Radiation levels at Half Moon Bay in California are registering at 8 times their normal levels.
After the Fukushima disaster occurred, there were predictions that a radioactive ocean plume would hit the west coast of the U.S. sometime in 2014. This could be that drifting plume.
Another cause of this could be from a large plume of steam that Tepco, the company managing the Fukushima disaster, stated came from reactor 3 last month. At the same time the US government ordered 14 million doses of potassium iodine.
Radiation get stuck in the atmosphere and comes down with rain and snow.