Multiple Storms Will Bring Significant Snow To Western Washington

Update 8:20 PM: Winter Storm Warning issued for King/Pierce County line southward. Alert below. This is for 1-3 inches of snow from 6 AM Thursday to 6 AM Friday.

Additionally, a Winter Storm Watch is in effect area-wide for the Friday/Saturday storm. Totals 6-13 inches possible. Stay tuned!

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Over the next few days, multiple weather systems will bring snow to Western Washington. There are currently 3 times to watch.

1. Thursday morning to Friday morning (weak system, not widespread)

2. Friday night to Saturday (widespread snowstorm possible)

3. Sunday and Monday (precipitation type and amount uncertain)

This blog post will examine the first two events and briefly preview the uncertain third event.

Snow will begin in the South Sound between 4 AM and noon Thursday. Below is the predicted snow forecast from NWS (National Weather Service) Seattle for Thursday morning to Friday morning. (Click to enlarge)

Expect 1-4 inches of snow with this system, ending by Friday morning. Isolated areas of more than 4 inches of snow are possible, but not likely, with this first system.

An important aspect of this first storm will be its weakness. This first storm is a cutoff low, which basically means it's a weak storm that's meandering its way onshore. Additionally, this storm will be battling dry air as snow moves north. The dry air won't entirely win the battle south of Seattle, but once you get into northern King County, snow totals dip under an inch to near nothing.

However, this first storm could be considered an "appetizer" to the second, more widespread snow event arriving Friday afternoon/evening.

A low pressure center will move onshore around the mouth of the Columbia River on Friday evening. With cold air in place, this storm will likely be near the "sweet spot" for a snowstorm in the Puget Sound area.

We'll look at multiple forecasts for this storm so that we can get the best idea of what to expect.

NWS Seattle predicts that areas south of Seattle will get 6-8 inches of snow. That's impactful snow, especially for our area. The heaviest snow is currently expected Friday night into Saturday morning. 

Below is the European forecast model, which has been consistently showing 8+ inches of snow in the Friday-Saturday system. The forecast below is valid through 4 PM Saturday, showing total snowFALL (what hits the ground), not snowDEPTH (what's left after settling, etc.)

Image from Cliff Mass Weather Blog and WeatherBell.

This forecast shows a total of 12-18 inches in the South/Central Sound! That will be enough to cause major impacts in the Puget Sound area! 

Before we pass this off as "certain" (which it isn't), we need to see the real probability of this happening. We can do that by examining the ensemble predictions. The ensemble predictions are many "members" of a forecast model coming together to make a prediction.

Below is the European Model's ensemble for SeaTac airport. Add a couple inches to these predictions if you live south of there.

Image from Cliff Mass Weather Blog and WeatherBell.

Nearly every member of the ensemble predicts a major snow event. The bottom half of the graphic shows the averages. Bottom line: the skillful Euro model is confident in at least 10 inches of snow at SeaTac (11-12 inches south of the airport).

Now, the slightly less skillful GFS (American) forecast model (also for SeaTac airport).

Image from Cliff Mass Weather Blog and WeatherBell.

The GFS model is also pretty confident in a major snowstorm, with at least 8 inches likely among the ensembles.

What does this mean?

Well...nothing is absolutely certain yet, but be prepared for a major snow event, with totals of 6-15 inches, starting Friday afternoon and ending on Saturday.

What happens after midday Saturday is still uncertain. A changeover to rain or rain/snow mix by Saturday afternoon is possible, but confidence isn't high enough to determine that yet.

The next system, slated for Sunday/Monday, is too far out for us to accurately know what will happen.

As far as impacts to roads go...the temperature will play a big part. In the forecast for Puyallup (below...from weather.com), we see that temperatures on Friday will stay near/below freezing, keeping the ground cold. That means that snow is more likely to accumulate on roads.

Similar, or colder, temperatures are likely around the area. Don't be surprised if either one of our 2 storms brings snow accumulating at least on side streets.

Please stay tuned for updates! I will have nightly updates until this event is over.