Modified Arctic Air Reaches the Northwest, Colder Air Is Yet to Come

The coldest air of the season has reached the Pacific Northwest, and some western Washington locations are already reporting precipitation in the form of ice pellets!

You can see the impact of the cold air on visible satellite imagery taken on Monday afternoon (see below).  As frigid cold air from Alaska and northern British Columbia moves over the warmer water of the Pacific, an unstable situation develops that produces cumulus convection......towering cumulus clouds with brief showers.

You can see the cumulus showers on the satellite image (white blobs with clear spaces between them), and they are aligned with the wind direction (red arrows).

Why does cold air moving over water produce showers?  

 Because it creates a large lapse rate, a change of temperature with height, that causes the atmosphere to convect, with upward motion producing clouds and precipitation.

You could see the convective showers coming onshore late Monday afternoon, as viewed by local weather radars:

As the atmosphere cooled and showers moved in, snow started to fall in the mountains, such as at  Hurricane Ridge at around 5000 ft in the Olympics. 

Late Monday afternoon, the freezing level was about 1600 ft in the Olympics, which means the snow level was roughly 600 ft.  It will decline further over the next few days.

During today (Tuesday) and Wednesday, very cold arctic air will push into southern British Columbia, with some of it leading into western Washington through the Fraser River valley.  

The situation Wednesday AM is shown below, with the purple and white colors indicating the coldest air.

Wow.  The Arctic will be getting very close to us.

A closer view on early Thursday AM (below) shows very cold temperatures over Washington...cold enough for lowland snow.  The only thing missing will be precipitation.

The latest UW model forecast showing predicted snowfall totals through Thursday morning does show some lowland snow from Tacoma southward.  Details uncertain, but there is a good chance that some lowland folks will see some flakes, with some localized minor accumulation.

Lowland snow will occur in the Willamette Valley as well, which is more isolated from the warming effects of the Pacific.

Finally, expect particularly cold air moving southwestward down the Fraser Valley into Bellingham.  This is illustrated by the ensemble of many high-resolution forecasts at Bellingham (below).  Temperatures will drop way below freezing.  Windy as well.  

Some wind chills may decline below 0F.  BRRR.

Light Lowland Snow Coming to Western Washington and Oregon

 I have held off talking about this until we were closer in time and uncertainty had declined, but there is a very good chance that some of you living in the western Washington and western Oregon lowlands will see some flakes on Tuesday and Wednesday.

I don't want to paint this as a snowy apocalypse, but there is a significant chance for some light snow, with marginal accumulation.

It won't be this bad

Let's begin with the latest UW WRF model snow forecasts. 

Let me be VERY clear about something. 

Snowfall is NOT the same as snow depth.  Snow melts on the surface and gets compressed.  Snow depth is generally MUCH less than snowfall.

The predicted snowfall total through Thursday morning is shown below.

The Olympics get hit hard, with much of the lowlands experiencing some flakes, including some bands of a few inches of wet flakes.  

Oregon will get even more snowy bounty with lots in the mountains and some accumulation in the southern Willamette Valley:

The lower-resolution European Model is similar, but has less snow over the western Washington lowlands.

When will the snow start, and what is the uncertainty in the forecasts?   

A good tool for answering these questions is ensembles of many forecasts.

The National Weather Service global model ensemble forecasts (below) for Bellingham are very generous with snow (I suspect too much). The gray lines show that the various solutions were all over the place (substantial uncertainty), with the highest resolution model indicated by blue (and the average of all the forecasts by black).   

Starts on Tuesday, with the greatest amounts on the 20th.

So what is going on?

As shown by the upper-level (500 hPa, about 18,000 ft) forecasts for late Monday, a strong trough of upper-level low pressure will move southward down our coast.

This is a well known as a cold pattern for our region.

As shown by the surface forecast for Wednesday morning, this will bring VERY cold air into British Columbia (purple colors, blue is cold enough to snow) and low pressure just east of Astoira.  

This configuration will pull cold air from Canada into northwest WA, particularly through the Fraser River Valley.

To illustrate, below are the predicted winds and temperatures on Thursday morning.  You can see the cold northwesterly flow coming out of Canada.

Give me the chills to look at it.

During the next few weeks, there will be substantial snow in the mountains, as shown by the predicted totals through March 2 (below).    

Hopefully, this bountiful snow will calm down some of the current panic in the media and social media about the current low snowpack.