Major Cool Down

For those of you unhappy with the unseasonal, but wonderfully warm and dry, weather of late,  a smile will soon be on your face.  An extended period of cool, cloudy, and sometimes rainy weather is in store.

The key transition is a profound change in the upper level flow pattern.   For the past week, the circulation aloft  has been dominated by a ridge of high pressure over the western U.S., something documented in the upper level (500 hPa) height anomaly map (difference from normal) shown below.  A large positive anomaly (yellow/orange colors) is evient over the western states.  The means high pressure (or heights)

But the situation is rapidly changing right now.   Let me show you the 500 hPa height anomalies predicted by the U.S. GEFS ensemble system for Friday afternoon (5 PM) and for the following Thursday:  substantial negative anomalies over the western U.S. coastal zone, which means troughing or low pressure.

Reflecting this upper level pattern, the latest NOAA Climate Prediction Center 6-10 day forecast is for cooler than normal and wetter than normal conditions over the Northwest (see below).  

As I will describe in my next blog,  the global circulation pattern has experienced a profound shift during the last few weeks.   El Nino has collapsed and high pressure over western North American has shifted.  None of our global models indicate a return of ridging (high pressure) over our region during the next several weeks.

Tonight (Tuesday), an upper level trough is approaching the West Coast, with the ridge retreating to the east (see graphic)

An approaching trough produces upward motion, which helps force thunderstorms, and many were observed over eastern Oregon during the afternoon and early evening (see radar for 5 PM).  This trough will also force more marine air inland.

Tomorrow will be a transition day and after that we will be in the upper 60s for at least a week. This cool/wet period will reduce wildfire potential east of the Cascade crest and reduce evaporative loss for agriculture.

Smoke and Fire

Last night, enjoying a warm evening outside, I noticed a very strange "dark cloud" moving southward over Lake Washington.    Getting a better look, it had the appearance of a smoke plume and I quickly learned about a huge fire along the Everett waterfront (see image from KOMO TV).

A series of images from the wonderful SpaceNeedle cam, showed the southward movement of the smoke.   The first image is at 6:50 PM Saturday;  you can see Mt. Baker---and keep that location in mind during the subsequent images.

 By 8:00 PM the plume was evident on the horizon.

At 8:30 PM it had reached northern Seattle

And by 8:50 PM it had moved past the city.

Now you might ask, why did the plume of smoke not spread in the vertical?  What made it stop at one elevation in the low-level northerly flow as a VERY well-defined plume with little vertical depth?

Good question.    As shown in the picture above, the smoke plume initially rose upward from the fire.  This makes sense because the fire-heated air was warmer than the environmental air at the same level and thus was buoyant.     But as many of you know, rising air parcels cools as they ascend to regions of lower pressure aloft, where they expand (called adiabatic cooling).  But Saturday evening there was something that blocked the upward motion of the warm air plume from the fire:  a strong inversion, in which temperature increases with height.

On Saturday, there was a shallow layer of cooler, northerly flow over the Puget Sound region that was topped by easterly flow aloft, which was quite warm due to its origin over warm eastern Washington and adiabatic compression as it sank down the western slopes of the Cascades.   A time-height cross section of winds and temperatures over Seattle on Saturday shows this (the y axis is height in pressure--850 is about 5000 ft, red lines are temperature in C, time increases to the left--in GMT)

As shown in the vertical sounding over Seattle on Saturday, there was a strong inversion between roughly 400 and 600 meters above the surface.   Such warmth aloft kills the buoyancy of air rising from below and blocked the upward motion of the smoke plume.

In contrast, to the Everett Fire, the oil train fire in the Columbia Gorge had a smoke plume that rose and mixed through substantial elevation (see pictures)

The reason for the difference?   No inversion blocking the plume rise.

Let me end, with an editorial comment.   Moving large amounts of oil by train is crazy stuff.  Oil trains move through densely populated regions (like Seattle) and next to environmentally sensitive areas (such as right next to Puget Sound and the Columbia River).   Trains derail all the time.   There are frequent slope failures next to train tracks.     Why is our region is taking such a large risk for our population and environment to help move Bakun and other oil to market?  And all these oil (and coal) trains make our traffic significantly worse.

Finally, today (Sunday) will be very warm (hitting 90F around Seattle), but changes are already in place that will bring substantial cooling on Monday.  And a MAJOR cooling is going to occur mid to late week.   

If you were worried that this year is going to be a repeat of last year, don't.  The atmospheric circulation is very different this year and high pressure should not hold in place like 2015.

Weekend Heat Wave

We should see some very warm temperatures this weekend, and certainly some daily records will fall.  But the heat will be short-lived and more temperate conditions will return on Monday.

A strong upper level ridge (high pressure) has been in over our region and will continue to amplify for the next day.     Here is the 500 hPa (around 18,000 ft) heights (like pressure) for 5 PM Saturday to illustrate.  Huge, broad ridge over the West Coast.   In contrast, a cool trough is over the eastern U.S.

This ridge brings warmer temperature aloft and the development of a thermal trough that will progressively extend up the West Coast.

Here is the 24 h forecast of sea level pressure, surface wind, and low-atmosphere temperatures (925 hPa) for Saturday at 5 PM.   There is a tongue of low pressure in western Oregon that extends into western WA...that is the thermal trough.   VERY warm temperatures in Oregon. The northern portion of the trough is associated with strong offshore and downslope flow.

A close up view of the temps at the same time shows that Portland should be over 100F on Saturday. and southwest WA will reach the 90s.   Puget Sound is cooler.  Why?  Because with the thermal trough centered south of us, there will be a north-south pressure gradient that will drive northerly winds and cooler air from north.  Low to mid 80s around Seattle.  Cooler over NW Washington.

At Sunday at 5 PM, the thermal trough will extend north and start to transition over the Cascades.  This will be the warmest day around Seattle.

 The temperature forecast for this time shows 90s around Olympia and over the eastern suburbs of Seattle, which temperature will peak around 90F.      Eastern WA will warm up on Sunday as the thermal trough starts making the move. The coast will start to cool down that day.

Monday at 5 PM is a different world.   Cooler air has moved in west of the Cascades crest and you can see the resulting strong pressure gradient over the western side.   Eastern WA has warmed considerably.

The temperature at 5 PM Monday illustrates this...cooler over western WA--perhaps reaching 80F in Seattle.  But 100F in portions of the Columbia Basin.  That should ripen some crops.

Temperatures will further moderate during next week, dropping into the 60s by Wednesday.

Rainbow Clouds Seen Around the Region

I received about a dozen emails yesterday (Tuesday) from folks seeing the most beautiful, multi-color clouds stretching horizontally above the horizon:  let me show you a few examples:

Stuart Lochner sent me this one from near Naval Station, Everett

Kim Rochat forwarded this one from Camano Island

Sherry Scherer sent this from Stevens Pass

And another one from Stevens Pass, from Chelan Robbins

Notice that in all these pictures, red is on the top and that the color bands are oriented relatively parallel to the horizon...this will be important.

Yesterday, the sky was full of ice crystals associated with cirrus and cirrostratus clouds.   You could see the thin clouds on the visible satellite image.

Or even from the Space Needle Cam during the early afternoon:

So what was happening here?  

 I believe we had a circumhorizontal arc, in which the sun's light was refracted (bent) by ice crystals in the thin upper clouds.   To get the arc, the ice crystals need to be in the form of "plates" that float downward with a similar horizontal orientation (drop a small paper square and you will see that this can happen).     The sun's rays are bent (or refracted) as they pass through the ice plates (see image), passing through the narrow edge and passing out of the broad lower section.  

Different wavelengths of light are bent differently (something called dispersion), resulting in a rainbow of light, with red at the top.  A familiar example of dispersion occurs in a prism.

To produce this rainbow arc, the sun must be at least 58° above the horizon (any less and the light will pass overhead and not be visible).   Today, the sun got to 64° at noon, so there there was a limited time window to see the arc.  Due to our northern latitude, the circumpolar arc is only viewable from early May through early August.    So keep watching during the next few months--there is a good chance this phenomenon will occur again.

Washington State is Losing Its Signature Views. Can We Restore Them?

Meteorologists really care about views:  there is nothing we love better than a expansive vista of clouds and mountains.   And Washington State is the home of some of the most stunning viewpoints in the nation, often located on local peaks or near major water bodies.  

But inaction by State government, either deliberate or not, is resulting in the loss of some of the best viewpoints in the State, mainly because of the growth or encroachment of trees.   A bit of judicious pruning could fix the situation in a few hours, something advocated in this blog.   Let me give you a few examples.

Historically one of the grandest view of our region, and perhaps the best in the State, has been found at the top of Mount Constitution on Orcas Island (see map).

A stone tower built on the summit by the WPA during the 1930s used to provide a stunning 360° view, with the lofty height of the Olympics, Vancouver Island, Mount Baker and the Cascades, the islands of the San Juans, and Puget Sound surrounding the lucky visitors.   One of the great views of the world.

Unfortunately, the trees around the summit have been left to grow and the views to the south, west, and north have been lost or severely degraded.  To appreciate the loss, here is an information sign on the top of the summit structure, showing the vast and impressive scene to be enjoyed (at least several decades before).  The problem:  you can't see any of it today because the trees have grown up.

Or perhaps you have enjoyed the hike to the top of West Tiger 3 near Issaquah, one of the most popular hikes in the State and only 30 minutes from Seattle (see map).  The view from the top used to be amazing, with the entire Sound laid out below you.

But, again, trees have been allowed to grow, wiping out nearly the entire view (see below)

Or what about Chuckanut Drive, south of Bellingham?   It WAS the most beautiful coastal drive in the State with amazing views of the San Juan Islands (see map).

 You guessed it.   Trees have been allowed to grow, blocking the stunning views.

Now I could give you many more examples, but you get the point.  One of the reasons that folks love living in Washington State and why tourists like to visit are our amazing views of the water and mountains.  But we are losing many of our most precious viewpoints because the State seems to have a policy of letting trees grow, even if such growth is robbing us of some of the most uplifting and inspirational views.

Is there a policy not to protect views?  To always let trees grow?   Is ANYONE in State government responsible for ensuring that views are maintained?   I love trees as much as anyone, but views are important and it would be a shame if future generations are robbed of the views that many of us have valued and savored over the years.  Let's do something about it.  And if you know some other good examples, please leave a comment or email me.

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.