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.