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Following its hottest summer on record, Phoenix experienced its 3rd warmest September on record with a mean temperature of 91.5° and warmest June-September period on record with an average temperature of 95.4°.
2020 was the 13th year on record where Phoenix recorded a mean temperature of 90.0° or above in September. 9/13 (69%) of those years have occurred 2000 or later and 4/13 (31%) have occurred 2010 or later. September 2020 is the most recent example demonstrating that anthropogenic climate change is producing what amounts to a lengthening summer.
September 2020 Summary:
Average high temperature: 104.1° (2nd highest)
Average low temperature: 79.0° (9th highest)
Average temperature: 91.5° (3rd highest)
In addition, all five September cases with a monthly mean temperature of 91.0° or above occurred 2000 or later. Four (80%) of those cases occurred 2010 or later.
During the 1980s, the 30-year moving average mean temperature for September broke out of the multi-decade range 84.0° – 85.0° that had held since 1950. The September mean temperature now exceeds 89.0°. The average high temperature now stands at 100° or above. The number of days on which the temperature reaches 100° or above has increased by 2.3 days since 2000. The number of days on which the temperature reaches 105° or above has increased by 2.0 days. The frequency of low temperatures at or above 80° has increased by 3.6 days since 2000. Such elevated minimum temperatures now account for nearly 3 out of every 8 days in September.
Table 1: Select September Data (30-Year Moving Average)
Table 2: Record High Maximum Temperatures
Table 3: Record High Minimum Temperature
June-September 2020 was, by far, the warmest June-September period on record for Phoenix. The four-month mean temperature was 95.4°. The old record was 93.9°, which was set in 2011. That four-month average exceeds the hottest summer temperature prior to 2020, which was 95.1°. That prior record was set in 2013 and tied in 2015.
All 9 June-September periods with a mean temperature of 93.5° or above have occurred in 2000 or later. 6/9 (67%) have occurred during 2010 or later.
The average June-September period is now as warm as the average summer was during the 30-year period ending in 2000. The last time Phoenix had a June-September mean temperature below 90° was 1984 when the four-month mean temperature was 89.8°.
Table 4: Average Summer and June-September Temperatures (30-Year Moving Average)
Just as summer 2020 provided a foretaste of the kind of summers that are expected to become routine by 2050 on account of climate change, September 2020 and the June-September 2020 period offered a glimpse into the future of longer summers that will continue to emerge from the evolving climate regime.