May 28, 2017

Spring Stratus Inundates the Eastern Pacific

Every spring it happens---the eastern Pacific, offshore of the West Coast, fills with stratus--and this year is no different.  This norming's Norhtwest visible satellite imagery show stratus/stratocumulus over the offshore waters, with some of it extending eastward  into the Strait of Juan de Fuca and south of the Olympics.   This onshore extension is being encouraged by a weak onshore pressure gradient (about 1 hPa higher at Hoquiam than Seattle).  Small variations in the east-west pressure differences controls our summer weather.

Pulling back to see the entire West Coast,  there are clouds offshore from southern California to British Columbia.  We are all in this coastal soup together.  And for the same reason:  an extensive area of high pressure!

A few days ago, a ridge of high pressure started building offshore (see upper level, 500 hPa, map for 5 AM Thursday.

But as the days passed, the ridge slowly drifted eastward (see map for 5 AM Saturday).  But still impressive.

Building high pressure offshore is good for coastal stratus development in many ways. High pressure aloft is associated with sinking (or subsidence) that weakens towards the surface.  Sinking causes warming by compression, so there is more warming aloft.  Plus, air near the surface is cooled by contact with the cold Pacific.  Both of these mechanism tends to create an inversion (warming with height) above the surface, which is very stable (fights against vertical mixing).  That leaves a shallow layer near the ocean that is cold and moist--and full of stratus/stratocumulus.   Higher pressure offshore at low levels helps to gently push the marine air into western Washington.

Let me illustrate what is going to by showing you a vertical sounding at 5 AM this morning from a balloon-borne radiosonde at Quillayute, on the WA coast. Red is temperature, blue dots indicate dew point, a measure of moisture. Very nice inversion at low levels (temp increasing with height) and shallow saturated layer (temp and dew point the same) near the surface.  The air is actually quite dry aloft (dew point and temperature are separated).

High pressure becomes more persistent in June over the eastern Pacific and low clouds will be a familiar sight.  The result for the Northwest?   June gloom with lots of low clouds and temperatures in the 60s.  Enjoy.


  1. It seems to me the skies are not as blue as I recall them during the 60's and 70's. Wondering if there is data on how "blue" the sky is and if it has changed to the extent we can see it. In the immediate days after 9/11/2001, airplane traffic was grounded. I remember reading a very short article on how dramatically the clarity of the sky improved due to lack of airplane contrails. I figured for sure we would see alot more research and public information on this topic, but it seems to have fallen in some kind of scientific black hole. Anyone know more about this? Thank you.

  2. June Gloom is one of the primary things "wrong" with our climate in my view. May, on the other hand, is one of the nicest and most "balanced" months, with most Mays having plenty of sun AND plenty of rain. June, though- more clouds AND less rain. But not always.

    But it is not OUR high pressure that creates the problem- it is the high over the sea working with the thermal low in Eastern WA- plus the up-welling which creates fog when the air from over warmer water (and higher dew-point) gets cooled- the right? The up-welling is the real culprit.

    On a related note, cliff, can you tell me why the sound breeze often isn't well developed until well after 5 PM- when the land is already beginning to cool? It is frustrating for us sailors- not much wind all day, then a nice breeze about the time we want to drop anchor and fix dinner... like this past holiday...


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