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.