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This is going to blow the July 2009 heat out of the water and into geostationary orbit. Dew points will also be high, between 17-20 Celsius. This goes beyond Arizona-during-the-monsoon-heat and into East Coast heat dome territory. What is the synoptic setup that is causing this combination of deadly heat and oppressive humidity, considering that the Rockies block Gulf moisture from crossing into the West and raising dew points?

ECMWF temperature forecast for June 27, 2021 at 4:00 pm PDT

ECMWF dew point forecast for June 27, 2021 at 4:00 pm PDT

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One might say it's a "perfect storm", heat-wave version. High pressure aloft and wind patterns driven by a low-pressure trough on the ground conspire to raise surface temperatures.

According to a summary from the Washington Post:

The intensity of this heat dome, based on projected atmospheric pressure at high altitudes, “is forecast to be near the strongest on record at Quillayute, WA upper-air station,” tweeted the Oregon Climate Office.

The Weather Service office in Seattle wrote: “temps aloft will warm to readings as warm as we ever see — even in mid summer.”

A look at the tweet:

enter image description here

In other words, pressures above half the normal atmospheric pressure of about 1000 mb are climbing as high aloft as they have ever been.

But wait, there's more from the Post:

The heat’s intensity will be augmented by what’s known as a “thermal trough,” a zone of low pressure near the ground that increases air flow from east to west or land to sea, effectively cutting off any cooling sea breezes.

So there is the second element of the heat wave, not only are overland temperatures raised by the pressure pattern aloft, but cooling breezes from the sea are blocked from competing with the overland heat dome.

Perhaps the editors of the Post could not resist making their opinion of climate change known given this weather situation:

Evidence suggests heat domes such as this are becoming more frequent and intense due to human-caused climate change and the buildup of heat-trapping greenhouse gases in the atmosphere.

We'd like to see the evidence before pronouncing our own opinion on such a grand cause, but the pressure distribution is certainly there.

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    $\begingroup$ @OscalrLanzi A classic Rex block requires a low pressure south of the high pressure over the North West Pacific. Is there a low pressure over southern or SW USA right now ? $\endgroup$
    – gansub
    Jun 26 at 3:40
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    $\begingroup$ The map shows low pressure southwesrwards from the high, over the ocean. $\endgroup$ Jun 26 at 3:52
  • $\begingroup$ I don't remember the rex being so strong this early in the year. Max solar input + rex is not helping. Turn on the AC! $\endgroup$
    – f.thorpe
    Jun 26 at 5:38
  • $\begingroup$ @f.thorpe There isn't any consensus that it is a Rex block - cbsnews.com/news/heat-dome-2021-seattle-portland-weather Others say it is a Omega block. $\endgroup$
    – gansub
    Jun 28 at 6:02

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