Rain Shadow Winds

During the past day or two there have been moderately strong southerly winds over the central and southern Sound.    We have been cloudy, but with very little rain. In fact, this is some of most cloudy weather and the lowest solar radiation we have ever experienced in March when it was not raining.

 In contrast, winds on the coast have been westerly and its been much wetter there, as has been the case in the Cascades.

What is going on?   The answer:  A profound rain shadow over Puget Sound, and what I call rain shadow winds.  A new term to add to the western Washington weather lexicon.

Let's start by looking at the winds in Seattle (at the UW) for the last three days (ending noon today).  The second panel is wind direction.  We have been stuck in southerly flow.  And for over a day the wind speeds have been nearly unchanged... gusting to 10-15 knots. Virtually no rain for the period  (.01 inches)

The winds aloft along the coast have been persistently from the west (westerly) to southwest in the lower atmosphere, something indicated by the radiosonde soundings at Quillayute on the Washington coast.  A sounding at 5 PM yesterday shows this (below). The wind barbs are in blue and heights are in terms of pressure 850 is about 5000 ft.  The winds were westerly at 10,000 ft (700) and southwesterly below.

With westerly and southwesterly flow in the lower atmosphere, the air rose on the western sides of the Olympics and coastal mountains, producing precipitation.  But then it sank over the eastern side, producing warming and a rain shadow over central and western Puget Sound.   And then it rose again on the western side of the Cascades producing more precipitation.  

The NOAA precipitation total for the 24h ending 5 AM this morning shows this precipitation pattern.

And the sinking air and rain shadow were even evident in the visible satellite imagery this morning (see below).  Small rain/cloud shadow to the east of the Olympics, big one to the east of the Cascades.

How does this all explain the relatively steady southerly winds from roughly Lynnwood to the south Sound?

With strong southwesterly/westerly flow approaching the Olympics, there was substantial sinking on the eastern and northeast sides of the barrier.  Such sinking causes warming by compression and warm air is less dense than cold air.  Less dense air results in lower pressure to the lee of the barrier. 

A forecast of sea level pressure (solid lines) and winds from the WRF model, valid 2AM this morning shows this pattern--with a lee low or trough of low pressure east of the Olympics.  South and southeast of the low there is a relatively strong north-south pressure gradient because of the lee low, and that difference in pressure drove moderate southerly winds.

The bottom line:  there is a direct connection between the rain shadow and the strong southerly winds, with both forced by sinking on the lee (eastern) side of the Olympics.

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Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the main annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.
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