Using EPA Radnet data, I've identified periods of high radiation levels in southern, US west-coast cities (excluding initial releases in spring 2011):
PERIOD 1: October 2011 – January 2012 (this period was documented as a high release period in Japan by Japanese scientists - see here)
PERIOD 2: November 2013 – January 2014
PERIOD 3: November 2014 - present
These periods cannot be explained by seasonal variation alone or plant refueling in the US alone because levels were not elevated during the fall/winter of October 2012 – January 2013.
The jet stream is lower over the US in the fall/winter period so higher radiation levels could be partly from Fukushima, although its possible that refueling at Palo Verde NPP in AZ and Diablo Canyon NPP in CA are playing a role in producing the elevated winter radiation in southern west coast cities. However, as noted above, beta and gamma levels were not elevated in the fall of 2012 so there must be other factors at issue.
High Radiation Levels Winter October 2011 – Jan 2012. Beta levels decline spring 2012:
Low Radiation Levels June 30 2012 – October 28 2012:
Radiation levels start rising again fall of 2013, reaching high December 2013 and remaining high in January 2014:
Levels reaching highs again Nov 2014 through December 2014: