Page History

An inspection of Analysis increments can suggest to the user the areas where the  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 , and hopefully, the field.  Hopefully this modification results in an improvement of the field but they .  But the increments do not suggest that a perfect description of the field has been attained.   And in In any case, new data anywhere, unless fully rejected, will anywhere will always induce some, usually slight, changes some changes to the structure of the IFS model atmosphere. These changes are normally small but can amplify through the forecast.

 The influence of the any analysis modification of the analysis by latest data, both good and or bad, often travels faster downstream than the synoptic systems themselves.   For  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  There is also an ever-present influence from the subtropical and tropical latitudes.   In particular, when a subtropical depression, tropical storm or hurricane enters  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 will propagate propagates downstream faster than the ambient winds.  The speed of energy transfer may be considered very roughly as Typically, but very roughly, the energy progresses at about 30° longitude per day.  However, although case studies show that in individual cases 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 such an evolution that may not be adequately resolved by the IFS model and its downstream consequences.  Therefore, if the short-range forecast is initially poor or good over the area of interest, this does not mean that the medium-range forecast for the same area is necessarily poor or good also.  Any attempt to judge the medium-range performance, as distinct from the short-range performance, 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.

Fig43Fig4.A3-1:  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.
Jumpiness or errors in the forecast at Day3 over Europe typically have their origin over the eastern or north-eastern parts of the North American continent.  Similarly, jumpiness or errors at Day5 over Europe often have their origin over north-eastern North Pacific and adjacent coastal regions.  In rare cases, forecast failures at Day7 have been traced back even further.  The Madden-Julian Oscillation can also have a significant downstream effect.  During all seasons, but in particular during the summer and autumn, forecast errors associated with disturbances in the tropics or subtropics  Effects of disturbances at low latitudes or of extra-tropical transition can move into the zonal westerlies (shown schematically by the green arrows).  Extratropical transition of low-latitude weather systems into the mid-latitudes can often cause substantial differences in subsequent evolution and significant forecast variability in forecasts over European areas.