Thursday, May 2, 2019

Low Cloud Season Begins in the Northwest

It happens every year sometime during mid-spring.

The low-cloud season began in western Washington today.

You know what I mean, if you have lived here any amount of time.  But if you don't, this morning's Space Needle PanoCam tells the story in dreary detail.  On many  such days, the low clouds thin or burn out by lunchtime.  But on depressingly too many days, it holds in through much or all of the afternoon.


During the low-cloud period, which often stretches from May through mid- July, the northeastern Pacific fills will low clouds.  Here is today's GOES-17 full-color visible satellite image this morning--a huge area of the Pacific is full of these clouds!


Fortunately, you can generally escape the low clouds by crossing the Cascades into eastern WA or Oregon.

Ironically, all this low cloudiness is the result of high pressure building into the northeastern Pacific, something illustrated by the sea level pressure forecast for later Thursday afternoon (see below).  In contrast, there is lower pressure over the continent.  The map also shows temperature around 3000 ft., with warmer air being orange and red.


This weather pattern is a low cloud producer for several reasons. 

First, with high pressure offshore and lower pressure inland there is an onshore pressure gradient, which tends to push cool, marine air inland.

One strike.

High pressure extends aloft and it associated with sinking air above the surface, which warms by compression.  This sinking decreases towards the surface (since air can't go through the ground or ocean).   Such warming aloft tends to produce an inversion (temperature warming with height) aloft that acts as a cap on the lower atmosphere-- allowing the ocean to cool and moisten a lower layer, without bring down any drier air from aloft.  Good for clouds.

Here is the vertical sounding today at Forks, on the WA coast.  Temperature is on the x-axis and height (in pressure) is on the y-axis.   700 indicates about 10,000 ft.  The right-hand line is temperature, the left hand one is dew point.   The lowest 5000 ft is near or at saturation (the temperature and dew point are right on top of each other)--that is cloud layer.  An inversion is right above it.



Two strikes.

The high pressure also brings northerly (from the north) winds over the coastal zone, which causes upwelling of cold water from below the surface.  As warm air from farther out over the ocean moves towards that coast, it cooled by the coastal waters, resulting in saturation and clouds.

Three strikes.   Low clouds rule.

The good news is that in May there is still enough weather activity that high pressure is often displaced, moved about, or replaced by low pressure, which can get rid of the low-clouds for a while and even give us a temporary warm spell.  That should happen next week.

But then in June, when the weather activity quiets down and the high pressure become persistent and stronger, the low cloud settle in for much of the month.  You know what that means:  June gloom.

One good thing about the low clouds--it keeps many Californians away, thus keeping our real estate prices in check and the traffic from getting even worse than it is today.



11 comments:

Will Viharo said...

The clouds are exactly why I moved here from CA years ago and never looked back. I hate the sun. It actually depresses me. Thanks for the reassurance, Cliff.

Unknown said...

There is certainly June Gloom in coastal California. I believe the term even originates in SoCal.

Phil said...

Now Cliff, you must know it’s far too late to think of keeping Californians away..��..
Besides, as a former resident of the Bay Area I can vouch that the low cloud pattern far surpasses that of Puget Sound. Most of the summer there the weather forecasters just set the recording and leave town: “Low clouds and fog nights and mornings...”
At least we’re usually spared the late afternoon and evening fog.
As a matter of fact, didn’t you recently post a blog about how our summers here are the best in the country, better than California?
And yes, I agree the “June Gloom” term originated in SoCal.

Robert Smaus said...

Beat me to it - West Los Angeles and Santa Monica must have invented "June Gloom" which lasted, often, into the 4th of July so the fireworks disappeared into the clouds.

Unknown said...

What Will said. I moved here 30 years ago to escape Miami's oppressive sun and heat. Each summer for the 1st decade I had the opposite of SAD as summer approached

Kwa829 said...

At first, I had much the same reaction to the first four commenters about June gloom; that is, I lived in the San Francisco Bay area for 6 years (though to be clear, I am not a native of California; I grew up in North Dakota and Wisconsin) and remember the "June gloom" there.

But maybe the thing about keeping Californians away is that in a hypothetical scenario in which June gloom did NOT exist here, but did exist farther down the coast, then perhaps some Californians would be attracted to Washington State as an escape from their June gloom.

I'd also say that our usual stunningly beautiful weather from July through mid-September makes enduring "June gloom" well worth it. And, besides, it's not like we have June gloom every day.

Megan Baldwin said...

I'm the same way. I grew up in the low country of South Carolina, spent 15 months in South Texas just a stone's throw from the Mexican border, a year in Tempe, AZ and a collective 2 years in parts of Southern california. I've had my fill of sun and heat. I visited WA in 2003 and immediately fell in love (both with the area and my now husband). I knew then that I had to find a way back to stay and finally achieved that dream when I moved to the Seattle area 10 years ago. The gloomy, cool weather makes me SO happy and I'm grumpy when it's warm/hot and sunny (inverse S.A.D.)

Unknown said...

It's called May Gray this time of year in California and it started end of April.

Friends of the Garden of Ideas said...

Hey Cliff,

Any way you could cast your eyes towards the Northeast and tell us why we've been experiencing relentless rainfall since last August? Seems like every single weekly front has brought moisture, even through the winter. What's the cause of this pattern and do you see it breaking anytime soon? Thanks.

Ansel said...

But July actually has a better sunshine record than August. Once upon a time, before climate change back in the 70's, August was usually rainy and cloudy the third week. Now that those clouds appear to have been replaced by smoke, I coin the term "Smogust", if it hasn't already been invented. Let us hope that if there's a silver lining to climate change at all, it will reduce June Gloom to balance Smogust. Recent years suggest to me that it might.

Unknown said...

That's one of the very few things I dislike about the Seattle area: gloom. A period of blessed forgetfulness for 2.5 months, then BLEEEEEHHHH 9.5 months. Oh, and the sucky roads and traffic.