Observing and Forecasting Motions of Volcanic Emissions Shortly After the Initial Mt. St. Helens Eruptions


Roland R. Draxler


NOAA Technical Memorandum ERL ARL-95, February 1981


Abstract - The May 18, 1980 eruptions of Mt. St. Helens injected a considerable amount of ash into the atmosphere.  Staff at NOAA's Air Resources Laboratories prepared estimates of the path of the ash at various pressure levels. Forecast trajectory calculations were made for isobaric surfaces rather than isentropic to permit the forecasts to be updated more promptly by manually drawn post-facto trajectories.  A summary of the post-facto isobaric trajectories for various levels is presented in three figures representing the lower troposphere (to 3 km), upper troposphere (10 12 km), and lower stratosphere (to 18 km).  Additional information confirming the ash path is derived from regular 3-hr surface weather observations of blowing dust and smoke.  NOAA satellite photographs also showed the eastward progression of the ash in the upper troposphere.  Illustrations showing the estimated arrival and duration of ash as it crossed the U.S. are presented.  A global map illustrates the path of the 200 mb trajectory making a complete circuit by 31 May.


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