Overview

Extended range products provide only a general overview of forecasts out to Day46; they focus mainly on the week-to-week changes.  They give a broad-brush outline of likely weather rather than day-to-day definition, and no confidence should be placed on any smaller scale detail that might be indicated, particularly at longer forecast ranges on a single forecast run.  Only if there is a consistent signal of an identified feature from one forecast run to the next may any cautious reliance be placed on the indicated change.  Equally the amplitude of a feature will not necessarily increase with each successive extended range forecast; it may become less evident with time.  A broad trend may sometimes be evident providing useful information on general conditions (e.g. a gradual increase in temperatures or a lessening of rainfall).  However the user should not be tempted to extrapolate such trends unless they have very good reasons so to do.  This will only be the case in exceptional circumstances.

Significance and confidence

Significance

The anomaly, as shown on some of the extended range ensemble products, is the difference between the ensemble weekly mean of the real-time forecast and the corresponding ensemble weekly mean of the ER-M-climate.  On web charts, the regions where the significance of this anomaly (difference) is:

  • greater than 99% are delimited by a dashed contour (blue: wetter, colder or lower pressure; red: drier, warmer or higher pressure).
  • greater than 90% are coloured (blue: wetter, colder or lower pressure; red: drier, warmer or higher pressure).
  • less than 90% are left blank (taken as not significantly different from ER-M-climate).

Circulation patterns

A major goal of Extended Range forecasting is prediction, well in advance, of persistent, anomalous large scale circulation patterns that themselves can lead to severe weather events:

  • Strong, persistent large-scale high pressure systems are often associated with dry spells, with heat waves in summer and cold spells in winter.
  • Strong persistent quasi-zonal flow can be associated with very wet periods.

Extended range, sub-seasonal forecasts cannot be expected to accurately represent day-to-day weather variations.  However, they should be able to:

High confidence in forecasts of a circulation pattern over the fairly large region may not imply high confidence in forecasts of the associated surface weather in all parts of the area.   Factors that need to be considered are:

  • Whether the circulation applies to the whole region or whether a different flow is likely near the edge(s).  Confident winter-time forecast circulation can have a large spread in forecasts of 2m temperature due to atmospheric thermal gradients shifted away from their climatological positions.  
  • Atmospheric dynamical processes (Rossby wave propagations, weather regimes, etc).
  • Land Surface conditions: Snow cover, Soil moisture.
  • Ocean conditions: Sea-surface temperature, Sea ice.
  • Stratospheric initial conditions.
  • Teleconnections, particularly those resulting from the Madden-Julian oscillation (MJO), have a significant impact on the forecast skill scores.

Confidence

On extended range web charts the confidence is "over-stated", because reliability diagrams consistently show that forecasts are over-confident.  Thus where a high probability of a particular type of event is predicted, the "true" chances of that event happening are actually less than the probability that is shown. 

A particular consideration when using the precipitation charts is that the range of colours available for "below average" anomalies (red colours) is constrained by the local climatology in the ER-M-climate.  For example if all ensemble members showed a dry week, the mean (-ve) anomaly could be no larger in magnitude than the mean in the ER-M-climate.  So in some locations the strongest dry signal one can ever see will only be in the first or the second of the red shades.

Anomaly charts (500hPa N Hem anomaly and Global and Regional anomaly) show how extended range forecasts performed in the recent past in the lead up to a particular week.  The anomaly of one week of forecasts (relative to the ER-M-climate) is compared with the weekly mean anomalies (relative to the ER-M-climate) of recent extended range forecasts verifying over the same period.   They give information about the consistency between the monthly forecasts from one week to another.

Users can also reference metrics that describe how skill tends to vary in different scenarios.

Verification

The extended range products should always be used with historical skill metrics in mind.

Some broad indications of ensemble performance and predictability in the extended range should be taken into account when considering the extended range output.  Users are strongly encouraged to consult verification information:

An explanation of reliability diagrams and ROC diagrams is given in the annex to this guide).