Section 7 (ENS Products - Dealing with Uncertainty), and particularly Section 7.5 (Broad Guidelines), give suggestions on how to handle ensemble results from a series of forecast model runs.  Especially, it is unwise to follow a trend in forecast values.  This is because a change in evolution, due perhaps to a change in analysed upstream flow, can radically reposition location of features downstream (e.g. the position or development of a trough).  However, model forecasts can show increasing confidence in the range of forecast values as each model run becomes closer to the verifying time.


Fig9.2.1.5-1: Forecast evolution plot for 2-metre temperature around London-Heathrow VT 12UTC 26 May 2026.


Things to note in the temperature plot (Fig9.2.1.5-1):  

  • IFS ensemble mean (black diamonds), AIFS ENS control (red circles), AIFS Single (cyan circles) are quite erratic during the period 10 to 16 May.  IFS ensemble mean extrapolation suggests reaching about 26C on 26 May.  IFS control and AIFS Single are particularly variable and no trend can be seen.  It is not until about 20 May that a sharper more steady but still erratic rise can be seen.  AIFS Single shows very high temperatures.  Temperature forecasts are generally more steady after 22 May.
  • Box and whisker ranges (IFS blue AIFS ENS brown) are rather large implying low confidence until about 20 May.  Some ensemble members show temperatures below 15C while at the same time the upper extremity of the whisker is broadly about 27C for IFS, 30C for AIFS ENS.  Clearly there is uncertainty.  After 20 May the whiskers, boxes and range become smaller which allows greater confidence.
  • No ensemble or single results should be discarded.   High forecast temperatures even from forecasts made as early as 10 May cannot be ruled out and this might be useful early information for heat sensitive customers even though with low probability.  Equally, temperatures as low as 10-15C are possible until about 16 May, even with some support from the lower bound of the boxes, and this possibility might be useful to customers even though this proved to be very incorrect.
  • The latest model results cannot be taken as completely the best one without reference to earlier results from earlier model runs.  Confidence can grow as the box and whisker plots become shorter.


Fig9.2.1.5-2:  Evolution plot showing 3-day average temperature for Reading 24-26 May 2026 in forecasts.  The plots show the variation of normalised ensemble mean and standard deviation for Sub-seasonal, IFS and AIFS ensembles leading up to the high temperatures well above climatology on 26 May.  The sub-seasonal ensemble mean anomaly normalised with the standard deviation of the sub-seasonal model climate (pink, solid) and the normalised ensemble spread (pink, dashed).  IFS-ENS (red) and AIFS-ENS (blue) are normalised with the IFS model climate.  

For very long lead-times in sub-seasonal forecasts there is no indication of the coming large temperature anomaly.  The normalised ensemble mean anomaly is close to 0 and the normalised ensemble spread close to 1.  Around 15 days before the event the ensemble mean anomaly starts to increase and crossing the ensemble spread 11 days before the event.  The anomaly plateaued around day 7-9 before steadily increasing again and finally reaching an amplitude of more than 4 standard deviations. The ensemble spread remained large for longer and started to decrease around day 8.  (This could be a symptom of the possibility of having a large anomaly).  The signal for very high temperatures started to appear around 9-10 May (about 15 days before the event) and gradually gained strength.

AIFS-ENS was ahead of IFS-ENS in growth of the signal in the ensemble mean.  It also kept an higher ensemble spread for longer.  


(FUG associated with Cy50r1)

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