Page History

...

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.  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 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.

...