A Snow-Shadow Snow Event

 You have all heard of a rainshadow.  

Air approaching a barrier rises on the windward side, producing lots of precipitation, while air sinks on the lee side, causing drying and a lack of rain (see graphic).


On Thursday, we are going to have a profound snow-shadow situation for large portions of the western Washington lowlands, particularly around Puget Sound, and snow lovers like myself are going to be disappointed.

Below is the latest super-high resolution UW snowfall forecasts for the 24-h ending 4 AM Friday morning.  Light green is less than an inch, while purples and darker colors indicate more than 6 inches.

Plenty of snow on the windward (western) sides of the Olympics and Cascades.  Bellingham is going to be snowed in!  But little around Puget Sound and virtually nothing from north Seattle to north Kitsap.  No snow near Yakima on the eastern side of the Cascades.


Looking at snowfall for the entire region for the same period (below), you can see little snow on the lower eastern slopes of the Cascades and then a HUGE snow dump on the Blue Mountains of southeast Washington.  The dryland wheat farmers will be smiling.



So why so little snow around Puget Sound? The problem will not be the temperature--it will be cold enough to snow. The problem will be a lack of precipitation.

And that deficiency is due to the orientation of the winds in the lower atmosphere, which will be from the northwest, resulting in downward flow on the lee side of the Olympics.

To show this, here are the heights (like pressure) and winds at the 850 hPa pressure level...roughly 5000 ft at 1 PM Thursday.  The winds are from the northwest over the Olympics, resulting in a snowshadow over Puget Sound.  Bellingham and vicinity are not in the lee of the Olympics (or the mountains of Vancouver Island) for this direction, so they will get plenty of snow.  There is a small chance that portions of Seattle might get some light convergence zone snow....but that is uncertain at this point.


Well, at least there will be plenty of snow in the Cascades for recreation and water supply--all good.

Temperatures will moderate over the weekend and snowy roads in the lowlands will become a memory.  Life will return to normal--except for the COVID situation.



Comments

Post a Comment

Popular posts from this blog

The Time of Year You Can See the Air Move

Image
  Most of the time, you can not see the air move.   You can feel it, but the complex, turbulent motions are invisible to you, except when it interacts with leaves and branches on trees and plants. But for a few weeks of the year, the sky is full of small, cotton-like tuffs that like a natural MRI machine reveals the complex three-dimensional motions of the atmosphere.   Cottonwood season.  And we right in the middle of it.   Being cottonwood aware is of particular value this year.  It reveals the active dispersion and movement of air that makes the outdoors essentially COVID safe, and thus should be of some comfort for those nervous about transmission outside. Cottonwood seeds are embedded in small cotton-like fibers.   A million seeds weigh about 3 pounds.  Yes, each seed and tuff weights about .000003 pounds.   And that fact, plus the cottonwood tuff, means that these seeds fall VERY, VERY, VERY slowly.   So slowly that they are excellent markers for the three-dimensional flow of the
Read more

Why so little lightning in the Pacific Northwest? And a very nice weekend ahead.

Image
During the past 24-hours, there have been a few lightning strokes over the region (see below) but nothing to write home about (see below). It turns out that the Pacific Northwest and the remainder of the West Coast have some of the lowest frequencies of thunderstorms in the entire country (see below). But why?    My podcast provides the answer. And I also give the weekend forecast....and I think most will like what I have to say. To listen to my podcast, use the link below or access it through your favorite podcast service. Some major podcast servers:     Like the podcast? Support on Patreon 
Read more

Strong Atmosphere River Heads into British Columbia and Southeast Alaska

Image
While the Northwest has been drier than normal, it has been drenching wet a few hundred miles to the north. An unusually strong atmospheric river ....a long current of moisture stretching southward to the subtropics...has been in place for days, bringing large amounts of water vapor to Alaska.   And the atmospheric river will be in place for several more days, slowly moving southward. To illustrate, the scale and beauty of the atmospheric river, here is a water vapor satellite image for 11 AM today. The lighter areas have more water vapor.  You can see the current of moisture extending from near Hawaii into central BC.  This current is the result of low pressure over the Gulf of Alaska and high pressure offshore of California.   A forecast of total water vapor content in the atmosphere for 11 PM tonight shows a healthy atmospheric river (white and blue colors indicate the higher amounts of moisture). As this atmospheric river slams into the mountainous West Coast, the air is forced to
Read more