Monday Snow Update

Well, the new model runs are in, so I thought I’d give another forecast on the snow (or lack thereof) expected for Monday morning. Each individual model has changed little over the past several runs, but the different models show vastly different scenarios. The American model (GFS) paints a couple inches on the higher hills with wet, non accumulating snow below 200 feet or so, and the European model (ECMWF) is much more pessimistic about our snow chances, with less than an inch for most places.

Let’s start out with the most important models: the ensembles!

Here’s the American ensemble, also known as the GEFS. The ensemble average (black line) shows an average of nearly two inches for Sea-Tac beginning Monday morning, then nearly 3 inches for Thursday afternoon/evening. Comparing today’s run to the GEFS ensemble yesterday, there is better agreement between the models that we will see some snow at the airport tomorrow. Again, remember that the GEFS has very course resolution, and with the mountains nearby, it may think that Sea-Tac is higher than it is in reality (452 feet). There is a chance the ensembles may think it is lower… I really don’t know.

I do not have access to the ECMWF ensembles, but word on the street is that they show much less but still give a trace to Sea-Tac.

Now, let’s look at some “deterministic” model charts, i.e. charts from the “main” GFS run. These are taken from the WRF-GFS model run by the University of Washington department of atmospheric sciences.

Above is a “sounding” valid 7 AM Monday morning. It shows the predicted temperature (red line), dewpoint (blue line), and winds (small barbs = 5 knots, large barbs = 10 knots, and the barbs point in the direction from which the wind is blowing).

What we’re looking for here is not only the temperature, but how close together the temperature and dewpoint are. When precipitation falls, it evaporates into the air around it, cooling it in the process. The drier the air, the more evaporation can take place. Here, the air is nearly saturated (dew point and temperature are very close to each other), so any precipitation falling will have a tougher time cooling the surrounding atmosphere than if the air was drier.

The sounding above shows a situation that would give snow to Sea-Tac. Because temperatures are near freezing and road surfaces are still quite warm, snow will have a very tough time sticking on sidewalks and roads and will primarily accumulate on grassy surfaces.

Elsewhere around Puget Sound, I expect there will be a large “snow gradient” with elevation. Locations like Queen Anne Hill could see a couple inches on grassy surfaces, while downtown Seattle and areas near Lake Washington/Sammamish and Puget Sound will see very wet snow or a rain-snow mix.

Confidence today is higher than yesterday that a few flakes will fly throughout the Puget Sound region due to better agreement in the GEFS ensembles. Snow will have trouble sticking due to ground surfaces being relatively warm, particularly on asphalt. As such, I am not too worried about the Monday morning commute.

Finally, here is the predicted 24-hour accumulation of snowfall from the WRF-GFS from 4 AM Monday to 4 AM Tuesday. This particular chart has extremely high resolution – 1.33 km – and is able to more accurately resolve features such as Puget Sound. It generally shows 1-3 inches over the Puget Sound lowlands.

You can see a band extending from the King/Snohomish County Line southeastward into the Central Cascades near Snoqualmie Pass. This is our trusty Puget Sound Convergence Zone. At this point, it appears likely that one of these will develop, but it is too early to pin down exactly where. Temperatures are almost always cooler in convergence zones due to heavier precipitation which cools the air via the aformentioned evaporative cooling as well as frozen precipitation melting (it takes energy to melt ice into water). If a convergence zone sets up, areas within the zone could see several inches on grassy surfaces, even down to sea level.  I believe the convergence zone will play a big role in whether we get snow or not.

I’ll have another update on Sunday with more detailed information, and then we can focus on the cold weather ahead and the possible snow event Thursday.

Even if we don’t get snow, it’s fun to be talking about it once again! It has been far too long.

~ Charlie