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.