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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 will only be the case in exceptional circumstances.

Example: Information from extended range plumes

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. 

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