Mt. Baker has a geographic funnel that shoots wet ocean air directly at the ridge that the ski area is on. When the air rises it makes a lot of snow.
The geographic track is from the Pacific, down the Straight of Juan de Fuca, up the Skagit River, and then the Baker River where it gets squeezed between Mt. Shuksan and Mt. Baker, and is forced into the ridge made up of Shuksan Arm, Austin Pass, and Table Mountain. Austin Pass is inbounds at the ski area; the center of the vortex.
Stevens Pass Ski Area has two common meteorological patterns that I notice regularly effecting snowfall. Stevens has the fifth highest snowfall of ski areas in North America, so it's significant.
The first one is when cold air from the interior of the continent meets warm, wet air coming in off of the Pacific. As the ocean air rises up to cross the Cascade Mountains it not only is cooled by altitude gain, but also by hitting the continental air mass. Because it's cold and dense, the continental air slides under the warmer ocean air, cooling it even more and causing it to dump lots of snow. Many times Stevens is the only only one having snow while everyone else has rain because of this, and it also makes the snow better on average than other regional places on the west slope.
The second one is called the Convergent Zone, which is a localized effect that moves around the central Puget Sound region. Stevens Pass is located dead east of the Olympic Mountains which are on the west side of Puget Sound. The Olympics are roughly a square which creates the distinctive shape of the NW corner of the state of WA. The air from the ocean flows in around the south and north ends of the Olympic Peninsula and often during a wet weather cycle, those winds meet in the middle on the east side of the mountains. When this happens there can be some very localized severe weather. Stevens Pass is in the area where this happens, and when the Convergent Zone sits on the pass, there can be epic snowfall of sometimes great quality.