Every snow event, it's the same thing -- we weather forecasters toss out all these caveats about how it might snow more here, and might not snow as much here, and if this and that happens, you might get more or less snow.
But the area's terrain is so complex, coupled with the fact that with our mild winter climate, cold events typically barely get us down to near freezing, and it's an incredible challenge.
How much of a challenge? That's where this blog entry comes in.
Let's take the typical winter snow event around here. If we took these atmospheric conditions and were someplace flat with little change in topography and no water -- like say, Oklahoma -- it'd be about 35-36 degrees across the region, and probably a straight rain -- maybe a rain/snow mix.
But you can see we're just a couple of degrees away from freezing -- get us down to even 33 or 34, and you can get an accumulating snow if it's heavy enough.
And there's many ways for some spots to get those bonus couple of degrees cooling. Let look at some scenarios:
Under normal circumstances, the temperature drops roughly 3-5 degrees per 1,000 feet of elevation (more toward 3 degrees in humid conditions, and closer to 5 in drier conditions).
So being at 400-600 feet elevation, which many locations are around here such as the Seattle hilltops, the Cascade foothills, and the Alderwood Mall/South Everett area, will get you about 2 degrees colder than it would be at sea level in Seattle. That explains why many times if you drive I-5 from Seattle to Everett, it's bare in Seattle, and increasingly higher snow totals once you get to Shoreline (about 375-400 feet) up through south Everett (500-550 feet.)
That's also why there's typically more snow, say, atop Admiral Hill in West Seattle than just a couple miles away down on Alki Beach below, which many times is also bare.
(Check your elevation here.)
Proximity to Water:
The water temperatures of Puget Sound in winter are typically around 50 degrees. (Lake Washington is usually a skosh cooler, but not significantly so.) That keeps areas near the water cooler in the summer, but in winter, and especially during a snow event, it acts like a bit of an electric blanket. That's good for a degree or two of warming right on the shorelines.
Thus, areas near water get a double no-snow whammy of being at a lower elevation, plus standing next to a big source of "heat".
Air flowing up or down mountains can increase or decrease snow as well. As air rises, it condenses and squeezes out its moisture. So if you're in a mountain foothill area where the wind is blowing right at the mountain, you can get an enhanced area of snow.
This is why the Hood Canal area and those spots along the southeastern and eastern side of the Olympic Mountains are such a snow belt. Typically in cold air situations, we have a wind blowing from the northeast or east. (Cold air is denser and has higher pressure than warm air. When we have the big pools of arctic air in B.C. and/or Eastern Washington, or a storm passing offshore to our west and southwest (or both), that air typically pushes toward the west through the gaps in the Fraser Valley and Cascade passes. As that air rushes across the Puget Sound area out toward the ocean, it'll ram into the Olympic Mountains, where it'll rise and squeeze out its moisture, depositing heavy snow right along those foothills. Areas that get snow from this scenario include Shelton, Brinnon, Hoodsport, Belfair and Quilcene.)
On the other hand, when it's a cool, westerly flow, you can get this upslope snow over in the Cascade foothills. (A westerly flow is milder though, since it comes off the ocean, so unless it's a really cold air mass, this scenario usually doesn't translate to a big snow maker for the lower Cascade foothills. Instead, it's your higher elevation that mainly contributes to more snow over there.)
In the same realm, a cold north or northeasterly wind can cause this similar upslope snow along the northeastern side of the Olympic Mountains, affecting the foothills behind Port Angeles east through Sequim and over to Chimacum.
OK, so we've gone through how the topography can jiggle your temperature a degree or three to make it snow or not snow, let's add in some common atmospheric quirks that can also tweak a temperature.
The most common one is how the intensity of a precipitation event can drop temperatures for a brief period.
In winter, when precipitation moves into an area and begins to fall, it comes out of the cloud as snow, as it is typically well below freezing at the height of the cloud base. As that snow falls toward the ground, it encounters the warmer air at lower elevations and begins to melt.
Here, two factors come into play. If the air had a low humidity before the snow arrived, some of the first burst of precipitation will evaporate into the drier air. But the process of evaporation requires energy, which is "stolen" from the surrounding air, to over-simply atmospheric physics. As that energy is taken from the air, the temperature drops. (Temperature is actually a measure of average heat energy in the atmosphere.)
Once the air eats up enough moisture to where the humidity is closer to 100%, the evaporating process slows, but if the air is still above freezing, the snow will melt into rain.
This also requires energy, and aids in cooling the air a bit.
The combination of these two events can knock down the air temperature near the ground a couple of degrees, and could be the deciding couple of degrees to make it snow. The drier the air before the event begins, the more efficient this cooling process is and the temperature can drop farther.
(A lot of times in weather discussions during potential snow events, you'll see forecasters refer to the dew point -- the temperature at which the air would be 100% saturated. The lower the dew point means the drier the air. If the regular temperature is, say, 39 degrees, but the dew point is 21, that's a good sign that this evaporative cooling process could be quite effective in dropping the temperature quite a bit. As a basic rule of thumb, take the average of the two temperatures and that gives an idea of how low the temperature might drop. In this case, that's 30 degrees -- and signals a potential for snow. So if the temperature in your city is 39 with a dew point of 21 before precipitation moves in, it has a much better chance of snow than a temperature of 39 degrees and a 35 degree dew point.)
And secondarily, the greater the intensity of the precipitation, the more efficient the melting process is, and the farther the temperature can fall. A light shower and 37 degrees might stay as rain and maybe drop to 35, but a heavier shower at 37 degrees might be enough oompf to drop it to 33 or 32 and now you've got snow. And even more maddening, it can change back and forth from rain to snow as the intensity lightens and strengthens.
In this situation, the cooling process is tied to the intensity of the shower, and once the shower passes, the temperature usually climbs back up to the ambient temperature. In this scenario, it might be 37 before it snows, heavy shower rolls over and drops temperature to 32 and puts down 1-2" of snow, the shower passes and the temperature gradually climbs back to 37 and the snow melts.
Puget Sound Convergence Zones can combine several of these factors into one, since it can be an intense band of precipitation, and its usual roosting place is between Northgate and Everett, which is around 300-550 feet, adding some elevational cooling to the mix as well. That's why these areas tend to frequently get snow when others don't.
All together now!
So in these situations when we're on the fringe of freezing, factor in how variable our topography is, and then factor in how each individual little rain or snow shower can be affected by its intensity or amount of dry air around, and you can see just how incredibly variable a snow event can be.
At 8 a.m., you might have a light shower that, due to elevation, brings snow to Shoreline but rain to Ballard, and then a heavier shower moves over Ballard, bringing snow there, but misses Shoreline. The shower lightens and it changes back to rain in Ballard, but might still be snow atop Queen Anne Hill. Meanwhile, it's been all rain on the shores of Magnolia right along the water. Try and put that into a forecast :)
Thus, that's why we're a bit general in snow forecasting totals and regions, usually highlighting those in higher elevations and away from water as having better chance of snow than those near water and sea level. And it's also why you might be at home with a green lawn while your friend a mile away up the hill is bragging about the snowman he just built.
And it's also why we'll never be 100% right for everyone. But we're usually right for someone :)