July 17, 2018

Marine Air Surges Inland

For those of you unhappy about the recent warm days, living in a city where AC is rare, relief is at hand.

As I speak, marine air is pushing rapidly into western Washington.  Trees are swaying, temperatures are rapidly falling, the air is moistening, and my wind chimes are ringing.   Life is good.    Our natural AC is turned on, and a night of comfortable sleeping is in store.

It has been substantially warmer than normal for the past week, with several days of high temps that were 10F or more above normal (see plot, purple line shows the average highs, cyan, average lows).  There was a bit of marine air leakage today...resulting in highs dropping 5-7F from yesterday.

All meteorologists know to watch the onshore pressure gradient--the difference in pressure between the coast (say Hoquiam, HQM) and Seattle (SEA)-- to get an idea of the amount of marine influence.  Just a small change from offshore to onshore pressure gradient can make all the difference.  Here are the pressure differences today.  Note that the Hoquiam to Seattle pressure difference rose to 3.5 hPa--that is enough to guarantee a good surge of marine air.

Winds have responded to the onshore pressure gradient. At Smith Island, in the eastern exit of the Strait of Juan de Fuca (and just offshore of northern Whidbey Island), winds climbed to 25 knots as of 9 PM.  At Race Rocks, near Victoria, it hit 40 mph.

At the University of Washington, winds have also picked up to around 10 knots (top panel).  Temperature at the UW is falling rapidly (third panel) and dew point rises (more moisture in the air).  As a result, there has been a huge increase in relative humidity (fourth panel) from 30 to 75%.  This will help reduce fire risk in the west.
The onshore flow is also bringing in the low clouds that have been waiting their chance offshore.  The visible satellite at 8 PM Tuesday shows stratus/stratocumulus pushing inland, particularly south of the Olympics through the Chehalis gap (see below).  If you live in western Washington expect to see lots of clouds when you wake up Wednesday morning.

The next few days should be typical for mid-summer--low clouds in the morning and temperatures rising into the upper 70sF during the afternoon.  Perfection.

There is a cloud in the silver lining though....with cool air and higher pressure in western Washington, winds over the eastern slopes of the Cascades (e.g., Ellensburg) should strengthen considerably, resulting in an increase potential for stoking any fires.   Even with the minor cool-down today, the winds at Ellensburg revved up quite a bit, with sustained winds reaching 23 knots and gusts to 37 knots  this evening (see plots)

Expect even stronger winds there tomorrow.

Morning update.  The cool air moved in, but was quite shallow.  Here are the winds and temperatures above Sea Tac Airport (time on x-axis, pressure on y axis, 850 is about 5000 ft).  Above that level, nearly no change in temperature, but at around 2000 ft the temperature it  is about 10C (18F) cooler.

The visible satellite photo at 6 AM shows low clouds reaching the Cascade foothills, but it is still clear in the mountains.  A hike this morning to a low peak would be glorious.....going from cool to warm and looking down on the low clouds.    But hurry!  They will burn back quickly.


  1. Can I ask a stupid question? When there's a pressure gradient, doesn't the Coriolis Effect mean that the air should move perpendicular to the gradient (as it does around the low-pressure center of a cyclone)? Is this a scale issue—over these short distances, the effect is too small to matter?

    1. Pressure gradient winds are more of a secondary flow, and are more of a result of the Coriolis effect. Air at the surface flows from high to low, those highs and lows being created by areas of circulation. The air is moving to equalize the pressure.

      So it’s a mixture of things. At the surface, winds are more influenced by topography than coriolis.


  2. Cliff - any way to predict if we're getting another 'blob' that will give us 90F+ for 20 straight days?

  3. Hi Cliff,

    Looking at the first graph, I can see a small bump appears around evening-night on most days, where the temperature drops slows, stops or even reverses like on the night of the 10th.

    Is there a meteorological reason for that little bump? It seems pretty consistent.


  4. I'm curious about something: why do we get low clouds off the Pacific, but in California they get fog? Topic for future blog perhaps?

    1. In the winter time the marine layer is mostly fog. Fog is just clouds “on the ground”.

      I think in California’s case, the “cool” layer, is shallower, allowing the atmosphere to “push” it down closer to the surface easier. Also, heights are usually higher, compressing things further.

  5. Awoke this morning to several pressurized household products, specifically cooking spray and stainless steel appliance cleaner, hissing and discharging in our kitchen. So strange?? Could a rapid change in barometric pressure explain this?

  6. I’ve been wondering the same thing too - seems like the heat wave is going to last forever now! Also when can we expect our last heat wave this year ?

  7. Seriously? We have a chance of hot weather 2 measly months a year and all people can hope for is clouds and wind??!

  8. Christie... I moved out here 16 years ago. Cool summers until 2015. I did not sign up for this.


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