“High concentrations” of radiation hit US and Canada — Plume was rich in Cesium-137 and “close to the surface” from Vancouver southward — See also Hawaii, Florida (MAPS)

SOURCE: Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant, Stohl, A., Seibert, P., Wotawa, G., Arnold, D., Burkhart, J. F., Eckhardt, S., Tapia, C., Vargas, A., and Yasunari, T. J., October 20, 2011
Here are some excerpts concerning North America [Emphasis Added]:

“Already on 15 March, a first isolated 133Xe cloud reached western North America, followed by the arrival of high concentrations of both 133Xe and 137Cs on 19 March.”
“The main part of the radioactive plume entered western North America on 17–18 March. On 18 March at 12:00UTC, the head of the plume had already arrived over the North Atlantic, but the main part was located over the eastern Pacific Ocean and western North America, where it could be detected at monitoring sites. This part of the plume was also rich in 137Cs, as it was still close to the surface south of 50 [Most of US/Canada border is 49°]. At the same time, the plume penetrated the subtropics and arrived at Hawaii on 19 March.”
“A map of the simulated surface concentrations of 133Xe for 22 March shows that all of western North America was engulfed by the FD-NPP plume, as well as parts of eastern North America and eastern Asia.”