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.