Freezing rain is an extremely rare event in Western Washington, especially to the extent of the ice coverage seen on Thursday, which will likely go down as among the worst in Seattle's history.
And for many, it's a source of confusion -- how can it be 29 degrees and raining, not snow?
To get freezing rain you need a complex meteorological situation where you get a warm layer of air sandwiched in between two layers of freezing air.
When the water droplet falls from a cloud, it's in the form of snow. Then when it encounters warm air on its journey to the ground, it melts into rain -- a process that is pretty much 99.9% of our lives in Seattle.
But if you have another layer of freezing air right along the ground, the raindrop gets super cooled to where it's not quite frozen but is about to be. Then when that drop hits a surface -- a cold road, tree branch, power line, car window, etc. -- it instantly turns to ice.
As the freezing rain keeps falling, the ice will accumulate upon itself, and get thicker and thicker, adding a frozen glaze to road surfaces and adding weight to trees and power lines. This extra weight is often too much for trees and limbs and they'll topple over.
Freezing rain events are often marked by large power outages from those falling trees and power lines, airport closures and traffic accidents.
Sleet is somewhat related -- that occurs when the layer of cold air near the ground is thicker, so the rain has a chance to freeze back into an ice pellet -- much less headache worthy.
How Did We Get Here?
We have a mild area of low pressure coming in to Western Washington, but we also still have a cold, north wind blowing arctic air from the Fraser River Valley through Bellingham south into the Puget Sound area.
Cold air is denser than warm air, so it will sit on the ground until winds or enough warm air comes along to scour it out. That just hasn't been happening and it's being made worse by lower pressure from the incoming storm passing to our south, drawing in even more of a cold, north wind to reinforce the arctic along the surface.
So to recap, we have cold air aloft, then warmer air moving in from a milder storm melting the snow to rain, then the cold air stuck right near the ground.
Snow in Snohomish County
Much of Snohomish County and northern Kitsap County had both ice and snow -- warm air nosed just far enough north to bring them some freezing rain, but then as we get into later Thursday morning, the cold wind started winning the battle, pushing the warm air back south and changing the freezing rain back to snow.
Areas around Everett, Mukilteo, Lake Stevens, Mill Creek and Edmonds were reporting a very heavy snow once again. Good news in that it stops additional ice accumulations, bad news in that it meant the atmosphere was cooling again, not warming.
Not uncommon in spots, but rarely regionwide
Western Washington does get isolated freezing rains spots in the winter -- typically where they are near an outflow of cold air continuing amid a warmer rain event.
The greater Bellingham/Whatcom County areas sometimes get freezing rain when warm storm comes in over arctic air leaking out of the Fraser River. And the Cascade foothills get a similar situation when cold arctic air from Eastern Washington leaks through the Cascade passes .
But to get this perfect situation of arctic air mixing at just the right ratio with a slightly warmer storm that brings just a smidge of warm air but also reinforces the cold air on the surface for such a widespread freezing rain -- it just hasn't happened in decades.
Portland no stranger to these type of events
Seattle can perhaps look to our neighbors to the south for some guidance on how to get through the event.
Unlike Seattle, Portland has had several freezing rain events in their history, due to their proximity to the outflow from the Columbia Gorge. Cold, arctic air bottled up in Eastern Oregon gets drawn through the Gorge to provide that layer of arctic air along the ground as a warm storm moves over it.
Portland Airport has had several days in its past akin to what Sea-Tac is dealing with Thursday.