Analysis increments charts indicate areas where the analysis scheme may not have completely captured the true state of the atmosphere.  They show where observations have led to a modification to the field.  Hopefully this modification results in an improvement.  But the increments do not suggest that a perfect description of the field has been attained.   In any case, new data anywhere will always induce some changes to the structure of the IFS model atmosphere. These changes are normally small but can amplify through the forecast.


 The influence of any analysis modification by latest data, good or bad, often travels faster downstream than the synoptic systems themselves.  For example, for forecasts over Europe:

  • a two-day forecast may be affected by initial conditions over most of the North Atlantic.
  • a five-day forecast may be affected by the initial conditions over the North American continent and easternmost North Pacific.


There is also an ever-present influence from the subtropical and tropical latitudes.  In particular where:

  • subtropical depressions, tropical storms or hurricanes enter the mid-latitude westerlies.   Extra-tropical transitions can happen in any season, but particularly in summer and autumn.
  • large areas of severe convection modify the upper flow (e.g. as in MCS over the United States).

Extra-tropical transition of low-latitude weather systems into the mid-latitudes can often cause substantial differences in subsequent evolution.  This can cause significant variability in forecasts over European areas. 


The energy of the distorted flow propagates downstream faster than the ambient winds.  Typically, but very roughly, the energy progresses at about 30° longitude per day.  However, case studies show that the value can be very different in individual cases .


Jumpiness or errors in the forecast over Europe:

  • at Day3 typically have their origin over the east or northeast parts of North America.
  • at Day5 often have their origin over northeast North Pacific and adjacent coastal regions.
  • at Day7, in rare cases, forecast failures have been traced back even further.

The Madden-Julian Oscillation can also have a significant downstream effect.  Forecast errors associated with disturbances in the tropics or subtropics can move into the zonal westerlies in any season, but particularly in summer and autumn.


It is important to assess the potential influence of any inadequately resolved features and the subsequent downstream evolution and consequences.  A judgement of the medium-range performance should be based upon large upstream areas and also involve the upper-air flow.  A good or poor short-range forecast in an area does not mean the medium-range forecast there is necessarily similar.


Fig4.3-1:  Schematic illustration of the typical propagation of forecast errors over the northern hemisphere towards Europe in situations with generally zonal flow.  The errors propagate mainly along the storm track, which during the warm season, on average, is displaced polewards. Note however that in individual cases the propagation speed can vary greatly from what is shown here.  Effects of disturbances at low latitudes or of extra-tropical transition can move into the zonal westerlies (shown schematically by green arrows).