Yet Another Pattern Change Update

I must admit, it’s a little difficult coming up with varying titles for your blogs when you are simply blogging about the same thing. It’s not as difficult as forecasting snow in Seattle, but it’s definitely up there.

Models are still adamant with a major pattern change beginning Saturday evening as a deep upper-level-low drops down from the NNW.

But before going any further, I want to touch on what I mean when I say an “upper level-low” vs. a “surface low,” as a lot of meteorologists use those terms without explaining what they represent. Simply stated, an upper-level-low is an area of low pressure situated in the upper atmosphere, while a surface low is an area of low pressure situated in the lower atmosphere.

More specifically, surface lows are dependent on the mass of the entire air column above the low and are generally associated with cyclogenesis (storm formation), while upper-level lows are dependent on the mass of the air column above that particular level of the atmosphere and the temperature profile of the atmosphere via a relationship known as the hydrostatic relationship. If you are curious, you can read about this relationship here as part of a study guide/blog post I wrote while I was in college.

Our extratropical cyclones are known as “cold-core lows,” meaning that the center of low pressure has colder air than its surroundings since it draws down air from the pole. Since cold air is denser than warm air, this leads to both an upper level-low and surface low. Meanwhile, tropical cyclones are “warm-core lows” that have warmer temperatures at their center than their surroundings, and actually have high pressure aloft even though the pressures at the surface are the lowest found on the planet.

Because all of our cyclones are cold-core lows, they have both upper-level AND surface lows. The reason that I’m highlighting that the system dropping down from the NNW is an “upper-level low” is simply because it is associated with a colder air column and therefore has a stronger upper-level low than surface low. Our really strong surface lows generally come from the SW and are associated with warmer air throughout the air column, with our strongest (like the Columbus Day Storm of 1962) having tropical origins.

OK. Let’s get back to our forecast.

In the near-term, snow continues in the mountains tonight, tapering off in the early morning hours of Friday. Friday will be a day between systems, with partly cloudy skies and highs in the low 40s for Seattle. The North Sound will be slightly cooler, while the South Sound will be slightly warmer.

Saturday is when things get interesting, particularly Saturday evening. Let’s take a look at the 500 mb heights, which is a topographical map of elevation at the 500 mb pressure level of the atmosphere (approximately 18,000 feet).

Our beloved Rex Block that I talked about in my previous two posts is still positioned around 150 W, and you can see the upper-level low digging into our region. Compared to last night’s model run, this low is a little bit more defined.

At the surface, you can see a weak low over NE Washington.

Compared to last night’s models, very little has changed here.

Now, for the question on everybody’s minds: snow.

Models are coming into better agreement about the possibility of snow Saturday night into Sunday. This will be a minor event because temperatures will be very close to or slightly above freezing. As such, higher accumulations will generally be limited above 500 feet and away from the water.

All of the GEFS ensembles show snow for Sea-Tac airport now. However, remember that these have low resolution and commonly overestimate snow.

The higher-resolution WRF-GFS run by the UW shows some light snow and hints at two convergence zones – one over the NW interior and one over the King/Snohomish County line extending to the SE.

The setup  at this time is favorable for convergence zones to form, with northwest flow around the Olympics and the Strait of Georgia. If you look closely at the graphic below, you can see the effects of terrain funneling winds and creating convergence in the areas where snow is shown. Pretty cool stuff!

Right now, the National Weather Service is calling from anywhere from a trace to three inches of snow, with the most snow to the north. I generally agree with this forecast, but would add that there could be locally higher amounts in certain areas if a convergence zone or two forms.

We cool off Sunday into the mid-30s and become downright frigid early next week, with highs struggling to reach freezing. Sunny conditions will prevail. Our next chance at snow comes late next week, but there’s still a lot of uncertainty with this feature.

In my last post, I talked about how I was very interested in the weather in the far extended as a polar vortex approached the area. Models have been consistent in stalling the vortex north and slightly west of our area, which would spare us severe cold and give us northwest flow, but vary greatly in the details. Under the above scenario, snow levels would be low, but not low enough for snow in Seattle. The mountains would see a ton of snow, and it still certainly bears watching at this time. As the GFS ensembles above show, there’s still a ton of uncertainty once we get past mid next-week.

Thanks for reading! Enjoy the pattern change ahead.
Charlie