A Change in the Sky

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Yesterday from the Puget Sound Clean Air Agency Visibility Camera

Today from Dale Ireland's web cam

Notice that the sky has a very different look the past two days? Yes, there are clouds...but the clouds have a very different look...with the sky filled with cotton-ball, cumulus type clouds (see above two images). Some of the cumulus have grown into cumulonimbus with rain and lightning (see satellite picture below, particularly over the eastern Cascade slopes)

What has caused these changes? Destablization of the atmosphere. And what has caused that?--moving in of cool air aloft.

There really has been an amazing decline in the lower atmospheric temperatures this week. For example, at roughly 4000 ft...the height of Stevens Pass...the temperatures of the free atmosphere has declined 20F or more from the warm period of a few days ago. With the sun and surface heating still being relatively strong, and cool air moving in aloft, there develops a large change of temperature with height. This destablizes the atmosphere, producing convective mixing, like you see in a saucepan when you cook some oatmeal by turning on the burner.

Tomorrow will be like today, perhaps with more convection over the Cascades. And then things go downhill as a series of system move across the region. Looks like the coast and the southern half of western WA (and Oregon) will get seriously wet as the jet stream dives south of us. I would avoid hiking in the Oregon Cascades for a few days starting late tomorrow unless you are ready for wet stuff.

And keep in mind an important safety issue: the first real rain of the season often produces highly slippery conditions as all the oil that has accumulated during the summer emulsifies with the water. Nasty stuff.

In a future blog I will talk about the upcoming La Nina winter. Hint: this will be a better year to secure an annual ski pass.
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