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In what has become a seemingly regular occurrence beginning in 2020, Phoenix has experienced its hottest summer on record. Since 2020, Phoenix has eclipsed the summer mark in 2020, 2023, and again this year. A major theme, as was the case in 2020 and 2023 when the heat set in, was its persistence. Summer 2024 got off to a blazing start with its hottest June, by far, on record. The sizzling June was followed by Phoenix’s second hottest July and second hottest month on record. July featured an 11-day extreme heat event (Clarke et al., 2014 methodology) during which the temperature reached or exceed 115° on six days. That was the second longest such event on record. Summer concluded with the third hottest August on record. On account of the historic heat, Phoenix saw its longest stretches of 80° or above lows, 100° or above highs, and 105° or above highs: 80° or above lows: 74 days, June 5-August 17; old record: 51 days, July 1-August 20, 2023 100° or above highs: 97 days (as of August 31 and ongoing); old record: 76 days, June 10-August 24, 1993 105° or above highs: 63 days, June 5-August 6; old record: 56 days, June 24-August 18, 2023 Anthropogenic climate change amplified by the urban heat island effect is driving a warming of Arizona’s and Phoenix's summers. Data from a joint May 2024 report published by Climate Central, Red Cross Red Crescent Climate Centre and World Weather Attribution revealed that the influence of climate change has resulted in a 231.1% increase in Arizona’s days with temperatures above the 90th percentile (1991-2020 baseline) over the figure expected without the influence of climate change. The warming has accelerated in recent decades. Between 1950 and 2023, summers have warmed by 0.6° per decade in Phoenix. Since 1980, summers have warmed by 0.9° per decade. As a result, Phoenix has set summer records in 2020, 2023, and now 2024. As a result, the annual number of days on which the high temperature has exceeded the 90th percentile for summer maximum readings (1991-2020 baseline) has increased from 6.5 days during 1961-1990 to 9.99 days during 1991-2020. The most recent 30-year moving average (1995-2024) is 12.7 such days per year (through August 31, 2024). Select highlights: Updates: Phoenix records 100th 80° low temperature of the year Progression of Phoenix's Record Streak of 100° Highs Phoenix and Surrounding Area 100° Highs and 80° Lows U.S. Sites with 110 or More Consecutive 100° Highs and Population Hottest Last 7 Days of September Phoenix Experiences its Hottest September on Record Phoenix's Extreme September 28th and October 1st High Temperatures Phoenix Experiences its Hottest First Week of October on Record Phoenix Sets or Ties Daily High Temperature Records on an Unprecedented 21 Consecutive Days Climate Change not the Urban Heat Island Effect drove Phoenix's Record Autumn Heatwave Southwest Region Summer Trends Phoenix Follows its Hottest Summer with its Warmest Fall on Record Phoenix Caps Off its Warmest Year with Its Warmest December on Record

After a brief hiatus following three consecutive record hot summers (2016-2018), Las Vegas demolished its summer heat record. Summer 2024 got off to a blistering start with its hottest June on record. July became that City’s hottest month on record. August also became its hottest August on record. July featured a 12-day extreme heat event (Clarke et al., 2014 methodology) during which the temperature reached or exceed 115° on a record seven consecutive days. That extreme heat event was highlighted by Las Vegas’ hottest reading on record (120°) and three other temperatures that surpassed the old all-time mark of 117°. On account of the historic heat, Las Vegas saw its longest stretches of 80° or above lows, 105° or above highs, 110° or above highs, and 115° or above highs: 80° or above lows: 63 days, June 21-August 22; old record: 33 days, July 4-August 5, 2020 105° or above highs: 43 days, June 20-August 1; old record: 25 days, June 15-July 9, 2017 110° or above highs: 11 days, July 3-13; old record: 10 days, June 17-26, 1961 and July 14-23, 2023 115° or above high: 7 days, July 6-12; old record: 4 days, July 16-19, 2005 Anthropogenic climate change amplified by the urban heat island effect is driving a warming of Las Vegas’ summers. The warming has accelerated in recent decades. Between 1950 and 2023, summers have warmed by 0.8° per decade in Las Vegas. Since 1980, summers have warmed by 1.0° per decade. Select highlights:

Looking into the near future it looks like we will see a few shots of colder air and some more cold mornings with frost and possibly a freeze. Really the daytime highs will be perfect.

Last year, Phoenix experienced a historically hot summer. Although the extreme heat held off through May this year, a severe heatwave developed toward mid-June. The June 2021 heatwave was among Phoenix's and Tucson's most severe June heatwaves and their most severe heatwave this early in the season. This heatwave developed as an extreme upper air ridge evolved during an era of rising June temperatures and an ongoing drought. An extreme heat event commenced at Tucson on June 11 and it commenced at Phoenix on June 13. Such events are likely to become even more frequent in the years ahead. Anthropogenic warming is creating a growing likelihood of long-duration extreme events through more frequent wave resonance events (Kornhuber et al., 2016 and Mann et al., 2017). Already, the frequency and intensity of compound summertime hot extremes (events that combine daytime and nighttime heat where such temperatures are above their 90th percentile for their calendar) has been increasing especially in geographic locations that include the U.S. Southwest (Wang 2020). The increase in forcing associated with anthropogenic greenhouse gases is the dominant driver of this outcome (Wang 2020). Table 1: Phoenix's High Temperatures during June 12-20, 2021 Table 2: Phoenix's Average June Temperatures (30-Year Moving Average) Table 3: Phoenix's Record High Maximum Temperatures Table 4: Phoenix's Record High Minimum Temperatures A portion of this heatwave qualified as the 8th Extreme Heat Event (EHE) on record for June, as at least 3 days saw the maximum , temperature exceed the 97.5th percentile (Phoenix: 115° or above; Tucson: 110° or above), the high temperature average exceeded the 97.5th percentile (Phoenix: 115° or above; Tucson: 110° or above), and all high temperatures exceeded the 81st percentile (Phoenix: 110° or above; Tucson: 105° or above) in June-August high temperatures for the 1971-2000 base period (Clarke, et al.,2014). The extreme heat event began on June 11 in Tucson and June 12 at Phoenix. Table 5: Phoenix's Extreme Heat Events in June (1896-2021) Additional Records: Earliest 116° or above high temperature: June 17, 2021 (old record: June 19, 2016 and 2017) Earliest 4 consecutive 115° days: June 15-18, 2021 (old record: June 19-22, 1968) Most consecutive 115° days: 6, June 15-20, 2021 (old record: 4, June 19-22, 1968; June 25-28, 1979; June 25-28, 1990; July 26-29, 1995; July 28-31, 2020; and, August 16-19, 2020) Earliest 4-day average high temperature of 115° or above: June 14-17, 2021 (old record: June 18-21, 2017) Earliest 5-day average high temperature of 115° or above: June 14-18, 2021 (old record: June 17-21, 2017) Earliest 6-day average high temperature of 115° or above: June 13-18, 2021 (old record: June 18-23, 2017) Earliest 7-day average high temperature of 115° or above: June 13-19, 2021 (old record: June 18-24, 2017) Earliest 8-day average high temperature of 115° or above: June 13-20, 2021 (old record: June 18-25, 2017) Earliest mean temperature of 100° or above: June 15, 2021 (old record: June 17, 2008) Earliest 2-day average mean temperature of 100° or above: June 15-16, 2021 (old record: June 17-18, 2008 and 2015) Earliest 3-day average mean temperature of 100° or above: June 14-16, 2021 (old record: June 17-19, 2015) Earliest 4-day average mean temperature of 100° or above: June 14-17, 2021 (old record: June 18-21, 2017) Earliest 5-day average mean temperature of 100° or above: June 13-17, 2021 (old record: June 18-22, 2017) Earliest 6-day average mean temperature of 100° or above: June 13-18, 2021 (old record: June 18-23, 2017) Earliest 7-day average mean temperature of 100° or above: June 13-19, 2021 (old record: June 18-24, 2017) Earliest 8-day average mean temperature of 100° or above: June 12-19, 2021 (old record: June 17-24, 2017) Earliest 91° minimum temperature: June 17, 2021 (old record: June 22, 2017) Earliest 92° minimum temperature: June 18, 2021 (old record: June 25, 2017) Earliest 2 consecutive 90° low temperatures: June 16-17, 2021 (old record: June 21-22, 2017) Earliest 3 consecutive 90° low temperatures: June 16-18, 2021 (old record: June 29-July 1, 2013) Earliest 4 consecutive 90° low temperatures: June 16-19, 2021 (old record: July 8-11, 2020) Most consecutive 90° low temperatures in June: 4, June 16-19, 2021 (old record: June 26-27, 1990; June 29-30, 2013; June 26-27, 2016; June 21-22, 2017; and, June 25-26, 2017) Earliest 2-day average low temperature of 90° or above: June 16-17, 2021 (old record: June 21-22, 2017) Earliest 3-day average low temperature of 90° or above: June 16-18, 2021 (old record: June 24-26, 2017) Earliest 4-day average low temperature of 90° or above: June 16-19, 2021 (old record: July 4-7, 2018) Earliest 5-day average low temperature of 90° or above: June 16-20, 2021 (old record: June 22-26, 2017) Earliest 6-day average low temperature of 90° or above: June 16-21, 2021 (old record: June 21-26, 2017) Highest average June minimum temperature after over 4 days: 91.0°, June 16-19, 2021 (old record: 89.8°, June 24-27, 1990; June 25-28, 1990; June 26-29, 1990; June 27-30, 1990; and, June 23-26, 2017) Highest average June minimum temperature after over 5 days: 90.6°, June 16-20, 2021 (old record: 90.0°, June 26-30, 1990 and June 22-26, 2017) Tucson also experienced extreme heat. As with Phoenix, Tucson has seen a steady increase in June minimum and maximum temperatures. This outcome has increased the statistical probability of extreme heat events. Table 1: Tucson's High Temperatures during the June 11-21, 2021 Heatwave Table 2: Tucson's Average June Temperatures (30-Year Moving Average) Table 3: Tucson's Record High Maximum Temperatures Table 4: Tucson's Record High Minimum Temperatures Table 5: Tucson's Extreme Heat Events in June Additional Records: Most consecutive days with high temperatures of 110° or above: 8, June 12-19, 2021 (old record: 6, June 24-29, 1994) Most consecutive days with high temperatures of 111° or above: 7, June 13-19, 2021 (old record: 6, June 24-29, 1994) Most consecutive days with high temperatures of 112° or above: 7, June 13-19, 2021 (old record: 4, June 25-28, 1990) Highest average high temperature over 8 days: 112.5°, June 12-19, 2021 (tied record set during June 23-30, 1994) Updates: Phoenix has its hottest June on record