A record warm day goes out with a bang

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August 16 will certainly go down as one of the more memorable weather days of 2020.

Lets start with the heat. Highs were in the upper-90s across much of Puget Sound, with a few stations hitting 100. Perhaps the most noteworthy measurement was the high of 96 at Snoqualmie Pass at 3,010′ elevation.

High temperature in western WA, August 16, 2020

Widespread 100s were observed in eastern Washington, with temperatures approaching 110 in the Columbia Basin near Moses Lake. I’m not sure if that 112 reading is legitimate or not.

High temperature in eastern WA, August 16, 2020

A number of daily record highs were broken on both sides of the state, as well as across most of the western US. A few areas (e.g. Oregon, northern California) avoided record highs, mostly thanks to cloud cover.

Locations experiencing record temperatures on August 16, 2020.

It might be hard to believe after the hot summers of the late 2010s, but Sea-Tac airport had not reached 98 degrees since 2009.

What was especially remarkable about that high of 98 is that it occurred with (1) cloud cover and (2) a south-southwesterly wind (i.e. an onshore wind plus no local boost from the hot runways to the N-NW of the weather station).

The cloud cover might have been responsible for keeping Sea-Tac below 100 degrees. It also was the first sign of an exciting evening that followed the warm day. It is worth taking a closer look at how we went from record heat to thunderstorms in a span of just a few hours.

The water vapor satellite image yesterday afternoon (below) shows the moisture content of the upper atmosphere. The first thing that sticks out to me is the high amplitude ridge (the upside-down U in the image) over the all of the western US and Canada. Underneath the ridge there was a huge stream of moisture (red arrow) coming straight from the tropics. Finally, the small white arrow over the Pacific indicates an upper-level disturbance that was responsible for the lift needed to trigger the thunderstorms.

Water vapor satellite imagery at 4:30 PM PDT on Sunday August 16

So two of the three ingredients for thunderstorms were present (moisture and lift). What about the third ingredient (instability)?

Experienced meteorologists and weather enthusiasts probably noticed the presence of altocumulus castellanus clouds in western Washington yesterday afternoon. The significance of these clouds is nicely explained by Michael Snyder below:

Sure enough, thunderstorms went off just prior to sunset:

Radar GIF from the evening of August 16

I was out for a bike ride at the time and captured the developing thunderstorms from West Seattle. The combination of a high cloud base and the setting sun was remarkable.

Photo (by Joe Zagrodnik) of the developing thunderstorms from West Seattle around 7:45 PM Sunday.

Not only did Michael nail the forecast but he also captured the photo of the day. Absolutely stunning!

With all of the lightning and relatively little rain, one hopes that no major wildfires will result from yesterday’s excitement. Fortunately, cooler temperatures are expected this week with some light rain possible by the end of the week.

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