Submitted by Bruce Hecht:

Portland, Oregon is known for many things; coffee, hiking, biking, snow skiing, water sports, walking, bike riding and great homes for great prices!  We are not known for our snow storms, however, once in a while when the following sets up and the stars are aligned, we will get some snow.  When we do... be prepared.  Here are the reasons why we sometimes end up with an unexpected snowstorm:
 
The perfect set up for snow in Portland starts with offshore flow of cold dry Arctic air coming down from the Gorge, from the east.   It begins spreading  through the Willamette Valley. Then, a storm comes in from the west with moisture, but the center of the storm stays south of Portland so  the cold offshore wind  is maintained. We can get several inches very quickly over a broad area under that set up. It can also change into a freezing rain situation really fast, as the cold air  at the surface is eroded from above by the warmer  winds from the west.

I personally hope each Winter for a good snow storm, but I usually don't get excited about a lowland valley snowfall when we're still dealing with onshore flow. That means the winds are coming in from the ocean, which has a warming influence on cold air masses from the north. That influence may only reach 500 feet or so above sea level, but usually, with onshore winds, it's enough to keep most of Portland Oregon from seeing widespread snowfall. Last  time, after the first front passed, the air wasn't cold enough and the moisture wasn't solid enough to deliver a big snow storm. Sure, there were areas that received some, but  more folks saw it above 500 ft and those below were mostly rain showers. The difference being that showers are smaller in  coverage, and while they can produce intense precipitation: rain or snow with hail, they tend to be short-lived and don't impact the whole area.
 
But we still have 30+ days to try to get more snow!  Keep your fingers crossed!