview of the atmospheric evolution for the 32-day period (Day15 to Day46).  Standard 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).  The anomalies are mainly shown as 7-day means for calendar weeks Monday to Sunday.  From these one can infer whether 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.

Extended Range forecasts are coupled to the Wave model (ECWAM) and the Dynamic Ocean model (NEMO) run with resolution of ~28km.

The medium range and extended range models have the same structure and physics etc. but different spacial (but not vertical) resolutions and differing land-sea masks and orography.  Users should be aware that the output is effectively from different models.  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 extended range.  Users need to 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.

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.  It is important as it 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). 

 The MJO has an influence on the forecast skill over the extra-tropics and MJO predictions are skilful well beyond Day20..  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)