July 03, 2017

Why is Puget Sound cloudy when the rest of the region is clear?

It just isn't fair.   While nearly the entire region basked in sunshine, the Puget Sound basin had considerable cloudiness, breaking to partly cloudy skies for a few hours.   The satellite image at 3:40 PM tells the story (see below).  Even the coast had less clouds!

All the clouds caused the temperatures to cool down today, with much of Seattle not getting beyond the lower 70s.  This chilling story is illustrated by the UW Seattle weather observations for the last 3 days (below, time increases to the right).  The third row show temperatures (black line)--clearly cooler today.  The bottom row shows solar radiation.  On Saturday and Sunday we had clouds in the morning, but they were pretty much gone by noon.  Today, the solar radiation was suppressed much of the day.
This Puget Sound cloudiness in summer is not rare...here is another example, from June 20.  I could show you a dozen more.

So why all the cloudiness over Puget Sound?  A plot by Jeff Bezos to keep his minions working?  No.  It is the result of northwesterly flow and a very big obstacle--the Olympic Mountains.

The flow approaching the coast this morning and early afternoon was northwesterly (from the NW), as shown by this short-term forecast for surface winds and sea level pressure (solid lines) for 8 AM.

An expanded version over Puget Sound shows the convergence quite clearly (the winds are parallel to the wind barbs):

 NW flow is blocked by the Olympics, moves around the barrier to the north and south, and then converges over Puget Sound...a summer version of the Puget Sound convergence zone.  If air converges at low levels, it is forced to rise, causing cooling, increasing relative humidity, and eventually condensation into clouds.
      Here is the relative humidity forecast for 11 AM at 5000 ft. White is at saturation (100% RH).  You see the problem?  High relative humidity over Puget Sound and the western foothills, where northwesterly flow is forced to rise by the Cascades.

What you really want to know is about tomorrow.  Will the Puget Sound gunk stick around?    I have good news.  The answer is no.

Temperatures should increase by about 5 F on Tuesday and there will be far fewer low clouds.    The latest model run shows that some high clouds, now over the Pacific (see satellite image Monday evening), will move in over the region...but won't be a problem for the fireworks or anything else.

So enjoy the fourth..conditions should be excellent for the fireworks anywhere in our region.


  1. Why is it not possible to forecast the weather for this region, even on the same day? Very frustrating!

  2. Luckily some of us like the clouds. :)

  3. These details are going to become especially important down in Oregon for the week of August 21st. Imagine a scenario where all of the eclipse watchers in Lincoln City and the Willamette Valley suddenly realize they will miss it due to stratus and they suddenly head en masses across the Cascades! The Seattle Times had a scary article about this last week. Cliff, we are going to depend upon your guidance!!!

    I personally am going to make my choice at midnight the night before and head out from our hotel rooms in Kelso. I will have a party of Japanese paleontologists in tow.

    A good tip is to stay 2-4 miles off the centerline. You will only miss about 6-10 seconds of totality. As soon as the shadow bands disappear you are only observing a partial eclipse so if you want to get out of there before gridlock ensues, leave then 5 minutes after totality. The traffic jam will be mostly behind you!

    They are calling this the Largest Human Migration in history. Each state between Lincoln City and Raleigh is expecting a million visitors or more.


  4. You may not like the clouds, but in Portland they often don't show up for weeks at a time in summer, leading to ninety - degree days like today. I'd rather take the clouds, if given a choice.

  5. Cliff, I have an interesting cloud formation photo I'd like to send you. What email can I use?


  6. Catherine--the example Cliff gave shows the UW model did pretty good that day. This morning brings an example of a very good forecast--compare the satellite picture at 7am to a low cloud forecast for the same time--they match up extremely well!


    The models do pretty good sometimes, and sometimes they miss. There is also a problem with communicating what the model shows, you can be sure that a lot of good information for the computer simulations is lost in translation. But there are also times that all the fine detail in the model has to be thrown out. It is tricky. IBM or Panasonic or NOAA or maybe the UW will figure out how get you a better forecast someday soon. There are competing interests, maybe Google ventures can fund your startup to get it right, heh.


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