Dr. Jeff Masters ·

Above: Motorists stop at an intersection where a sign displays the temperature on June 20, 2017 in Phoenix, Arizona. The Phoenix airport hit 119° that day, their fourth hottest temperature on record, and just 3° below their all-time hottest temperature ever recorded. Photo by Ralph Freso/Getty Images.

The great Southwest U.S. heat wave of 2017 is gradually diminishing, but it has left behind hundreds of smashed heat records, including at least four all-time hottest temperature marks for major stations. According to wunderground weather historian Christopher C. Burt, this week’s event has been the most intense heat wave yet recorded to affect the Southwest so early in the summer, coming about a week earlier than the previous great June heat waves that have affected the Southwest, like those of 2016, 2013, 1994, 1990, 1961 and 1940.

All-time hottest temperatures tied or broken during the heat wave:

Las Vegas, NV: 117° on June 20, tied the all-time record for the airport, which has a Period of Record (POR) back to 1937. However, there was a 118° reading measured by the official USWB COOP site on July 26, 1931.

Needles, CA: 125° on June 20, tied the all-time record. 126° was attained for a minute or two at one point, but not the 5-minute period needed to be deemed official. Previously, 125° was measured on June 20, 2016, and on July 17, 2005. POR: back to 1940.

Ocotillo Wells, CA: 124° on June 21, hottest temperature ever measured in San Diego County.

Prescott, AZ: 105° on June 21, tied the all-time record set on July 17, 1925. POR back to 1898.

Notable readings from June 18:

San Jose, CA: 103°, their hottest temperature since July 22, 2006, when it was also 103°. The last time it was hotter than 103° was on June 14, 2000, when the all-time San Jose record of 109° was achieved. POR since 1898.

Notable readings from June 19:

South Lake Tahoe, CA: 90°, tied for their hottest June monthly record along with June 25, 2015. All-time record for the site is 99° on July 22, 1988. POR only back to 1968. But this is, of course, a huge summer destination and keep in mind how a temperature like this at 6000’ is affecting the massive snowmelt now underway in the Sierra!

Notable readings from June 20:

Death Valley, CA: 127° (actually 126.5°, but rounded to 127°). This is the hottest temperature ever measured in the Western Hemisphere so early in the year.

Palm Springs, CA: 122°, 1° short of all-time record.

Phoenix, AZ: 119°, 3° short of all-time record, and the city’s 4th highest temperature on record. The extreme heat caused dozens of cancelled afternoon flights in Phoenix. As Dr. Marshall Shepherd explained in a Tuesday Forbes article, the cancellations were primarily due to the fact that air is less dense at hot temperatures. This results in less lift for an airplane, requiring a longer runway to achieve takeoff and a reduction in climb performance.

Tucson, AZ: 116°, 1° short of their all-time record. Their morning low of 87° gave Tucson an average daily temperature of 101.5°, their first triple-digit average temperature on record.

Reno, Nevada: 104° tied its all-time June monthly record set on June 16, 1940 . Reno’s all-time any month record is 108° set on July 10 and 11, 2002 and also on July 5, 2007. Reno has a long POR going back to 1893.

Tombstone, AZ: 110° tied its all-time June monthly record (along with three other earlier dates), and was 2° short of the all-time record of 112° set on July 4, 1989. POR since 1893.

Tonopah, NV: 102°, 1° short of the all-time June record, and 2° short of all-time record of 104° on July 18, 1960.

Thermal, CA: 123°, an all-time June monthly record, and 2nd hottest temp ever measured, following 126° on July 28, 1995. POR since 1950.

Notable readings from June 21:

Mexicali, Mexico: 51°C (123.8°F). According to weather records expert Maximiliano Herrera, this is the hottest reliably measured June temperature on record in Mexico. The only hotter temperatures in Mexico were measured in July 1995: 52.0°C (125.6°F) at Mexicali Sur and Rancho Williams, and 51.4°C (124.5°F) at Ejido Nuevo Leon (all located in Baja California near Mexicali.)

Winslow, AZ: 108°, tied its June record (also set on June 20, 2016). This fell just short of the all-time heat record of 109° set on July 13, 1971.

Notable readings from June 22:

Tucomcari, NM: 108°, tied as its 2nd hottest temperature on record (any month). Record is 109° set on June 21, 2013. POR since 1904.

Notable readings from June 24:

Palm Springs, CA: 122°, again just 1° short of all-time record.



Redding, CA: 113°, tied with June 19th for their hottest day of this heatwave (and another daily record of course).



Death Valley, CA: 126.7° (rounded off to 127°), the hottest temperature of any station in the heatwave.

Quillayute, WA (right on the coast of the Olympic Peninsula): 92°, 2nd hottest June day after 94° on June 27, 2015.

Notable readings from June 25:

Palm Springs, CA: 122°, yet again just 1° short of all-time record.

Portland, OR: 101°, 1° short of their June all-time record (102° on June 26, 2006).

Seattle, WA: 96°, 2nd warmest June day after 98° on this date in 1925 and 97° on June 30, 1942.



Olympia, WA: 98°, tying the June record of 98° set on June 18, 1982.

Burbank, CA: 111°, a tie for June record along with June 20, 2016 and June 27, 1976. Just 2° short of all-time record 113° set on Sept. 12, 1971. POR since 1934.

The minimum temp at Phoenix was 93°. This ties for the highest min on record for June along with June 27, 1990 (the day before Phoenix reached its all-time record of 122° on June 28, 1990).



With 125° on June 25th, Death Valley has now had five days this month with 125°+ temperatures. This ties 2013 for the record for such (even including the bogus early 20th century observations).

Long streaks of record-breaking temperatures

The multi-day nature of this heat wave has been adding to its dangerous nature: prolonged heat waves are especially hazardous because there is less chance for people to cool down at night, thus increasing the stress on those who do not have access to air conditioning. A few of the notable record streaks of hot weather from the heat wave:

Las Vegas, NV has seen 9 consecutive days of 110° readings, June 17 - 25. If the city hits 110° on Monday, this would give Vegas ten consecutive days with highs of 110°+, tying the record set in 1961.

Redding, CA had five consecutive days that broke the record high for the date (June 18 - 22), with a peak of 113° on June 19th. POR since 1893.

Needles, CA tied its record for consecutive days of 120°+ on June 19 - 21. Previous such streaks: June 28 - 30, 2013 and July 16 - 18, 2005.

Flagstaff, AZ hit 90° or hotter on sevem consecutive days, June 18 – 24, which is the 2nd longest such streak on record.

Prescott, AZ hit 100°+ on six consecutive days, June 19 – 24, the tied for longest such streak on record. The streak ended on June 25, when the high only hit 99°.

Winslow, AZ hit 105 °+ on four consecutive days, June 19 – 22, longest such streak on record. POR since 1898.

Figure 1. Peak ozone pollution levels for Friday, June 23, 2017, as measured using the Air Quality Index (AQI). An AQI with red colors is “Unhealthy”, meaning everyone may begin to experience health effects, and members of sensitive groups may experience more serious health effects. An AQI in the orange range is “Unhealthy for Sensitive Groups”, meaning members of sensitive groups like the elderly and those with lung disease may experience health effects, but the general public is not likely to be affected. Image credit: EPA.

With the heat--air pollution

This week's heat wave brought high levels of ground-level ozone pollution to the affected region. Ground level ozone, which is created from chemical reactions between volatile organic carbon (VOC) compounds and nitrogen oxides in the presence of sunlight, is created more readily at warmer temperatures. The heat wave’s record heat helped these chemical reactions occur faster, resulting in ozone pollution that topped out in the “Unhealthy” range in the valleys of California. The heat was accompanied by light winds, which allowed pollution levels to build. These concentrations were as much as 50% higher than the federal standard for hazardous level of ozone, and were capable of causing increased risk of stroke, heart attack and breathing problems for sensitive groups. Even people not unusually sensitive to air pollution could have seen adverse health effects from pollution levels this high. The other main pollutant of concern, tiny particles known as PM2.5 (those that are less than 2.5 microns or 0.0001 inch in diameter)--which cause over 80,000 premature deaths each year in the U.S.—was not as big of a concern, since PM2.5 levels stayed below the federal standard.

The forecast: the heat wave will diminish Sunday - Tuesday

The heat wave will weaken in California beginning on Sunday, when an upper-level low pressure system off the California coast deepens and moves closer to the coast. Further cooling will occur across the entire Southwest U.S. on Tuesday, when a weak trough of low pressure moves across California. Near-normal temperatures are expected over much of the region late in the week.

Longer and more intense heat waves are a key indicator of our planet's warming climate, as noted in a website from Climate Signals that puts the evolving Southwest heat wave in the context of recent observations and research.

Kudos for contributions to this post go to Christopher C. Burt, Maximiliano Herrera, and Guy Walton (guyonclimate.com).