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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, which .  This will only be the case in exceptional circumstances.

Example: Information from extended range plumes

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Fig8.2.4.1: Extended Range ENS plume showing the increased spread of forecast values at longer ranges.  The forecast 500hPa values lie between 590dam and 540dam by Day30.  The forecast 850hPa temperature spread is between 2°C and 20°C and the central 50% of ENS values lie between 5C and 11C.  Clearly there can be no confident forecast of values for any individual day.

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Fig8.2.4.2: A series of Extended Range forecasts for 850hPa temperatures made at weekly intervals ( data times 00UTC on 3, 10, 17, 24 July 2017) for Bucharest.  The significant fall in temperature at Days1-4 on the 24 July forecast is a much weaker fall at Days8-12 on the 17 July forecast and is not really discernible at all at longer leads..  The significant rise in temperature at Days 0-5 on the 17 July forecast can still be seen at Days 7-11 on the 10 July 17 July forecast, but is almost indiscernible where it might be expected at Days 14-18 on on the 3 July forecast.  Therefore minor variations at long ranges are worth noting, but it is important to observe their evolution during successive extended range forecasts.  Significant changes may not be clear, even early in the extended range period.   Too much reliance should not be placed on any one feature.  It should also be noted that the scale of temperature on the y-axis is not the same on every plume; it alters according to the spread of the data, so direct comparison can be difficult.

Deductions from MJO diagrams relating to extended range predictability and reliability

Inspection of Hovmoeller and Wheeler-Hendon diagrams can give a guide towards the consistency and intensity of an MJO event.  The distortion of the upper flow associated with an MJO affects monsoon activity and, equally important, modifies the northern and southern hemisphere mid-latitude jets, impacting on predictability of extratropical patterns. This can be expressed in terms of weather regime impacts. 

Some broad remarks can be made:

• The probability of the development of a positive NAO signal is significantly increased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly decreased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
• The probability of the development of a negative NAO signal is significantly decreased about ten days after the MJO is in phase 3 (i.e. enhanced convection over the Indian Ocean), and significantly increased about ten days after the MJO is in phase 6 (i.e. enhanced convection over the Western Pacific and suppressed convection over Indian Ocean).
• The impact of the MJO on two other Euro-Atlantic weather regimes (the Atlantic Ridge and Scandinavian blocking) is much weaker.

Research has shown that in the winter half of the year the reliability of 2m temperature forecasts for Europe is influenced by whether or not there is a substantive MJO at analysis time.  The IFS can model the evolution of a pre-existing MJO quite well and also can also capture the consequent influence at mid-latitude effects.  However, the IFS has trouble generating new, substantive MJOs in the right place, which then impedes the predictive skill when there is no MJO at start time.

Thus the 2m temperature anomaly forecasts for Europe tend to be:

• more reliable if there is a substantive MJO at analysis time, (i.e. lying outside the circle on the Wheeler-Hendon diagram).
• less reliable if there is not a substantive MJO at analysis time.

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:

• capture large-scale circulation patterns that typically last longer than about a week (circulation patterns or regimes are described in this guide).  
• roughly indicate the timing of a change or transition from one circulation type to another.  

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:

• monthly forecast reliability diagrams (for 2m temperature, surface temperature, total precipitation, mean sea level pressure)
• monthly forecast relative operating characteristics (ROC) diagrams (for 2m temperature, surface temperature, total precipitation, mean sea level pressure).

An explanation of reliability diagrams and ROC diagrams is given in the annex to this guide)Forecasters should be able to use these results to good effect when examining the monthly forecasts.