What concerns me is if the computer model is inaccurate as it appears to have been in this case, could it get it wrong the opposite way round and end up with aircraft flying through an ash cloud not predicted.
From Eurocontrol :
"These charts enable States to establish NO FLY zones or other restrictions where necessary. Ongoing work by the UK Met Office and the UK CAA has confirmed the effectiveness of the model used to determine the areas where ash concentration could be above engine tolerance levels. For that reason, the 60 nautical mile "Buffer Zone" which was initially added has been removed from charts published as from 11-1200 UTC. States retain the prerogative to add the 60nm buffer if they believe it is necessary to do so. "
So apparently the model is good enough to remove the 60nm buffer zone, but so faulty that it closed a large area around London yesterday...
Really hope that the VAAC computer is set to "Over cautious"-mode and not just calculating wrongly as green granite is wondering too...