I have a background in air quality meteorology, and had a look at the weather patterns past two days over the region of interest. It does broadly fit the pattern of a long-distance source because the area has been under a sprawling high pressure area that is increasingly coming under the influence of mid-level westerly winds from the Pacific after being more in a northerly arctic flow (that’s why it was so cold earlier this month in the Great Basin region).
When inversions slowly break down, whatever’s trapped in the levels just above the surface can slowly mix down to the surface. The only other plausible explanation for the pattern seen on the map would be a moderate release of radiation from the Hanford WA nuclear power station.
I have the feeling that this Japanese nuclear problem is somewhere between a crisis (as speculated on some websites) and no big deal (as our governments are telling us). It is a situation that is similar to the low-level increases in radiation seen after Chernobyl at relatively large distances and people are right to be concerned about it.
Tepco just admitted that most of the fuel in Reactor #1 escaped the reactor pressure vessel RPV and entered the containment vessel CV. And they admitted that more fuel then they had thought has escaped the RPV of #3. So two melt outs confirmed now by Tepco.