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.



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