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Ensemble: 'Meteogram'

This is an example of a 'Plume' or 'Meteogram' (or Meteogramme in German), that is available on the Wetterzentrale site. For each site, selected from the 'drop-down' menu, output from the Ensemble run of the NCEP GFS model can be viewed, specifically the temperature at 850 hPa ("850 hPa Temperatur") and 12hr accumulated precipitation ("Niederschlag"). [ Although not stated on the page, I assume the precipitation is rainfall-equivalent, as there is no allowance for explicit phase-change within the diagram. ]

Example Meteogram for Manchester

AT THE TOP OF THE DIAGRAM (TEMPERATURE AT 850 hPa):-
The thick red line shows the 30 year average value.
The thick blue line shows the output from the Control Run of the Ensemble model (lower resolution than the operational!)
The thick white line shows the ensemble mean (i.e., very roughly, an 'eyeball' guide to the trend over the period).
The thin differently coloured lines are individual ensemble members (11).

AT THE BASE OF THE DIAGRAM (PRECIPITATION TOTAL FOR PREVIOUS 12HR):-
The thick white line shows the ensemble mean.
The thin differently coloured lines are the individual ensemble members.

Cautionary notes:
+ ensembles are best used to detect future trends rather than to define the precise outcome on a particular day, particularly beyond about 3 days.
+ where the ensemble members cluster tightly together (as they should in the first couple of days of any output), then confidence in the outcome will be high.
+ where many ensemble members cluster tightly together, with only a single 'outlier' to either side, then confidence in the mean of the dominant cluster can be regarded as reasonably high (but not a 'perfect' solution).
+ the greater the 'spread of the fan' as forecast lead time increases, the more uncertain is the outcome. This will be the normal pattern in mid-latitudes.

[ For more on Ensembles, Operational runs, Control runs etc., see the Glossary ]