Long-Range Forecast Trajectories of Volcanic Ash from Redoubt Volcano Eruptions

The Redoubt Volcano in Alaska began a series of eruptions on 14 December 1989. Volcanic ash was often reported to reach heights where, as it moved with the upper-level flow, it could affect aircraft operations thousands of km from the eruption. In an agreement between the National Oceanic and Atmospheric Administration (NOAA) and the Federal Aviation Administration, the NOAA Air Resources Laboratory (ARL) was assigned responsibility for providing long-range forecast trajectories of volcanic ash during a volcanic hazards alert. An ARL immediate-response program was implemented for the Redoubt Volcano eruptions. The response products, in the form of tables, maps, and written messages are discussed. An evaluation of the forecast trajectories is included. The evaluation is based on after-the-fact trajectories from analyzed wind fields and on actual ash cloud sightings. For 90% of the cases verified at 300 mb, the average forecast error was less than 25% of the downwind distance from the eruption (this often included distances beyond 5000 km). At 500 mb, the forecast error was less than 50%. Errors were inversely proportional to wind speeds.