Calm Through Saturday, Huge Uncertainty in Extended Forecast

 Update on Saturday as forecasts get more in focus. Stay tuned!

FastCast—Thursday, Dec. 15 to Sunday, Dec. 18:

A partly to mostly sunny and calm remainder of the week is expected for Western Washington. Expect brisk conditions, with highs in the low 40s and lows in the mid 20s to low 30s. Some areas of freezing fog are possible, so be prepared for icy spots on roads. Eastern Washington and the Cascades will be much colder, with highs in the upper 20s to mid 30s and frigid lows in the mid teens to low 20s, colder in the mountains. Clouds return on Saturday, with highs remaining in the low 40s and lows rising to the mid 30s. After Saturday, huge uncertainty exists in the forecasts. Arctic air will move into British Columbia by Sunday, but the extent that it moves into Western Washington is yet to be determined. What is known is that it will be cold in Eastern Washington, temperatures will be decreasing in Western Washington, and there is a potential for snow. News and social media reports about lowland snowstorms are to be taken with a grain of salt given the uncertainty in the forecast. The first chance for lowland snow would be Sunday into Monday, with additional chances through next week. However, with uncertainty on the placement of systems and the extent of Arctic air, forecasts range from cold rain to heavy snow. That means we will have to stay tuned to the forecasts for the next few days while substantial uncertainty remains. Stay tuned!

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Continue reading the full blog below!

After a few days of cloudy and calm weather, northerly flow will bring partly to mostly sunny conditions to most of Western Washington through Friday! This will also bring chilly overnight lows. Below is the European model forecast for lows on Friday morning.

Things will be relatively the same for Thursday and Friday mornings. Expect lowland lows in the mid 20s to low 30s. Freezing fog and icy roads are possible. Eastern WA will be much colder, with lows in the low teens to low 20s. 

Clouds return and overnight temperatures rise back to the mid 30s on Saturday. After Saturday, there is substantial uncertainty, as Arctic air moves into British Columbia. (Seen below on the European model for Sunday…note the uncertainty with the southern boundary).

The southward extent of Arctic air and how it interacts with warmer air from the Pacific Ocean will determine if Western Washington gets any snow. As of Wednesday night, situations range from heavy rain to heavy snow, with lots of potentials between those.

The first potential chance of lowland snow is from Sunday to Monday, with more potential chances as we go through next week.

To illustrate the uncertainty, primarily between the American (GFS) model and the European model, we will take a look at upper-air heights (determines lower atmosphere weather patterns) next Tuesday.

Below is the American GFS model (usually less accurate). We’ll be looking at the feature in the black box.

The outlined feature is called a polar lobe, a part of the polar vortex that moves south and produces cold weather outside the poles. On the GFS model, this feature is weaker and further north, thus limiting impacts to far northern WA and BC.

Take a look at the same feature at the same time on the European model below.

On the traditionally more reliable European model, this polar lobe is stronger and further south. It is generally a bit too far north for significant impacts, but a lot of that could change.

Forecasts have differed strongly from each other and have generally differed from run-to-run over the past few days. Stay tuned to the blog, my Twitter, and the Pacific Northwest Weather Watch YouTube channel, linked on the right side of the blog under “Helpful Weather Websites.” There, local weather expert Michael Snyder details the forecasts in daily briefings, well worth 10-20 minutes of your time! Stay tuned!