Those dreaming of a White Christmas in Seattle had to be jealous of their friends out west in the Hood Canal are. A week after getting anywhere from 4-10" in a snowstorm last Wednesday, that area is one again on the hook for several inches of snow on Christmas Day.
Meanwhile, Seattle gets lucky to spread around a dusting here and there, despite being only about 40 miles east of that snow wonderland.
How? You can thank/blame the Olympic Mountains.
It's an opposite effect of the Olympic Rain Shadow where the mountains typically prevent precipitation from reaching the greater Sequim "metropolis" area (I say that as someone who lived near there when there was one traffic light).
In our usual fall rainy pattern with southwesterly flow aloft, air ramps up the southwestern side of the Olympic Mountains. That lift creates condensation and essentially wrings out the air's moisture in heavy rains -- the source of the Olympic Rain Forest that get over 200 inches of rain a year.
But once the air crosses the summit, it sinks down the northeastern side of the mountains, which has an opposite drying effect -- creating a zone of clear, dry weather over what happens to be Sequim.
For these Hood Canal snow events, we essentially spin the weather pattern around about 130 degrees to the east. In this case, very high pressure builds in Eastern Washington due to cold, dense air in place. As the lower pressure of storm approaches -- like what is happening Christmas Eve night -- east winds will race from Eastern Washington through the Cascade Mountains (thus the warnings of 40-50 mph winds there Christmas Day), across the Puget Sound area and then slam into the Olympic Mountains.
But as the winds approach the Olympics -- we get lift, which creates enhanced precipitation along the eastern slopes of the Olympic Mountains -- that Hood Canal area -- with gradually lesser effects as you go east from there -- why Central Kitsap might get some snow but not as much, and why Kingston sometimes misses out, and Edmonds always misses out (at least due to the east wind effect)
The enhanced precipitation is they key to the snow because when we are in a marginal lowland snow situation with snow levels flirting just above ground level, a heavier shower can bring the snow levels down a few hundred feet due to a process called evaporative cooling. This is caused by the process of precipitation falling into drier air and getting evaporated. That evaporation takes energy from the air to do so and in turn, cools the air. And that process takes a marginal snow event into a... regular snow event.
And thus how that Hood Canal snow engine works.
This map shows the forecasted snow for Christmas Day but also paints a great illustration of where the Hood Canal snow belt typically lies: