Near-Record High Atmospheric Pressure Followed by the Big Chill

If you feel some pressure in your forehead today, there may be a cause.

Right now, the Northwest is experiencing unusually high atmospheric pressure.

In fact, the third highest pressure in the past 25 years, hitting around 1042 hPa, something shown by the plot of pressures at SeaTac below (the dashed red line shows the highest pressure observed yesterday).

This uber-high pressure has been suppressing water levels, resulting in the high tides at Seattle being far less than would have occurred under normal pressure (see below, red is observed, blue is predicted)

Later tonight and tomorrow morning, a front will move through, bringing some rain to the lowlands and snow to the mountains.   Nothing exceptional.

And then the chilly fun starts on Saturday.  That morning, cold air (purple and blue colors) will start moving in from the Northeast.

And by Sunday morning will have spread over the entire region.

The cold air will bring high sea level pressure and large pressure differences that will drive strong winds in and downstream of gaps in our terrain.  

As shown in the wind gust forecasts for Sunday morning, powerful northeasterly winds to push out of the Fraser River Valley into Northwest Washington, and downslope easterly winds will descend the Cascades from North Bend to Enumclaw, moving westward in and south of Sea-Tac.  Some bumpy landings can be expected.

And now I am going to disappoint some of you.  Virtually no lowland snow will accompany the cold air.  The set-up is just wrong.

Below is the accumulated snowfall through Sunday at 4 AM. 

Perhaps a few flakes between Portland and Chehalis, but other than that, all snow will be at higher elevations.   Good amounts in the Rockies.

Sunday and Monday mornings will be quite cold (in the mid-20s over the western lowlands), so disconnect hoses and keep your pets indoors.