That's the meteorological explanation: a ridge of high pressure over the West Coast that has been blocking weather from the Pacific, driving it north, leaving the entire West Coast in drought.
But what explains the so-called "Ridiculously Resilient Ridge?".
That we really don't know yet and the climatologists are loathe to speculate. One spot of good news: Paul Rodgers for the San Jose Mercury News discusses the Ridiculously Resilient Ridge brilliantly.