After a week-long blast of arctic air that managed to somehow leave the area with hardly any snow despite temperatures in the teens and 20s, we're looking at a potential for snow after spending just a few hours in the low-mid 30s, and a Winter Storm Watch has been issued for many areas, including Seattle.
We have some cooler air moving in later Wednesday into Thursday, and it'll be in place when our next storm arrives Thursday night and early Friday morning that the Puget Sound area could see a period of moderate snowfall, provided warm, marine winds don't get here first.
This is a typical mild storm that would normally just bring a mundane, rainy day. And it still will... eventually. But temperatures are expected to drop into the low 30s Thursday night into predawn Friday morning, and that's when the weather system is expected to arrive.
Thus, forecast models indicate that the precipitation will begin as snow. But as for how much? It will be a race between how long it can snow before the storm's warm winds mix out the cold air and eventually push temperatures well into the 40s, changing it all to rain and washing away whatever does fall.
But with at least the potential for the snow to fall for a period before the changeover, the National Weather Service has issued a Winter Storm Watch from Thursday evening through Friday afternoon for the greater Puget Sound Metro area, the Northwest Interior and the Cascade foothills from Pierce County north to the Canadian Border. (A "watch" means conditions are possible, but not imminent.)
The watch is for potential of 1-2 inches of snow around Tacoma, to 2-4 inches from Seattle-Bellevue northward, with potential for a few spots to reach 5-6 inches north and east of Puget Sound. Outside these areas such as the coast, Olympic Peninsula and areas from Olympia southward, there is also at least potential for 1-3 inches of snow but these areas have a greater chance for getting a much quicker switch form snow to rain so the odds of getting a few inches of snow here are less and no watch has been issued.
As for timing, it looks like snow will begin around 4 a.m. Friday in the Puget Sound area and last until mid-late morning (maybe until noon or early afternoon in the Northwest Interior) before changing over to rain. Models indicate a period of heavy snow around 7 a.m. which of course, puts snow squarely into the heart of the Friday morning commute and those with kids should plan on potential school delays or closures (many school districts are just a 1/2 day Friday anyway so they may have a lower-than-typical threshold to close.) Snow will change to rain from south to north as the morning progresses and the warm, southerly winds eat away at the cold air.
Now for the caveat: There is a decent chance that the southerly winds will be strong enough to eat away at the cold air fast enough that the snow changes to rain a lot quicker and this is a shorter event with little to no accumulations. In other words, this has potential to bust -- it's always a tricky call to predict who will win the "snow falling" versus "warm wind approaching" race. But if the wind takes longer to work its magic, the snow totals could lean toward the higher end of the forecast scale and the Winter Storm Watch was issued to cover this possibility. But again -- just a watch, meaning possible, but not slam dunk. This is a very difficult forecast.
In any event, this will be one of those 33-36 degree wet, sloppy snows with no freezing concerns and by the time we get into Friday afternoon, temperatures will be steadily warming into the 40s and snow will rapidly melt. I think any Friday evening/night plans should be fine and even the traditional Friday evening commute should be snow-concern free (just a typical slow rainy commute) unless there is still melting leftover slush on the roads.
Once we go above freezing Friday, we don't look back as temperatures are not expected to drop much below 40 for the next several days with highs even climbing close to 50.