El Nino will produce a warm NW winter. It wont be continuous warmth, but in the end, mainly lower elevations will have about 20% less snowfall. Recall, El Nino is an area of central Pacific Ocean temperatures, which are warmer than normal. But that warmth just doesnt migrate our way.
Here is what happens. The tropical warmth produces more thundershowers near the equator with El Nino. That creates a disturbed air circulation, which reaches northward to affect the position of the storm track in the NW. That modified storm track cuts off the coldest air from the north, making us warmer that normal. There is also a component of mild north Pacific air tending to come our way, because of this change in circulation.
As a result, its not directly El Nino warmth - but the cutting off the extreme cold spells from the north which warms us up.