Olympic Rain Shadow, Again

7:35 AM, Temp 52.1° F, Dew Point 46.1° F, Barometer 29.84 in, Wind 11 mph G/24, Humidity 80%


We speak occasionally here about the Olympic Rain Shadow.  It is also known as the San Juan Sunbelt, Banana Belt and the Blue Hole.  This is what accounts for a surprisingly low annual rainfall in the area of the western Strait of Juan de Fuca, northern Puget Sound and the southern Strait of Georgia.  You can see the rain shadow on the radar map from this morning.  It is literally the hole in the middle of the rain image.  The Olympic Mountain Range sits southwest of the city of Port Angeles.  The clear wedge in the radar image locates the mountains which the radar cannot penetrate.

The red box marks Fidalgo Weather's location.  Despite the large weather system all around us, we were bone dry at that time.  During these events, I can sometimes look southwest and see blue sky over Whidbey Island.  It really is a "blue hole" in an otherwise cloudy sky.  South Fidalgo and North Whidbey Islands receive about 20-22 inches (50-56 cm) of rainfall per year.  By comparison, Seattle receives around 38 inches (97 cm) annually.

As an aside, notice the empty wedges in the radar image north and east of Mount Vernon.  These are probably due to the Cascade foothills in the area.  Bow Hill to the north and Little Mountain southeast of Mount Vernon are probably partially responsible.  I would also venture a guess that Mount Pilchuck is producing the large gap between Mount Vernon and Everett.

A second and more profound rain shadow is created by the Cascade Mountain Range.  Out-of state visitors to eastern Washington and Oregon are often startled to find desert in a region famous for rain.  Some areas over there receive less than 10 inches (25 cm) of rainfall each year.  With deserts, forests, grasslands, mountains, river valleys and alpine snows, we have it all here.

9:15 AM, Temp 50.1° F, Dew Point 47.3° F, Barometer 29.91 in, Wind 10 mph G/22, Humidity 90%


By 9:15 AM, our rain shadow had closed up and it was about to rain very hard.  I drove downtown in a complete downpour.  Also at this time, the barometer reading of atmospheric pressure had begun rising rapidly.  It stopped raining by noon.  We would get a total of 0.13 in (3.3 mm), not that much to speak of.

In typical fashion, the rain was proceeded by wind.  As low pressure systems move inland north of us, southerly winds will roar up the Puget Sound basin, funneling between the Olympics and Cascades.  Once again these mountains are influencing our weather.  The winds will course up Saratoga Passage and Skagit Bay and slam right into South Fidalgo.  The Cliff Mass Weather Blog posted on the subject just in time, and explains it much better than I can.

Peak winds at my station would reach 33 mph (53 kph) at about 5:00 AM.  I woke to the sounds of rumbling and of fir cones and small limbs dropping on the roof.  A similar storm rolled through just three days ago on Thanksgiving Day bringing 35 mph (56 kph) gusts.  We can expect one or two of these wind events each week through December.  This morning, the winds would become calm with the arrival of the rain, which is another typical pattern for us.

Because windstorms are so common here, I have added a widget to the Current Weather page on a trial basis:



The widget shows wind predictions four days out and revealed this morning's event right to the hour.  I always check these widgets out carefully before making them permanent.  It doesn't link to anything nasty that I have found, but it may be disrupting page loads a bit.  I'll be keeping a close eye on it for a while.  I would appreciate hearing feedback about the feature, especially if it seems to be affecting page loads in different browsers.

Comments

  1. Cool post and cool site; yesterday was an interesting day for the rain shadow, but not one of those "classics" like the one Scott Sistek blogged about on November 16th...http://www.komonews.com/weather/blogs/scott/134008448.html.

    Yesterdaywe had .15 " of rain at my station in Sequim, which as a about 1/3 of Seattle's. Still it was cloudy overall...

    David

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