The Geophysical Institute at Fukushima University measured gamma and beta radiation at increasing altitude in April through August, 2011. They used a radiosonde (weather balloon) with a radiation measuring device on it to obtain the readings.
The first image is from April 2011, and shows increased beta readings at an altitude of 7-8 km. This is below the level of the jet stream. The second image is from August 27 and shows the beta level off the chart at the jet stream altitude of 9-12 km. We can see that the radionuclides had been rising in the atmosphere from April to August. There is also a significant amount of radioactivity up to a level of 63,000 feet. The maximum level on the chart is 20 cps, which is equivalent to 1200 cpm. This is in contrast to the April chart, which reached 500 cpm.
The third image indicates that the jet stream indeed was over Fukushima City on August 27. None of the scientific articles (that I have seen) that have discussed atmospheric dispersion model results, or measurements of Fukushima emissions, have incorporated this type of measurement, from weather balloons or airplanes. It is likely that a great deal of radioactivity is still in the jet stream, and will not completely come down to earth for years.