Saturday, January 2, 2016

Offshore Flow, Eastern Washington Cold Pool, Declining Air Quality, and California Floods

We are in the midst of a world-class period of easterly flow over our region, with powerful winds downstream of gaps, a stubborn pool of cold air in the Columbia Basin, crystal clear mountains, and ironically, declining air quality in places.   A radical shift from the persistent clouds and precipitation of December.

The key element of other weather the past few days has been strong easterly flow aloft.  Such strong offshore flow typically produces fine visibility (as long as you are not embedded in low-level fog) along the western Cascade slopes and this morning is no exception (see below at 7:30 AM from the Space Needle cam).   Below that is the view at noon Friday.



Why are the mountains so crystal clear under easterly flow?   Because the air sinking down the western slopes warms and dries as is descends, so no clouds and low relative humidity.  Low relative humidity prevents the growth of water-loving (hygroscopic) particles.

But why is there easterly flow?

We start with a large-scale upper level atmospheric configuration with high pressure to the north and and an unusually persistent low to our south.  This figure shows 500 hPa (roughly 18,000 ft )heights, winds, and temps forecast for 4 AM Saturday morning.  The wind barbs show strong easterly flow (from the east) over our region.


Closer to the surface, the easterly flow is revved up by the large difference in pressure across the Cascades, with higher pressure to the east.   Here is the seal level pressure and low-level temperatures at 4 AM this (Saturday) morning. The black lines are isobars (lines of constant pressure).  A very large pressure difference across the Cascades (about 10 hPa), which drives easterly flow down gaps, such as the Columbia River Gorge.  Why the big difference in pressure?   The big driver is temperature, with cold dense air (blue colors) trapped east of the Cascades.


To show this temperature contrast, here are the minimum temperatures the last 24 hours across our region.  Lots of single digits in eastern WA, and even some below zero, while twenties were dominant over western WA.


 What about wind?  Here are the maximum gusts for the last 24 h (ending 7 AM).  Very strong winds over the western side of the Columbia Gorge (51 mph) and the western side of the Strait of Juan de Fuca.  Air is accelerating down the pressure gradient in these gaps.  And you will also notice some strong winds along the eastern slopes of the Cascades.


But the sunny weather has a subtle down side:  our air quality is starting to decline.   Clear skies allow the earth to radiate infrared energy to space, thus cooling the surface and low-level air.  The easterly flow, and associated warming, is stronger aloft.   The result is the development of inversions, where temperature increases with height.  And inversions lessen vertical mixing and result in declining air quality.  You can see the changes by looking at the vertical temperature soundings at 8 AM, once a day, for the last several days at Seattle (see below).  Today (black line) there is a decided inversion.  Not so a few days ago.
Puget Sound Clean Air Agency has burn bans for Snohomish and Pierce Counties.  The air quality map (green is good, yellow is marginal, orange/red is poor) shows the situation.  Why is air quality better in a line from North Bend to Seattle?  Because of stronger winds pushing westward out of Snoqualmie Pass!


But changes are in the air.  A system will approach on Sunday, bringing clouds and rain to Oregon (but will hold off until Sunday night in most of Washington).    But nothing heavy.  Splitting systems.  Classic post New Year's situation under a strong El Nino.  But California is going to get hit, very hard.  Here is the GFS model total precipitation forecast for the next 10 days.  Biblical levels of rain.

2016 Northwest Weather Workshop On March 4-5, 2016

The Northwest Weather Workshop, the big regional meeting on Northwest weather will take place in Seattle on March 4-5, 2016.   At this meeting we talk about the latest advances in understanding our local weather.  There will be a special session on the OLYMPEX project.  It is open to all.   For more information go here.  

If you want to give a presentation, abstracts are due on February 1.

10 comments:

Judie said...

Thanks again Cliff for such a detailed amount of information! Even though I live in the Vancouver / Portland area, and you focus more on the Seattle area, the wide range coverage gives ample information pertinent to my area of the state as well. I have you bookmarked in my go to weather folder and check it daily. Happy New Year!!

Mark S. said...

Cliff, what's the chance of seeing Comet Catalina tomorrow morning, before we are enveloped in clouds?

Brian said...

Impressive videos of the wind on Granite Mountain just west of Snoqualmie Pass summit. Taken by an intrepid climber on 12/31/2015:

https://www.youtube.com/watch?v=rBB8NtzotoU

https://www.youtube.com/watch?v=iY7cV_H9JU0

Rebecca Dittemore said...

The NWS is forecasting a little lowland snow for tomorrow night. Do you think we'll wake up to more than just frost on Monday?

Jaxom92 said...

We had some wind come through the Fairwood neighborhood of Renton last night between 10 PM and 1 AM. Maybe coming through Snoqualmie pass and then the Cedar River valley?

Gary said...

Happy New Year, Cliff!

I read all of your blog entries and am very much appreciative for their information and entertainment! Your illustrative graphics make what is sometimes a pretty technical text for us non-atmospheric sciences folks understandable.

I want to point out a typo on the first line of the 3rd paragraph, where you talk about downslope warming. Where it says " . . . sinking down the eastern slopes " it should read "western".

Thanks for all the time and effort you put in to keep your public informed, not only about weather but also about science (math) education, shady public radio deals, etc.

It's been cold here in Olympia the past two nights--22F and 21F Sat night--and I live on the ridge right above Budd Inlet next to downtown. Although the dreaded fog was forecast for Tue - Fri, most of it was burned off by 9-10AM, giving us brilliant but chilly days.

Gary Schuldt
Olympia

rainycity1 said...

Sparse snow flakes just started in West Seattle: 12:55 pm 1/3

tenaiko said...

Cliff,

Why the low temps over the Northern Sound and over the North-Eastern slopes of the Olympics near Sequim compared to other parts of the lowlands? I would have thought the proximity to water would prevent temps from falling. Simply due to clear skies and good radiation in those areas?

Bryan Kerr said...

Found out about this blog from comments on Weather West's blog. I'm down in southern California, waiting to get slammed by rain this week. While impatiently waiting for El Nino to start influencing our weather down here, I put together a video of all the GOES 9 satellite imagery from the '97-'98 El Nino.

https://www.youtube.com/watch?v=t9s_EOsH6MU

Watch around January 14th , when the zonal flow sets up in the Pacific and the storms just pummel the west coast. Very similar to what's happening right now.

Ansel said...

Beautiful ski weekend, with temps down to 11 degrees at Stevens Pass.