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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