The Icelandic eruptions of Eyjafjallajökull 2010 and Grímsvötn 2011 caused major air traffic disruptions in Europe and beyond which had global repercussions. Aviation authorities and airlines required more timely and specific information on the horizontal and vertical extent as well as concentration of volcanic ash in the atmosphere. The Volcanic Ash Advisory Centers (VAACs) are responsible for using all available data (e.g. remote sensing, observational data) in conjunction with dispersion model output and forecaster skill to predict horizontal and vertical extent of the ash plume. Volcanic ash over a very dense network of air traffic routes and facilities over Europe combined with a high volume of air traffic placed increasing pressures on the airline industry who have been advised not to operate in volcanic ash. This pressure, in turn, was placed on the world's VAACs to re-examine their policies and procedures on the provision of volcanic ash services. New ash concentration charts were produced by the London VAAC during both the Eyjafjallajökull and Grímsvötn eruptions in an attempt to distinguish between non-hazard and hazard zones. This approach was borne out of crisis mitigation efforts in Europe to make airspace available during such an event. The global “standard” prior to this was zero tolerance, in other words, avoid all visible ash. The definition of exactly what visible ash is continues to be debated even today.

There are many forecasting challenges faced by the world's nine VAACs and the most recent Icelandic eruptions not only have caused some of these challenges to resurface, but has also raised some new and equally difficult questions particularly regarding what if any ash is considered "safe" to fly through. This presentation will discuss some of forecasting challenges and point to some possible future enhancements in volcanic ash forecasting.