Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

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The following is an excerpt from a recent article on the Weather West Blog. 

By Daniel Swain of Weather West

An *exceptionally* hot June across the American West

June has been an exceptionally hot month across the entire American West. In fact, the last 30 days was the hottest such period on record in over a century across most of the interior Southwest, including most in the California interior. (

I won’t belabor the point here–it has been incredibly, record-breakingly hot (yet again) across almost the entire American West in recent weeks. The immediate coast in California has, conspicuously, not been nearly as anomalously hot–and has actually been near average in some spots thanks to cooler than average ocean temperatures and a robust marine layer. But that’s a different universe from the interior parts of CA and the rest of the west, which have just suffered through what is likely to become the hottest June on record in many areas. And even more exceptional heat is to come across the Northwest–more on that below.

Incredible & historic heatwave across Pacific Northwest and southwestern Canada

This will be an *exceptionally dangerous* heatwave from a public health perspective, especially since this is a part of the country where structures are not designed to shed heat and where air conditioning is rare. Infrastructure/ power disruption is also possible. #ORwx #WAwx

— Daniel Swain (@Weather_West) June 23, 2021

The most extreme heatwave I can ever recall having watched evolve in my lifetime over the continental U.S. is presently unfolding along the West Coast of North America, centered on the Pacific Northwest and southwestern Canada. This coming weekend, virtually every high temperature record in existence will be broken across an enormous swath of land extending from central BC to central Oregon. This will most likely become the singularly most intense heatwave in modern history for much of the region, featuring temperatures that will probably shatter all-time record high temperatures in many cities and towns and mid-atmospheric high pressure that will be stronger than any previously observed in the region. This is a heatwave that has left meteorological observers in awe, and even caused some of us to question the validity of these extreme model predictions. Well, confidence has now increased to the point that I’m pretty comfortable saying that the models did a great job with this historically unprecedented pattern (save for a slight northward shift, relative to initial predictions, that will largely spare CA the worst of things).

This heatwave will have extremely high societal impacts across the Pacific NW and SW Canada, and will pose a very serious public health threat in a region where structures are not designed to shed heat and where active home cooling (i.e., air conditioning) is uncommon. Heat of this incredible magnitude will not only lead to many heat-related illnesses, but could also disrupt both transportation and electricity-distribution infrastructure–as well as turbo-charging wildfire risk in the densely forested areas that characterize much of the region.

An absurdly strong “Rex Block” will set up shop over southern British Columbia, bringing the most extreme heatwave on record to the entire Pacific Northwest and southwestern Canada.

What’s causing this spectacular, yet quite dangerous, extreme weather event? A “Rex Block” is rapidly intensifying “in place” over southern British Columbia–completely disrupting the mean westerly flow across a broad region and causing mid and upper-level winds to flow almost due east-to-west across WA and OR. As the associated ridge strengthens, very strong subsidence (downward motion) in the atmosphere will develop–causing dramatic adiabatic warming of the airmass as it descends toward the surface and completely suppressing cloud cover. The combination of long-duration offshore flow aloft, very strong subsidence, and near-cloudless skies will produce an incredibly hot airmass throughout the atmospheric column. Once it’s in place, this blocking pattern will likely persist for quite some time–persistent ridges in summer can be very difficult to “dislodge” as they tend to re-organize the hemispheric flow pattern in a way that favors ridge persistence (a “positive feedback”/self-reinforcing cycle). And indeed: the models suggest only a very slow breaking down of this high pressure system heading into the first week of July. This means that even after the record-shattering temps moderate somewhat later next week, very hot conditions will likely persist for many days across the affected areas and greatly compounding impacts (especially if wildfire activity becomes elevated, as seems likely).

Finally, and as many climate scientists have already pointed out, global warming is greatly increasing the odds of experiencing unprecedented heat extremes–both here in the Western U.S. and elsewhere around the world.

Upcoming extreme heat event across Pacific NW & SW Canada is looking truly exceptional. Ensemble modeling tools suggest high likelihood of a broad region of historically unprecendented 500mb heights (ridge strength) & 700mb temps (lower atmospheric temperatures).#BCwx #ORwx #WAwx

— Daniel Swain (@Weather_West) June 24, 2021

California will dodge worst of extreme PacNW heatwave, but interior areas will see prolonged period of very hot conditions

Northern California finally caught a break, of sorts–it no longer appears that the astonishing, all-time record shattering heat across the Pacific Northwest will extend across the Golden State. Given the state of the already extreme drought and the precarity of the wildfire situation, that is exceedingly fortunate.

But it is worth considering what might have been, since this was an exceptional weather event just narrowly averted. Some parts of Washington, Oregon, and British Columbia may break their all-time records by a margin of 5 or more degrees F–a truly astonishing margin in climatological terms. For perspective, in California, breaking all-time records by 5+ degrees would mean temperatures at or above 123F in Redding, 119 in Sacramento, 120 in Fresno, 111 in downtown San Francisco, 118 in downtown Los Angeles, 128 in Palm Springs, and even 104 in South Lake Tahoe. Had this extreme and historic ridge set up shop just a couple hundred miles south, some of these numbers could have been realized this weekend or next week.

But this time, at least, CA got lucky. Phew.

Absolutely astonishing temperature anomalies of 45-50 degrees F above average are present in the *ensemble mean* across Washington state and British Columbia during this historic heat event.

That said, a very prolonged period of fairly intense heat will still occur across most of the CA interior beginning this weekend and perhaps continuing all the way toward the 4th of July in some areas. Coastal areas will very likely be spared, and the heat will not be remarkable there. But over the Central Valley and SoCal inland valleys/deserts, where Excessive Heat Watches are once again in effect, a prolonged period of 100+ temperatures is highly likely. Daily records may still fall, especially across the northern Sacramento Valley and northern mountains closest to the record ridge (Redding and surrounding areas could still see at least a couple of days of 110+, but probably not 115+). This will round out what has already been a record-hot month across some inland portions of California–and will continue to dry out vegetation that is already running about two months ahead of typical dryness values for June.

Significant monsoonal surge could bring some summer showers/thunderstorms to CA next week–even outside of mtns/deserts

Models, and the GFS in particular, are suggesting the potential for a significant surge of monsoonal moisture westward across California and Nevada by the middle of next week.

In this midst of ongoing heat across the Northwest, a rather robust monsoonal surge of moisture may take shape–moving northwestward across California by the middle of next week. The GFS, in particular, is suggesting the potential for both a pretty robust increase in humidity throughout the atmospheric column along with at least one significant easterly wave. That easterly wave, if it actually materializes, has the potential to generate some convective activity even outside of the usual mountain and desert regions and perhaps across coastal/valley areas. Right now, the odds of this happening will be highest across SoCal, but there may ultimately be a risk of more widespread convective activity across NorCal as well heading toward the 4th of July weekend. Details with monsoonal surges are always hard to parse more than a day or two in advance, but this one has the signs of a potentially significant one for portions of CA.

Read the rest of the weather report here.

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a-1624925599 Jun 28, 2021 05:13 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

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MarcelK Jun 28, 2021 04:23 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

"The greenhouse gas with the greatest impact on warming is water vapour. But it remains in the atmosphere for only a few days."

And water vapor cycles constantly, returning to its sources (, whereas our industrial processes are releasing into the atmosphere over a very short time span the carbon from millions of years worth of organisms (

edney Jun 28, 2021 01:55 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

Why aren't we all dead and drowning?
We are now 30 years past the original predictions that we only had 10 years left, despite the emissions chart that shows a steady steep rise

They might be right about climate change, but they are awful at predictions of reprecussions and should stop.

MarcelK Jun 28, 2021 02:53 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

@Bumblebee Greenhouse gases are transparent incoming solar energy but scatter it when it leaves the surface of the planet in the form of infrared energy, resulting in a small imbalance of incoming and outgoing energy, with a slow increase of atmospheric temperature over time--global warming, with the majority of the increased energy being absorbed by the oceans. That increases the frequency of blocking events.

a-1624868912 Jun 28, 2021 01:28 AM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

Climate science deniers will consistently pull the same old tired arguments out of the playbook of big carbon.

Anthropogenic Global Warming is real, resulting from radiative forcing caused by our addition of carbon dioxide to the atmosphere.

You can find the various lies (like "It's water vapor", or "Climate is always changing", or "It's the solar cycle") with links to peer-reviewed climate science showing how the claims are bogus here:

MountainMan4865 Jun 27, 2021 08:20 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

I don't think I'm sufficiently afraid yet.... maybe some more words will do it. Pictures and colors, should totally do the trick.
This is an excellent resource. I posted it about 10 years ago and immediately got dog piled, still an excellent resource. Updates every morning

a-1624851948 Jun 27, 2021 08:45 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

Just to quote one of their more favorable reviews:

"Suspicious Observers (Suspicious0bservers) is a pseudoscience YouTube channel run by ex-lawyer Ben Davidson who has a degree in economics from Denison University and a Juris Doctorate from Capital University Law School, but no actual science training. His ideas have been debunked by genuine climate scientists.[1] He has various websites on which he sells his scribblings and tickets to his "annual conference".[notes 1] Davidson makes far-fetched claims about dozens of scientists attending his conferences and conversing secretly with him, but there has only been one notable name at any of his meetings — John Coleman, another notable climate denier."

From RationalWiki

MarcelK Jun 28, 2021 03:53 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

3:43PM: The point IS to deflect, mislead, misrepresent -- it's denial apologetics, a consequence of a corrupt or absent moral core. Note how far we have gotten from the original statement: "The most extreme heatwave I can ever recall having watched evolve in my lifetime..." and the completely bogus and irrelevant objection that the Earth is much older.

a-1624920579 Jun 28, 2021 03:49 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

From the Myth #12 bust:

"This statement does not tell the whole story. The initial changes in temperature during this period are explained by changes in the Earth’s orbit around the sun, which affects the amount of seasonal sunlight reaching the Earth’s surface. In the case of warming, the lag between temperature and CO2 is explained as follows: as ocean temperatures rise, oceans release CO2 into the atmosphere. In turn, this release amplifies the warming trend, leading to yet more CO2 being released. In other words, increasing CO2 levels become both the cause and effect of further warming. This positive feedback is necessary to trigger the shifts between glacials and interglacials as the effect of orbital changes is too weak to cause such variation. Additional positive feedbacks which play an important role in this process include other greenhouse gases, and changes in ice sheet cover and vegetation patterns."

ChemicalSuperFreak Jun 28, 2021 03:27 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

"CO2 didn't initiate warming from past ice ages but it did amplify the warming." This is from what link you posted and not only does it not refute what I wrote, but it agrees with me. In fact, it actually refutes Marcel's assertion about the "causal mechanism". Sorry Marcell, but you ended up being a victim of friendly fire here. Oops.

MarcelK Jun 28, 2021 02:57 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

As an intellectually honest informed individual, I understand these talking points that come from the opposite place but I'm not going to take the time to refute them all as it has done many times, e.g.,

ChemicalSuperFreak Jun 28, 2021 02:47 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

The "causal mechanisms", huh? Have you ever examined the temperature/CO2 data from the Vostok ice core? If you did, you'd note that temperature rise precedes CO2 increase by 800 to 1000+ years. That one has climatologists scratching their heads, and either trying to use isotopic analysis of moving air bubbles in the ice to adjust that lead time---or they ignore the stunning finding altogether. With respect to the adjustment attempts, one group shaved a few hundred years off that temperature rise lead, but could never get the rise in CO2 levels to come first. Oh well.

MarcelK Jun 28, 2021 02:34 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

Unless you subscribe to the wooish Gaia Hypothesis, the planet does not have a perspective. In any case, what matters is the human perspective ... human life was not possible on this planet for most of its history. And from a scientific perspective, what matters for establishing precedents is the contextual framework, which includes such things as what we care about (the consequences on human society) and the causal mechanisms (human industry increasing atmospheric green house gases, resulting in global heating).

yin yang Jun 27, 2021 07:29 PM
Weather West: California Dodges Worst of Historic Pacific Northwest Heatwave

I was introduced to Daniel Swain by a poster here. Thank You!!
I urged Edhat to regularly post his email updates. Thank you, Edhat.
His twitter feed is also indispensable.

"Worth noting that despite historically unprecedented nature of ongoing extreme heat across Pacific NW & SW Canada, numerical weather prediction models have actually done a very good job capturing causative large-scale atmospheric pattern--even at 7+ day lead times. #ORwx #WAwx

· Jun 21
Okay, folks. Both ECMWF and GFS are currently depicting what has *potential* to become extreme heat event across Pacific NW & NorCal this coming weekend into next week. But I'm not posting projected temperature map just yet (see thread below). Stay tuned... #CAwx #ORwx #WAwx

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