The reason for this prediction was clear--a weak to moderate La Nina would be in place for most of the cool season, which often leads to such a wet north/dry south pattern.
But reality turned out very different. The upper level flow pattern was dominated by a low-pressure anomaly (difference from normal) over the Pacific Northwest, that drove the jet stream into California and colder, but drier, air into the Northwest (see 500 hPa upper level height anomaly below, the purple area represents much lower than normal heights/pressures).
Lets look at the actual precipitation reaching the ground over the western U.S. The percentage of average precipitation for the last three months over the region (see below) clearly shows drier than normal conditions over Washington State and northern Oregon, but a way wet situation over California (150% and more!).
Looking at the actual difference in precipitation from normal (not percent, but inches), some parts of the Sierra Mountains and coastal range of California are more than 20 inches above normal---amazing. A real drought buster.
What about the next week? Over the next few days, a high-amplitude ridge of high pressure will build along the West Coast, drying out the entire region (see below).
The result, as illustrated by the UW WRF model precipitation totals for the 72 h ending 4 AM Friday, is lots of rain to the south of Washington State. The saga continues.
Finally, this winter's busted long-range forecast is ANOTHER good illustration that my profession has only minimal skill for our seasonal predictions. Last year did not work out well either. Clearly, a subject suitable for lots of research.