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.

The August 21st Total Eclipse: The Meteorology is Favorable

On Monday, August 21st, there will be an extraordinary astronomical event:  a total eclipse of the sun will cross our region, and specifically northern Oregon.

The map below shows the area of totality (between the two lines), which will only last for roughly 2 minutes.  Totality will occur around 10:15 AM along the Oregon coast and about 11:30 AM at the eastern border with Idaho.

For the big population centers around Puget Sound and Portland, the most convenient locations will be between Salem and Albany, Oregon...less than an hours drive from Portland and four hours from Seattle.  Rooms are impossible to get east of the Cascades, but some are still available around Salem.

The big issue, of course, is weather.  But I think the chances, based on climatology, are quite favorable in the Willamette Valley on that date.  And I will give you some tips on being meteorologically adaptive.

August is generally a warm, dry month in Salem, Oregon.  In fact, August is the driest month of the year there.

The risk of a major weather system coming in during that day is very slight.  The main threat is marine cloudiness, which is a far more serious issue for those hoping to see the eclipse along the coast.  The coastal mountains block a lot of the low clouds and the Willamette Valley is somewhat isolated from the coastal influence because there is no wide conduit from the ocean (like Seattle has with the Strait of Juan de Fuca and the Chehalis Gap).

To get an idea of the threat, I took a look at the visible satellite pictures at 11 AM on August 21st for the last 15 years.  Here is what I found around Salem, Oregon in terms of cloudiness:
    
Clear 50%, 43% Partly Cloudy, 7% Cloudy.

The partly cloudy days either had some thin high clouds or some scattered low clouds.  You would still be able to experience the eclipse.  The cloudy day had a continuous low cloud layer.    If I was in Salem, Oregon under such conditions, I would head up into the Cascades on routes 22 or 29 until I got above the low clouds (see map).

Eastern Oregon is obviously another possibility, with less chance of clouds.  But travel there is much longer, there are no rooms available, and the limited roads almost guarantee a huge traffic jam there.  And that region has another problem:  the potential for smoke, which has been an issue on that date in the past (see example from 2012).

NOAA, using airport stations, has provided a map of probability of clear conditions on August 21st (see below).  The suggest 60-80% chance in Salem and 80-100% east of the Cascade crest.

In the days before the event we should have good forecasts to work with, so stay tuned as we get closer in time.