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As an airline pilot I always check the VAFTADs for Colima and Popoctptl whenever I fly Toronto-Mexico City, however I'm a little uncertain as to just how much gravity I should treat them with. If I was flying, say, from Miami to Mexico City at 18000ft can you tell me just what I might encounter if VAFTAD has a big red "blotch" along my route? Should I be planning to avoid it altogether or is the ash likely to be too dissipated to cause a problem?

You raise a good point. We know that for some volcanoes, VAFTAD overpredicts the spatial extent of the plume. This likely occurs because of steam in the plume or other uncertainties in the source. For a real volcano, based on satellite imagery, a "reduced ash" version is run if it would be more appropriate. The "hypothetical eruptions" web page ( http://www.arl.noaa.gov/ready-bin/vaftadtst.pl) has VAFTAD output from both the full-ash case and a "reduced ash" case. The differences in these two products indicate some uncertainty in the forecast based on the uncertainty in the ash/steam mix in the source. Typically for a large eruption (greater than about 30000 ft) there are only small differences between the reduced and standard run, for smaller eruptions there is a greater difference.

Barbara Stunder