As our cool spring conditions fade and temperatures rise, we Arizonans dream of an early arrival of the summertime monsoon thunderstorms, which bring clouds and lower temperatures, sparing us from the heat that builds through June. It rarely does come early, though. Instead, the high sun and long days push temperatures regularly up above 100 in the low desert cities of Tucson and Phoenix.
Last past week was different. Instead of our typical June heat wave with a run of days with above-average temperatures, Arizona and much of the Southwest experienced a prolonged and dangerous period of extreme record-high temperatures. Record highs were set for five straight days in Tucson, and Phoenix observed temperatures hitting 115 with several daily records falling. Overnight lows were above 90 for several nights in Phoenix as the heat wave persisted, giving temperatures a head start to soar to record levels the next day.
This heat wave is a result of a supercharged version of what normally happens across the western United States in May. The typical upper-level westerly winds retreat north and are replaced by a ridge of high pressure that builds north as the mountains and plateaus heat up under the strong June sun. This shift in upper-level winds is an important precursor to the monsoon moisture that flows up into Arizona and New Mexico in late June and early July. Having the ridge overhead though leads to the heat. Sinking air, clear skies and long days lead to the perfect conditions for temperatures to soar and persist. Sometimes early monsoon moisture can sneak into the region, raising humidity levels ever so slightly, leading to higher overnight temperatures up and degrading the effectiveness of evaporative coolers, which many homes rely on to stay cool.
What is unusual about the current event is the magnitude and strength of this upper-level ridge of high pressure. Measurements of upper-level temperatures provided by weather balloons indicate that the ridge is especially strong, with warm and rarely observed readings occurring at standard levels tens of thousands of feet above the ground. Long-term trends in warming temperatures across the globe associated with climate change provide the foundation to turn a typical June heat wave into a dangerous, record-setting one. It may look similar to other heat waves on a weather map, but the magnitude, intensity and duration of the event is larger because of climate change.
The National Climate Assessment notes that the southwestern United States has observed some of the largest increases in temperatures in the country over the past several decades and is expected to continue to warm with an increase in the frequency and intensity of heat waves like this one. The implications are wide-ranging, including effects on public health as well as overtaxing the power grid. Extreme heat disproportionately affects vulnerable populations, including the elderly who live alone, those experiencing homelessness and anyone lacking access to refuge from the heat. The National Weather Service in Phoenix notes that more than 3,000 heat-related deaths occurred in Arizona between 2010 and 2020 — 100 times more than the second leading cause of fatalities: floods.
Extreme heat leads to extreme fire weather as well, drying out fuels more quickly, increasing overnight fire activity and creating dangerous conditions for firefighters on the ground. The convergence of heat events at the peak of fire season like we are seeing now in the Southwest hampers firefighting efforts and strains resources leading to fires that are larger and harder to control. High overnight temperatures and low relative humidity values with this heat wave helped the Telegraph fire in central Arizona, which started on June 4, to burn actively for days without slowing down at night as wildfires typically do.
This June as well several other recent Junes have shown us here in Arizona that these types of heat waves are becoming events that we’ll have to expect, plan around and prepare for in our changing climate. This year, the summer monsoon will probably show up on time in a couple of weeks, and temperatures will moderate as clouds and rain move in, bringing sweet relief. But the relief will be short-term. We mustn’t forget that extreme heat is an increasing threat here in the Southwest and will require both mitigating the causes of climate change and long-term adaptation strategies to protect the people, ecosystems and natural resources in the region.