June Gloom in May

June gloom came early this year, with a transition in mid-May to the typical June low clouds, sprinkles, and occasional sunbreaks.    Actually, this kind of weather in late May is not that unusual.   Here is a plot of Seattle-Tacoma temperatures for the past four weeks (with normal highs and lows shown as well).   Above normal temps through May 15th, and then a switch was flipped, with most days not even reaching the normal highs.

East of the Cascades, shielded from the low-level marine air by the Cascades, the transition was not as profound, as shown by temps at Pasco in the Tri-Cities.  Conditions have been very close to normal during the past two weeks, which is good for agriculture and those worried about wildfires.

Visible satellite imagery tells the story another way.   Take a look at the visible image from 12:30 PM on Thursday.  Classic late spring pattern, with a huge field of low clouds over the eastern Pacific.  Look closely and you will see the low clouds pushing to the Cascade crest, with coastal California from San Francisco southward socked in.

Why the low clouds?  Blame high pressure!  Yes, you have read that correctly.   Here is the surface pressure and wind forecast for 2 PM on Thursday.  A big area of high pressure over the eastern Pacific centered due west of the CA/OR border.

Why does high pressure offshore produce gloomy conditions west of the Cascade crest?

Many reasons!   First, simple pressure differences.  With high pressure offshore and lower pressure inland, low level cool air gets pushed into western Washington and Oregon.  

Next, high pressure is associated with sinking air aloft, something meteorologists call subsidence.  Now the sinking has to decrease near the surface.   Sinking air causes warming, so there is more warming aloft.  With more warming aloft, a stable layer or an inversion (temperature increasing with height) can occur.    In fact, if we look at the vertical sounding at Quillayuate (on the WA coast) on Thursday at 5 PM, that is exactly what happened at around 800 hPa--roughly 4000 ft (red line is temperature, blue dotted line is dew point).  Below the inversion the air is nearly saturated (temp and dew points are nearly on top of each other).  A stable layer aloft allows a layer near the ocean surface to moisten and get full of low clouds.

But as in those late-night commercials,  I have to say: WAIT, there's more!  The high pressure results in northerly flow along the West Coast, moving cooler water from the north southward and resulting in upwelling of cooler water from below to the surface near coast (particularly from southern Oregon down to central CA)--see sea surface temperature plot from yesterday below.   Cool water helps cool the air to saturation, enhancing clouds!

So high pressure  offshore gives us sustained, cool, cloudy weather.   Generally not that much rain--often just some light drizzle and sprinkles.   But hopefully enough to discourage Californians from moving up here (although our increasing traffic is our new secret weapon in that regard).

Now some of you might think that May or June gloom is going to decrease under global warming.

 Think again.   Some of our latest regional climate simulations suggest that global warming could make it worse!  (see graphic below of change in March-May low clouds, between 1990s and 2090s)  Blue is increasing low clouds.  Why?  Because the interior of the continent heats up faster than the ocean and warming temperatures causes lowering pressures.  So the onshore pressure gradient increases under global warming, result in enhanced marine air influx west of the Cascade crest.

 Another good reason why you should support the carbon tax initiation, I732, and do what you can to reduce your carbon footprint.