We've detailed some of the different scenarios below.
Scenario 1a -- Storms Coming From Northwest
Scenario 1b -- Fraser Winds And Overrunning
Scenario 2 -- Dry Air And Evaporative Cooling
Scenario 3 -- Heavy Showers Drag Down The Snow Levels
Scenario 4 -- Upslope Winds
Scenario 5 -- Timing Is Everything
Scenario 6 -- Strait Effect Snow
By far the easiest is to just have freezing temperatures in place when moisture arrives. If it's around 34 degrees or colder when the moisture arrives, it'll snow (Yes, it can be a couple of degrees above freezing and snow).
Within this scenario, there are a few ways this pattern can set up.
We have a storm track coming from the northwest that's bringing colder air and moisture from Alaska. This generally leads to a fringe-type snow event. The amount of snow in this situation depends on how far out over the ocean the storm goes as it slides southeast along the British Columbia coast.
If the storm stays inland along the coast, it will generally stay colder, but won't pick up as much moisture, so there won't be much oomph to the system.
If it tracks out further over the water, it'll tend to pick up more moisture from the ocean, but the flip side is, that ocean water is still 45-50 degrees, so it tends to moderate the cold air involved, leaving us with a wetter, but warmer system, and likely nothing more than a rain/snow mix or snow showers only down to 500-1000 feet.
The near-perfect snow scenario in this case is to have the system track over
land for most of the way on its trip along the Coastline, then curve out over
the water as it reaches the northern tip of Vancouver Island, and then come
inland around the Central Washington Coast or Northern Oregon (more on why
landfall there is important in a minute.)
That way, it stays cooler until the last moment, when it picks up some moisture over the Pacific right before it makes landfall, but doesn't spend enough time over the Pacific to warm up that much.
This scenario requires cold, arctic air to move south out of B.C. and Alberta to where it is parked just on the other side of the Cascades. Then, lower pressure moves in offshore, which will draw that arctic air southwest through the Fraser River Valley and into Bellingham and, eventually, the rest of Western Washington (think of it as an icy blow dryer.)
This scenario is the cause of almost all of our bouts with arctic air. Depending on the strength of the lower pressure offshore and the intensity of the arctic air, this can cause roaring winds of 50-70 mph in northwestern Whatcom County along with temperatures in the teens and lower 20s, combining for wind chills near or below zero. For the rest of Western Washington, this leads to usually the coldest temperatures of the year.
(Although we should point out, cold air doesn’t always have to come from the Fraser River Valley. It can come from the due north and northwest as well, but the Fraser River is the most common source.)
At this point, all it takes is any kind of moisture moving in to make it snow, since we're all likely below freezing. If it's a general warm Pacific-born system moving in out of the west or southwest, this is a scenario called "overrunning". Here, it starts as snow for everyone as the warm, moist air overruns the cold, dense air near the surface and the moisture turns to snow, or stays as snow, but then gradually turns to rain as the cold air scours out.
But! If we can combine Scenario 1a with Scenario 1b, now you've got yourself a snowstorm!
In that case, if you have the cold storm system moving south along the B.C. Coast, then along the Washington coast, *and* you have the large pool of arctic air in B.C., you have the double whammy. The storm will be the source of moisture and the low pressure pulling the arctic air from B.C. into Western Washington. If the storm heads into southern Washington or northern Oregon, but is close enough to spread moisture far enough north into Western Washington, now you've got a constant source of cold air and a ample supply of moisture.
That is what happened during the big snowstorm if Dec. 26, 29 and 30 in 1996.
One caveat though, if that storm moves inland over Western Washington or north of the state, now you're on the side of the low pressure where it draws in warmer marine air from the southwest and southeast, which will mitigate the arctic Fraser flow. It still might start as snow if there was some cold air already in place, but will quickly change back to rain and follow more closely to Scenario 1b.
And on the other hand, if you have a weak to moderate Fraser wind blowing, it might be enough to cause localized snow in the northwestern Whatcom County area (where the Fraser winds are keeping them around freezing) but rain everywhere else as the winds quickly warm as they spread out across the area.
Scenario 1c -- The Arctic Front
Also related to 1b -- Sometimes when the arctic air moves through Western Washington from the northeast and/or through the Fraser Valley, it can bring along an arctic front (since it's got cold air behind it much like a cold front), which is generally weak in regards to precipitation, but can dump a quick inch or two of snow, then freeze it solid in the front's arctic air wake.
This scenario can combine with 1b above when the arctic air isn't all that strong and still provide a decent snowfall.
Evaporative cooling is when moisture first falls from the clouds and into a layer of very dry air. Since the air has plenty of room to hold the moisture, the rain or snow will evaporate into water vapor. However, evaporating water requires heat and energy. As more and more drops evaporate, the air is using up more of its heat and energy, thus making the air cooler.
You can get a sense if this is possible by looking at the current temperature and dew point. (Dew points are the temperature at which the air would be saturated. See more about the Dew Point here. You can also find the current temperature and dew point at this link.)
If you take the mid point of the dew point and temperature, and it’s 32 degrees or less, then the precipitation could begin as snow. (For example, if it’s 40 degrees with a 20 dew point, that’s a midpoint of 30 degrees. That's because the temperature drops as the air cools, but as the air gains more moisture, the dew point rises. They generally meet in the middle.)
Have you ever noticed in spring or summer how just before, and right after it begins to rain, it cools off quickly. Sometimes heavy showers can bring along some cold air for the ride -- and in the winter, it can bring an unexpected snow.
Let's say it's 40 degrees on the ground in Seattle, but it's 25 degrees at the
base of a cloud sitting overhead. If that clouds lets loose and sends down a
heavy barrage of moisture, that initial burst of snow then rapidly melts into
the warmer air. But as we mentioned above in Scenario 2, melting snow requires
energy, so that intense melting actually zaps heat energy from the air,
effectively making the air colder and in some cases, cold enough to snow.
This effect is usually temporary, with temperatures then bouncing back up after the showers end, and they can be quite isolated. Also, how far the temperature drops is dependent on the strength of the showers. So one heavy shower could bring the snow level down to the surface and dump a quick 1-2 inches of snow, and perhaps the next weaker shower just drops the temperature a little and it stays as rain.
A Convergence Zone (See more about them here.) can enhance this effect in the south Snohomish/north King County areas as these tend to bring intense showers, along with the added bonus of that area generally being at 500 feet.
Also, this occurs somewhat frequently along the eastern slopes of the Olympics, where winds race up the eastern slopes of the mountain, then cool and condense into rain or snow (see more below in Scenario 4) and can increase the intensity of the storm enough to bring down the snow levels to the surface.
Then, of course, some areas have unique factors that help it to snow there.
Another general factor that can bring increased rain or snow is winds going up the side of a mountain range, which can then act like a sponge. As air rises, it cools and condenses, squeezing out its moisture on the windward side of the foothills.
This can bring enhanced snowfall to the Cascade foothills whenever we have a strong west to northwest wind coming along with our snow event.
But many times with our snow events, we have that cool air coming from the northeast out of the Fraser River valley and eastern Washington. In this case, you now have upslope winds going up the eastern and southeastern side of the Olympics.
That scenario usually brings heavier snowfall to the area along the Hood Canal, western Kitsap Peninsula, and Mason County (including Shelton, Brinnon, and Hoodsport.)
Upslope is also part of the "Strait Effect" snow listed below in Scenario 6.
Another one of those "drive weather forecaster nuts" scenarios. This is when we have moisture moving in at a very specific time frame -- say 3-6 a.m. For this to occur, you need it to be clear early in the night and the eventual clouds to hold off until at least after midnight, which would allow the day's warmth to radiate back into space and temperatures to fall to near freezing.
Then, when the clouds and eventual moisture move in, you're stuck at 31 or 32 degrees and now it can snow.
But as we said in the heading, timing is everything. If the clouds move in too soon (like, say 8-10 p.m.), they'll act like a blanket, keeping in some of the day's warmth and likely not allowing temperatures to get cold enough to snow.
If the clouds and moisture drag their feet and arrive too late (like 7-10 a.m.), now you've got sunlight warming us up and likely keeping it as rain.
So it's just a narrow window for snow in this case, and it's gotta arrive right on time.
This one's pretty rare and only affects a narrow area of the Northern Olympic Peninsula from, say, Port Angeles east to Port Townsend, but mainly the Olympic foothills between the Straits and the mountains.
But it's the same meteorology as the commonly-heard "Lake Effect" snows in the Great Lake areas. (More info on Great Lake effect snows here)
In this case, you have the strong northeast winds blowing out of the Fraser River valley southwest across the northern Puget Sound and San Juans and then into the northeastern side of the Olympic Peninsula.
As the winds cross the Sound, they pick up a little bit of moisture. As they slam into the Olympic Mountains, the air rises and condenses, dropping its newly-found moisture as snow.
This is along the same lines as the Upslope snow mentioned in Scenario 4, only it has it's own snow-mechanism going.
A Complex Pattern
Of course, it's not just either/or with these scenarios. Sometimes they can work in conjunction to enhance snowfall in certain areas. And then there's other factors, such as elevation, that can also vary snow totals.
So as you can see, forecasting snow around here is quite the challenge!