October 22, 2020

Strong Winds, Cold Air and Mountain Snows Ahead

 Note: my new podcast on Wind, Cold and Snow will be out tomorrow morning

The weather is about to get MUCH more active in our region, with the approach of a potent disturbance from the northwest, the invasion of frigid air from northern Canada, and the rapid increase in winds over Northwest Washington and the coast to 50-60 mph in places.  Even central Puget Sound will get a good taste of this wintry Zephyr.

The latest infrared satellite image shows a large area of clouds and precipitation approaching our region from the Northwest (see below).

This impressive feature is the result of a strong upper-level trough (area of low pressure) that is moving southeastward on the west side of large upper level ridge of high pressure over the eastern Pacific.  This trough will be approaching Washington State tomorrow afternoon (see upper level map at 2 PM tomorrow below).  Changes in the position of this trough (too far inland) now make it unlikely we will see snowflakes near sea level...but it will still bring a lot of weather action.

The surface reflection of the upper level trough...a small but potent low pressure system will move down the coast tomorrow (see the sea level pressure map at 11 AM tomorrow morning, solid lines are sea level pressure, colors are temperature, blue is cold).  This will bring strong winds to the coast and northwest Washington.

Below are the predicted gusts at that time:  some locations could see gusts to 40 mph or more.

But that is just the first stage in the upcoming weather adventure!

Twelve hours later (11 PM Friday), the low center will be crossing the southern Cascades and cold air will surge to the Canadian border, with some of it pushing into eastern Washington and Montana.  A huge pressure gradient (difference) will accompany the cold air (lots of pressure lines, also known as isobars).  In Montana the conditions will be severe.

Strong northeasterly winds will starting pushing through the Fraser River Valley and into Northwest Washington, something illustrated by the maximum gusts predicted by the UW/Seattle City Light WindWatch system (below) at 11 PM Friday.  Some gusts will push 40-50 mph and northern Puget Sound will get some of the winds as well.

By 5 PM on Saturday, eastern WA will be in the chiller, with cold accompanied by strong northerly winds.    The difference in pressure will build across the Cascades.  That is going to be important.

By 2 AM on Sunday, strong winds will continue to exit the Fraser River Valley and push out the Strait of Juan de Fuca, but they will joined by powerful finds moving across the Stampede Gap in the central Cascades that will accelerate down to Enumclaw, Black Diamond and out to Tacoma...and even the coast.
And did I mention snow?  With the upper level trough providing upward vertical motion and easterly flow providing more uplift on the northeastern slopes of the Cascades, there will be as much as a foot over and to the east of the crest of north Cascades.  A few flurries might reach sea level near Vancouver due to the cold Fraser outflow.  Snow will greatly lessen south of roughly Snoqualmie Pass.

Beyond snow, the accumulated precipitation  through Saturday morning at 5 PM (below) shows that there will be wet conditions in the Cascades down to southern Oregon (good for ending the fires)...but not that much over the Willamette Valley and Puget Sound, which will be rainshadowed by the coastal mountains and the Olympics. 

Precipitation will be over on Saturday morning and expect cool, blustery condition with lots of sun over the weekend.  Easterly flow will dominate and that usually brings dry conditions west of the Cascade crest.  You will sunblock and a jacket. There will certainly be power outages over NW Washington and around Enumclaw and vicinity.   Sunday morning will be particularly cold, with freezing temperatures over the region.

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  1. Cliff, could you explain what a surface reflection is? Also, how does a surface low pressure area become uncoupled from the upper level low?

  2. Wow--that post reads like an adventure movie! Mother Nature never fails to entertain... ;-) Thanks Cliff!

  3. "there will be as much as a foot over and to the east of the crest of north Cascades."

    Neat! Seems like a minor reversal of our normal rain shadow pattern.

    But windy and cold. It is going to be a bitter weekend.

  4. Cliff, I have an unrelated question. Do you think the short-term (24 hour in advance) Seattle weather forecasts have been less accurate over the last few weeks than they used to be? If so, why would that be? Thanks!


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