Summertime Low Clouds in Winter!

The satellite picture this morning did not look like mid-March.   Really reminded me of June or early summer.

A massive area of low clouds (mainly stratus and fog) covered the northeast Pacific, with some of the clouds pushing into western Oregon and Washington.

A view from a webcam at LaPush on the northern WA coast this moring was pretty dismal.

Viewing a local, high-resolution visible satellite image at 8:06 AM this morning shows the low clouds offshore, pushing into the Chehalis Valley and eastward down the Columbia.  Look closely and you will see some fog in the Snoqualmie drainage and around Bellingham.

Look even more closely and you will see the thin, tell-tale streaks of contrails, many going north-south and some oriented east-west.    There are some wide super-contrails (oriented east-west) along the coast, with their shadows on the low cloud below.

So why this summer-like low-cloud pattern?

Because summer-like high pressure has built offshore over the eastern Pacific (see surface pressure map for 10 PM Monday evening).  

A high-pressure area offshore results in northerly winds along our coast (typical of summer) and a low-level stable layer or inversion (temperature increasing with height), which is ideal for promoting low clouds.

High pressure is associated with sinking air aloft, particularly on the western sides of high-pressure areas.   Sinking air warms by compression (like in your bicycle pump, which is warm to the touch after you fill your tires).   Clearly, sinking is weaker near the surface (air can't move through the ground).  With greater sinking and warming aloft, an inversion often forms at low levels when high pressure is nearby.

Inversions are very stable structures, suppressing vertical motion, which is good for low clouds and fog.  They can also foster haze and air pollution.

Want to see the influence of the current high pressure on low-level temperatures? 

 Below is the lower part of the vertical sounding at Forks on the Washington coast, with the data coming from the radiosonde launched twice a day at that location.  The red line is temperature and the blue dashed line is dewpoint.   

Mama Mia!   That is a very strong inversion, with temperature increasing from the surface (around 1010 hPa) to around 950 hPa (roughly 2000 ft ASL) by about 13C (roughly 23F!!). 

 Temperature and dewpoint are the same in a very shallow layer:  This is where the low clouds are found.

Enjoy this taste of summer.   Much cooler temperatures and precipitation will soon return.