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

• subtropical depressions, tropical storms or hurricanes enter the mid-latitude westerlies (extra.   Extra-tropical transitions ) orcan 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 .

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.

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.  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 particularly in summer and autumn, forecast  Forecast errors associated with disturbances in the tropics or subtropics can move into the zonal westerlies (shown schematically by the green arrows) .  Extra-tropical 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 areasin any season, but particularly in summer and autumn, .