Taken from the BBC website., the 2011 plume.
Yesterday I said that the Grímsvötn eruption would not affect air traffic in the UK and Europe. It now appears that the weather conditions indicate that some ash will be transported to the northern areas of Scotland and possibly Ireland as early as Tuesday morning. However the decreasing column height (~10 km) and the large size of the ash being produced tells us that any effect on air traffic (if any) will be small and not as widespread as last year.
This illustrates the tempromental nature of volcanoes, volcanic eruptions and the weather and that nothing should be taken for granted as things can change momentarily!