Good morning everyone! Snow is spreading northward with snowflakes being reported at the south end of Puget Sound. Here’s the radar image as of 7:50am showing the northern edge of precipitation reaching Tacoma.

High res models haven’t shifted much since last night and are generally showing about an inch of snow for King County, with totals decreasing the further north you go. The HRRR is a bit more aggressive with spreading that precipitation northward, with the NAM keeping everything from the King/Sno Co. line south. Tacoma southward still looks to do well with this one according to our high res models, with locations generally receiving anywhere from 2-6″.

Here are the snow totals through 6am Friday on the HRRR and NAM

The Euro also wants to bring at least some precipitation up to even the far northern reaches of Washington. However, the Euro keeps totals lower through the south sound, generally maxing out around 3-4″ through 4am Friday morning.

Our consistently southerly leaning GFS still wants to draw a hard line on accumulation from about the King/Pierce county line southward, with some hefty totals along the I-5 corridor from Olympia to Longview.

Remember, the air is dry right now, so any precipitation will have to work a bit to saturate the atmosphere before we see snowflakes and accumulations at the surface. Snow in the south sound will increase through the morning, with snow starting in Seattle during the afternoon. I’m leaning towards the high res models for snow totals with this one, so expect about an inch or so of accumulation this afternoon/evening in Seattle, with totals decreasing once you get to about Everett. I still think 3-5″ looks like a good bet for Pierce county, with areas of 6″+ from Thurston county to Longview. While Seattle misses out on big totals with this one, it’ll be a beautiful afternoon to watch some snowflakes fall.

Moving onto Friday night and Saturday:

This could be a big one, but it also might not be a big one, but the odds of a big one is there. Right now its anyone’s game, and there will be winners and losers.

What we do know is that we have a ton of cold air to our north and east, combine that with a strong cross cascade pressure gradient, and you have a pretty solid engine to pump cold air into the western valleys. This chart below shows the 850mb temperatures, a slice of the atmosphere at about 5000ft in height coordinates. Check out those readings in the -20’s just to our north, that’s a lot of cold air.

That cold air coming down off the cascades will be drying us out bigtime around Seattle, and it remains to be seen just how much of an impact that will have. There is a chance that they will significantly reduce totals in Seattle and the eastside.

We also know that these storm systems will be tapping into a lot of tropical moisture thanks to their track, meaning that they’ll be juicy. The map below shows the total precipitable water as of 7am this morning with those two fetches of moisture on either side of Hawaii reaching up to the north Pacific.

Combine these ingredients together and you have the chance of some major snowfall in Seattle, but things have to come together just right. The deterministic Euro has shifted that second low further north to the point where it could bring warm air in early, bringing a faster transition to rain after a dumping of snow. However, the European track especially brings excessive precipitation into Western Washington. The deterministic GFS brings that low further south, reinforcing our cold air but limiting precipitation a bit.

When forecasting, meteorologists look at both the deterministic model runs and the ensemble runs. Ensembles work by taking the same set of initial conditions as the deterministic runs, and the pertubate’s them to slightly different values to see what happens. In significant weather events, ensembles are a great tool to get a sense of probability and the possible outcomes.

Each line in the chart below is a different ensemble member featuring slightly different initial conditions. Consensus has been growing in the ensemble members that this storm system could be big. Below are the European model’s 50 ensemble members over the course of the next 15 days.

There are a significant amount of members showing up to and over a foot of snow falling in a short time frame. While some of the ensemble members go way overboard, looking at you member #50…and some go way on the low end *cough* member #3 *cough*. The majority of them show a significant amount of snow. The mean at the bottom is a fairly possible solution.

Meanwhile, the GFS ensembles doesn’t have quite as many members showing massive snowfall due to that more southerly track, the theme of a major snowfall is still there.

I’m hoping that the models have locked onto a solution and wont shift too much over the next 24 hours, but regardless there is a chance of a major snowfall. What remains to be seen is how stubborn that cold air is, and whether the low tracks like the Euro is showing or the GFS. All models have trended a bit warmer which puts us closer to the cusp then I would like. The big IF for Seattle and the eastside is that easterly wind, its going to be strong and will do a good job of drying out the atmosphere. That could significantly limit totals in those regions.

The ingredients are there, but will they come together just right? I personally hope so! The low placement of the deterministic euro makes me nervous so we will have to see if it sticks with that. That easterly wind is going to be another massive wildcard that makes me nervous. There is still quite a bit of uncertainty which is making for a very exciting week of forecasting to say the least, but also a very challenging one. For all we know there is still a chance that the storm could bust entirely with out a couple of inches of snow…but the odds of that are slim.

Prepare for an impactful snowstorm, but also be prepared for snow totals that are less than what is shown above. Regardless a few inches will fall and that is enough to have significant impact. It’s anyone’s game right now…