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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output where shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Basic ensemble products

Basic products display only the raw ensemble forecast data, without any particular modification or post-processing. 

Postage stamps

Postage Stamps” (or "Postage Stamps Maps or Charts") show the individual forecast charts of MSLP and 850hPa temperature of all the ensemble members, including the ensemble control.  For ease of visual comparison these are shown all together.  They cover a limited area of the globe, normally Europe and eastern North Atlantic.  The charts are intended to be used for reference (e.g. to explain the spread in terms of the synoptic developments and, in particular, the reasons for extreme weather etc).

It might seem attractive to identify the member which verifies best in the early part of the forecast (say at T+12) and assume this member will continue to provide the best forecast during the rest of the medium range period.  But this is not true; the performance of any member during the first 12hrs of the forecast has little relevance to its skill beyond T+48hrs in the same area.

Clustering

In order to compress the amount of information being produced by the ensemble and highlight the most predictable parts, individual ensemble members that are "similar" according to some measure (or norm) can be grouped together.   This process is known as clustering.  

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Fig8.1.1-3: Clustering or the case shown in Fig8.1.1-1.  The three clusters for T+144hr are in the left column.  Clustering is based on 500hPa geopotential height pattern.  

Spaghetti diagrams

Spaghetti diagrams are available on ecCharts.  The charts display isolines from each ensemble member for MSLP, or 500hPa geopotential height, or 850hPa temperature.  The isoline values are selected by the user (e.g. 1015hPa, or 546dam, or 0°C).  The isolines are drawn for each member.  At short lead times the isolines are very tightly packed for forecasts because the spread of the ensembles is quite limited.  As the forecast progress they become increasingly spread showing the flow-dependent increase in forecast uncertainty.   Spaghetti diagrams are sensitive to gradients; in areas of weak gradient they can show a large spread of the isolines, even if the situation is highly predictable.  Conversely, in areas of strong gradient they can display a small spread of the isolines, even if there are important forecast variations.

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Fig8.1.1-5: Spaghetti Plot 500hPa geopotential height plotted at 560dam DT 12Z 26 May 2017, VT T+144hr at 01 June 2017.  The ensemble members (grey) have become more spread out, but retain the general pattern of an upper ridge over the northwest Atlantic and another over the North Sea but there are differences in position (or speed of movement eastwards) and amplitude.  The control ensemble member is shown in red.

Ensemble mean

The ensemble mean (EM) forecast is a simple average of the ensemble results.  It is an effective product because averaging reduces or removes atmospheric features that differ amongst the members.

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The ensemble mean is most suited to parameters like temperature and pressure, which usually have a rather symmetric Gaussian distribution at each point.  In the short range the ensemble mean is very similar to the ensemble control due to the symmetry (equal positive and negative) of the initial perturbations.   

Ensemble median

The ensemble median is defined as the value of the middle ensemble member where the members have been ranked by value.   Ensemble median might be more suitable than the ensemble mean for wind speeds and precipitation as these have skewed distributions.

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Fig8.1.1-7: As Fig8.1.1-5 but with the spread of mean sea level pressure (PMSL) by ensemble members (coloured - orange: high spread, blue: low spread).  Highest spread of PMSL is on the eastern side of the depression indicating greater uncertainty in the strength of a southerly wind.  There is lower spread to the west where most members suggest a fairly low pressure and higher probability of a northerly flow.  The smallest spread is near the centre of the depression indicated by the ensemble mean but wind direction is very uncertain here; it depends upon the positioning of the low in the of individual ensemble members.  Chart taken from ecCharts.

 Probabilities

The most consistent way to convey forecast uncertainty information is by the probability of the occurrence of an event.  The event can be general or user-specific regarding probability of exceeding an event threshold.  The event threshold may correspond to the point at which the user has to take some action to mitigate potential damage from a significant weather event.   Probabilities can be:

• instantaneous (e.g. probability 10m wind speed exceeds 20m/s as a given time),
• calculated over a time interval (e.g. probability precipitation exceeds 50mm during a defined 12 hour period).  This is possible because the values are themselves originally computed as values accumulated over some (shorter) time interval. 

Probability of precipitation

Probability of precipitation (PoP) totals include all precipitation types (rain, snow, etc. but not hail) in mm of rainfall or rainfall equivalent falling in 6 hour or 12 hour periods using colour shading.   As a rough guide 1 mm rainfall equivalent approximates to 1 cm of snowfall.

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Fig8.1.1-8: Probability of precipitation chart showing probability of total precipitation (large scale precipitation plus convective precipitation) exceeding 1mm during the 6 hours preceding the validity time.

Probability of extreme gusts

Probabilities for extreme wind gusts are computed as probabilities over 24 hours because it is considered more important to know that an extreme wind gust might occur than to know the precise time.

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Fig8.1.1-9: As Fig8.1.1-7 with the probability of wind ≥10m/s.  There is a higher probability (dark blue) in the area west of Ireland where the pressure gradient is uncertain although the direction is fairly certain.  The ensemble mean (Fig8.1.1-6) would not suggest such strong winds.  There is very low probability (white) south of Greenland where the gradient is generally light but the direction is uncertain.   Chart taken from ecCharts.  Light blue >25%, Blue >50%, Dark blue >75% probability.

Probability of combined events

Additionally, ecCharts can display the probability of a combination of events occurring together.  For example:

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Fig8.1.1-12: Chart taken from ecCharts showing ensemble probability of wind gust ≥10m/s and ≥2mm/12hr (ecCharts colour bands for this scheme denote Light blue 5-35%, Blue 35-65%, Dark blue 65-95%, Purple >95%).  There is a 31% probability of exceeding the thresholds at Munich (shown in the box).  The location of Munich is shown by the pin.

Forecast expressed in terms of intervals

Forecast intervals (e.g. “temperatures between 2°C and 5°C”, or “precipitation between 5 and 8mm/24hr”) can be used as a hybrid between categorical and probabilistic forecasts.  ecCharts provide a simple way of displaying probabilities above or below thresholds and by intercomparison can give a indication of probability of a parameter lying between the thresholds.   For example for maximum temperatures at Vilnius, (see Fig8.1.1-13) there is a 20% probability of being ≥20°C, and from Fig8.1.1-14 there is a 25% probability of being ≤15°C.  Therefore there is a 55% probability that the maximum temperature will lie between these two values.  Combinations of parameters are possible (e.g. the probability of combined events of wind gust and total snowfall is available on ecCharts as an aid to forecasting drifting of snow).

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Fig8.1.1-14: Chart taken from ecCharts showing ensemble probability of maximum 2m temperature ≤15°C during a 12hr interval (ecCharts colour bands for this scheme denote 5%-20%-40%-60%-80%-95%-100%).  There is a 25% probability of maximum temperatures ≤15°C at Vilnius (shown in the box).  The location of Vilnius is shown by the pin.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Watch a detailed theoretical presentation regarding Statistical Post-Processing of Ensemble Forecasts (30sec delay before start). 

(FUG Associated with Cy49r1)