Phoenix Registers Hottest Month on Record for Any U.S. City

[donsutherland1]

The heat… desiccates the unwary, its dryness sapping moisture from one’s mouth and eyes. Sweat evaporates too quickly to cool… –Thomm Quackenbush

Powered by a heatwave without precedent—christened “Heatwave Chevron” by former Weather Channel meteorologist Guy Walton—Phoenix experienced its hottest July and month on record. The dynamic city of 1.6 million spent day after day under a broiling sun in a raging storm of brutal, unforgiving, penetrating heat. Nights offered little relief from the fierce heat.

As a result, July 2023 left the prior hottest month, August 2020, in the dust much as Secretariat demolished the field in the 1973 Belmont Stakes. July 2023 surpassed the previous monthly mean temperature record by the largest margin by which any prior mark was surpassed. Phoenix also recorded the highest mean temperature and highest average low temperature for any month in any American city. The old records were a mean temperature of 102.2° and an average low temperature of 90.1° in Lake Havasu City during July 1996. Lake Havasu City also had an average high temperature of 114.4° during July 1996. Further, the lowest maximum temperature during July was 108°, which easily eclipsed the 104° mark from June 2013.

In addition to the records that melted under the unforgiving Phoenix sun, the 2023 heatwave produced Phoenix’s longest extreme heat event on record. Extreme heat events are determined using the Clarke et al., 2014 methodology, which applies percentiles to summer high temperature values from the 1971-2000 baseline. To qualify, a period must have at least three 115° or above high temperatures, an average high temperature of 115° or above for the duration of the extreme heat event, and no high temperatures less than 110° during the extreme heat event.

The Role of Climate Change

Anthropogenic climate change is driving a warming of Phoenix's summers. This ongoing warming is a global phenomenon with 98% of the world having experienced its warmest 51 years during the current 2,000 years. The IPCC’s Sixth Assessment Report found that heat and heatwaves are increasing on every continent. The primary driver is human-caused climate change. Phoenix’s unprecedented heatwave and record hot month are the result of a combination of factors that includes climate change, which has boosted temperatures and led to “stuck” patterns, the Urban Heat Island Effect, which has raised nighttime temperatures, and the synoptic pattern in which a powerful heat dome developed over the region.

The World Weather Attribution (WWA) Initiative found that the heatwave was “virtually impossible” without climate change and that temperatures were approximately 2°C (3.6°F) warmer on account of climate change. Event attribution studies calculate whether and the degree to which an event was made more (or less) likely and/or intense because of climate change.

The WWA warned, “Unless the world rapidly stops burning fossil fuels, these events will become even more common and the world will experience heatwaves that are even hotter and longer-lasting.” Heatwaves of the magnitude of the 2023 heatwave could occur every 2-5 years in a world that is 2°C (3.6°C) warmer than the pre-industrial world.

Summary

The record-setting summer of 2020 was a “summer from the future,” as it resembled the kind of summers that will likely occur on a regular basis by 2050. Similarly, the great 2023 heatwave can be said to be a “heatwave from the future.” On account of the unparalleled heatwave, July went on to become Phoenix’s hottest month on record, by far. In her poem, "Heatwave...Pleiades," Elizabeth Squires wrote of a heatwave "hotter than Hades... haranguing us from dusk to dawn." That was Phoenix in July 2023.

 

[TheClimateChanger]

Good post, Don.

Just a few comments:

(1) I believe Table 4 contains an error in the reported mean monthly minima for the 30-year periods ending in 2000, 2010, and 2020. To be specific, the values appear to be 10 degrees too high for each of those periods.

(2) The numerous records from the late 1980s to mid 1990s should be considered suspect due to the use of the HO-83 hygrothermometer, which imparted a known warm bias, especially under full sun and light wind conditions. Nearby Tuscon, Arizona set a large number of records during that era, which were considered to be inflated. There was a New York Times story about it at the time. I don't think we should toss records due to equipment biases; however, it does tend to put things even more into perspective. You can liken that era to the steroids era in baseball. There were certainly some hot years, but some of the extreme records were probably somewhat inflated. To continue with the baseball analogy, the current stretch would be like if someone, or several players, not taking steroids, suddenly starting knocking 80, 90 balls out of the park each season for several years in a row. So, anyways, it is interesting the two highest readings - 122F and 121F - both occurred during that period. Given the known bias, it wouldn't surprise me if it really was no more than 119F on those dates.

(3) If 2020 was a taste of summers to come in 2050, what year does this equate to? Perhaps, it's already here and things are progressing faster than expected in some spots.

[donsutherland1]

Thanks. Table 4 will be corrected.

[TheClimateChanger]

3 hours ago, donsutherland1 said:

Thanks. Table 4 will be corrected.

No problem. Thank you for compiling all of that data. Not a lot of people from that region on this forum, but those are some incredible numbers.

[Stormchaserchuck1]

There wasn't a strong 500mb ridge overhead, +30-45dm, that's it. 

Normally, hot July's in the SW precede warmer conditions in the eastern 2/3 of the US for Aug-Sept, but I wouldn't expect it as much. 

[Typhoon Tip]

1 hour ago, Stormchaserchuck1 said:

There wasn't a strong 500mb ridge overhead, +30-45dm, that's it. 

Normally, hot July's in the SW precede warmer conditions in the eastern 2/3 of the US for Aug-Sept, but I wouldn't expect it as much. 

There were a lot of heat aspects, air and sea, world over that were idiosyncratic like that this spring and summer.   Worrisome...   the notion that they are (thus) also not really fitting a predictive model, or able to be projected because of their emergent properties, should reverberate an uneasiness. 

I have written of "synergistic heat waves" as an increasing frequency of occurrence ...  They are a new distinction in the global warming empirical results "catalogue," but I it's not just heat, per se.  The weird inability in Antarctica to regenerate ice at failed climatological amount that's multiples of the erstwhile established -trend, occurring in a single season!   To the Labrador marine heat burst ... This stuff down in Florida...  And the March heat explosion over China.   

It's the lack of predictive skill prior to these short duration phenomenon that really should have folks spooked - more so than the scalar itself, as remarkable as the numbers are to talk about.    

[Roger Smith]

This event seems to be correlated in latitude, timing and intensity by the recent broiling heat wave in the central Mediterranean region, which peaked about a week ago, with temperatures near all-time highs in parts of Tunisia, Sicily, southern mainland Italy, Albania, Greece and western Turkey. For example it was 45.6 C on July 25 at Catania, close to their previous all-time high of 46.0. Values of 48-49 C were reported from central Tunisia including Sfax where the previous all-time record was 47 C. One report from Greece was actually closer to 60 C but may be either an error or fire-affected. Severe wildfires erupted all over the hot zone. Some believe that these super heat waves are the combined result of ongoing AGW climate change and after-effects of the water vapour eruption of the Tonga undersea volcano last year. 

The central Med heat wave (which was not quite as relentless as the Arizona version, a few days recently have been temperate) was the result of a depressed jet stream in July over the Atlantic forcing superheated Saharan air to escape northeast, with less of a west to north exit or a southwesterly exit over the Atlantic, that are more normal outcomes. In some past years Saharan heat has spilled into Spain and Portugal then on to France and western Europe in general. This year's setup with depressed heights over the Iberian peninsula has prevented that and Spain has been more moderate, while France and Britain, Ireland have been a bit cooler than average for July after quite a warm June. CET values were 17.0 June (5th warmest) and 16.1 July (near median of all data and 0.7 below recent 30-yr normal). An average for five locations in Ireland was 15.1 C, 1981-2010 average 15.5 C.

There are signs of a return to previous years' western Europe heat plume in mid-August however. 

My take is that the southwest heat dome will come back to life during August, spread north and then east. This may be an August similar to 1948, 1953 and 1973 all of which were fairly close to average in the east until the last week, then went into broiling heat waves with many records set. Then I speculate that this anomalous warmth will set up over eastern NA for most of the autumn, although the statistical correlation seems to be warm September and November, cool to average October. 

[donsutherland1]

4 hours ago, Roger Smith said:

This event seems to be correlated in latitude, timing and intensity by the recent broiling heat wave in the central Mediterranean region, which peaked about a week ago, with temperatures near all-time highs in parts of Tunisia, Sicily, southern mainland Italy, Albania, Greece and western Turkey. For example it was 45.6 C on July 25 at Catania, close to their previous all-time high of 46.0. Values of 48-49 C were reported from central Tunisia including Sfax where the previous all-time record was 47 C. One report from Greece was actually closer to 60 C but may be either an error or fire-affected. Severe wildfires erupted all over the hot zone. Some believe that these super heat waves are the combined result of ongoing AGW climate change and after-effects of the water vapour eruption of the Tonga undersea volcano last year. 

The central Med heat wave (which was not quite as relentless as the Arizona version, a few days recently have been temperate) was the result of a depressed jet stream in July over the Atlantic forcing superheated Saharan air to escape northeast, with less of a west to north exit or a southwesterly exit over the Atlantic, that are more normal outcomes. In some past years Saharan heat has spilled into Spain and Portugal then on to France and western Europe in general. This year's setup with depressed heights over the Iberian peninsula has prevented that and Spain has been more moderate, while France and Britain, Ireland have been a bit cooler than average for July after quite a warm June. CET values were 17.0 June (5th warmest) and 16.1 July (near median of all data and 0.7 below recent 30-yr normal). An average for five locations in Ireland was 15.1 C, 1981-2010 average 15.5 C.

There are signs of a return to previous years' western Europe heat plume in mid-August however. 

My take is that the southwest heat dome will come back to life during August, spread north and then east. This may be an August similar to 1948, 1953 and 1973 all of which were fairly close to average in the east until the last week, then went into broiling heat waves with many records set. Then I speculate that this anomalous warmth will set up over eastern NA for most of the autumn, although the statistical correlation seems to be warm September and November, cool to average October. 

Unless I am mistaken, the 60C figure was a surface temperature, not a 2 meter temperature. The ground is much warmer than air temperatures where the official readings are measured.

I agree concerning the Southwest heat dome. Indeed, near record to record heat is likely later this week. So far, none of the guidance brings the warmth into eastern North America, but that could happen when a fundamental change in the pattern takes place. Chile had some wild heat today with locations in the Andes experiencing all-time highs in mid-winter!

[TheClimateChanger]

All time high for Coquimbo province, in the middle of winter:

 

[TheClimateChanger]

And holy cow - a 35C diurnal temperature range. Wow.

[Roger Smith]

https://www.mag.ncep.noaa.gov/Image.php

 

I found the attached map in the GFS 00z run for yesterday evening, for S America. Don't know if this link will update to the next 00z run or not. If so perhaps somebody can freeze this image (it won't update before 0340z). 

(link not working, go to GFS maps and select S America, 00z run, any early map -- working on getting a link to show this)

Seems to show a sort of winter version of heat dome over central S America. There would probably be a bit of an easterly wind component in coastal Chile downsloping like a strong Santa Ana does in a California warm spell in late winter. 

Even so it's quite a remarkable map with the 582 dm thickness contours in their version of early February! Made me think of the remarkable warmth we had in BC around Feb 1, 1998, temps were into the 60s and I was out playing golf in summer clothing on a course that nine times out of ten is not open for play in Jan-Feb and can be covered in snow. (an analogue to season and El Nino) -- or the spectacular record breaking going on in Dec 1982 in eastern NA.  

[pazzo83]

The South American heat wave ongoing is one of the most insane weather/climate events I've ever seen.  Temps pushing 40C in the middle of winter at the mid latitudes.  WTF.

[SnoSki14]

It's really chilling to think what will happen once warming breaches 2C globally given what we're already seeing.

Many places will literally become uninhabitable. The fact that politicians are still acting like it isn't happening is insane. 

[Typhoon Tip]

6 hours ago, SnoSki14 said:

It's really chilling to think what will happen once warming breaches 2C globally given what we're already seeing.

Many places will literally become uninhabitable. The fact that politicians are still acting like it isn't happening is insane. 

It’s becoming easier to assume the  sociopathic divorce of ethics … when toward indifference is the notion of responsibility to future generations …

I’ve heard it leaked in the past “… I’ll be dead and gone …” so it doesn’t matter and there’s no guilt or moral ownership, then ?

One coveting this sentiment … wholly absolved in self … knows it is morally wrong - perhaps some in this detached group don’t get the connection … sure. Either way the suppressed/truer motive of not being empathetic to the notion of extinction merely because it makes no difference or existential threat to them and their life time is patently a sociopathic doctrine 

[donsutherland1]

18 hours ago, pazzo83 said:

The South American heat wave ongoing is one of the most insane weather/climate events I've ever seen.  Temps pushing 40C in the middle of winter at the mid latitudes.  WTF.

It’s unthinkable heat for this time of year. 

[Typhoon Tip]

We were in the 80s here in the mid Atlantic/SNE regions in two Febs over the last 7 years fwiw 

[LongBeachSurfFreak]

14 hours ago, Typhoon Tip said:

We were in the 80s here in the mid Atlantic/SNE regions in two Febs over the last 7 years fwiw 

Yet we can’t crack 100 during the summer. Most likely do to increased DP. Kind of like Florida, Hi capital but also really breaks 100.

[donsutherland1]

In Phoenix, July 2023 was so hit that it broke out of the 99.9% confidence interval for hottest month relative to the 30-year moving average of summer temperatures. This shows how extraordinary the month was. It could imply that the record will stand for some time, monthly extremes are growing faster than summer temperatures are rising, and/or climatic variability is increasing in the warming climate.

[Roger Smith]

In the ongoing heat wave BC has seen its previous August record (41.7 C, 2004) broken today (Lytton at 42.1 C, possibly a bit higher in end of day reports). 

I've had 41.2 C locally (106 F) and it feels about like the 2021 heat dome out there (we reached 44 C in that episode). 

Pretty sure there are some new August records for WA and OR as well. Brutal heat that will last to Thursday. The windy and perhaps stormy cold front on Friday is causing red flag concerns for current forest fire situation which is generally bad although under control in most cases. Wind gusts to 40-50 mph in that event will create a widespread dangerous situation.