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The three most populous U.S. states — California, Texas and Florida — are facing the worst of a punishing summertime heat wave, with 54 million Americans slated to see triple-digit highs this week. Some of the most intense heat is predicted in Arizona, and Phoenix, its largest city, could see its longest period of 110-degree high temperatures on record.

In California and the Desert Southwest, it’s a dry heat, but temperatures could skyrocket to over 120 degrees in spots. For Texas and Florida, the excessive heat will combine with stifling humidity to push heat indexes into dangerous territory.

The heat index is a metric used to gauge heat stress on the human body; it takes into account temperature and humidity to determine the extent to which a person will or will not be able to cool themselves through sweating — and subsequently how quickly heat will take a toll on them.

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We’re tracking dangerous heat waves across the United States daily. <a href="https://www.washingtonpost.com/weather/interactive/2023/heat-waves-map-us-tracker/" target=_blank>Look up your city to see extreme heat risks</a> near you.

Even more noteworthy than the magnitude of this heat is its longevity. El Paso is entering the 25th day in a row with highs topping 100 degrees, beating its previous record of 23 consecutive days set in 1993. With no immediate end in sight and as forecast highs hover between 105 and 110, the inescapable heat looks to continue. A number of other major cities, like Phoenix, are in a similar boat.

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The instigating heat dome, or ridge of high pressure causing the heat, is expanding and elongating. Think of a heat dome like a force field — it steers away any inclement weather, diverting storm systems and the jet stream northward over the Upper Midwest, Great Lakes and Canada.

Beneath the heat dome, sinking air will heat up and dry out. In addition to causing a marked warm-up, that also supports ceaseless sunshine, which will heat the air mass even more.

Warm air expands, which means the heat dome is essentially causing a vertical bulge in the atmosphere. In fact, the halfway point of atmospheric mass is now roughly a football field higher in altitude than it ordinarily would be — courtesy of the extreme heat causing air columns to grow.

By Thursday, the stretched-out heat dome will be anchored over California, but will stretch from the northeastern Pacific Ocean all the way to Florida. The hottest weather, found beneath it, will affect the Gulf Coast, the southern Plains and the Southwest.

Climate change has made the persistent heat in parts of Texas, the Southwest and Mexico at least five times more likely, according to an analysis from Climate Central. The nonprofit’s Climate Shift Index, a tool that quantifies how climate change has affected the likelihood of severe weather conditions, rated parts of the states a 5, indicating that the heat is an “exceptional” event.

Desert Southwest: Phoenix has logged 11 consecutive days at or above 110 degrees. Monday was expected to be a close call, but Sky Harbor International Airport reached 110. The National Weather Service is forecasting a high of 111 degrees on Tuesday; assuming that forecast is realized, the remainder of the week should be sure to hit 110 or greater every day. Friday, Saturday, Sunday and Monday are all expected to reach 116 degrees or higher.

Phoenix’s record for 110-degree readings lasted 18 days in 1974. That record could be in jeopardy.

California’s Death Valley will climb above 120 degrees on Friday, and could reach the upper 120s to near 130 degrees this weekend. A high of 130 would <a href="https://www.washingtonpost.com/weather/2021/07/09/death-valley-record-high-temperature/?itid=lk_inline_manual_23" target=_blank>match the mark it set on July 9, 2021, the highest in reliable modern records</a> worldwide.

Excessive heat watches have been expanded through most of California, and now cover <a href="https://twitter.com/NWSSacramento/status/1678779454322274304" target=_blank>the Central Valley</a>. That’s where the National Weather Service is warning of “dangerously hot conditions with daytime high temperatures 98 to 113 with overnight lows in the upper 60s to low 80s possible.” The harshest conditions will arrive Friday into early next week.

Texas and the southern Plains: Temperatures will hover 7 to 12 degrees above average for the southern Plains; that will keep highs in Texas in the 100- to 105-degree range, except in Central Texas, where readings will spike between 105 and 110 degrees. Heat indexes could climb as high as 109 to 115 in Houston, Dallas and Austin through Wednesday.

In Florida, a <a href="https://www.washingtonpost.com/weather/2023/07/10/florida-ocean-temperature-heat-records/?itid=ap_danstillman&itid=lk_inline_manual_26" target=_blank>marine heat wave</a>, with water temperatures off the Southwest Florida Peninsula above 95 degrees, will continue to add extreme moisture to the air. That will combine with air temperatures in the mid-90s to push heat indexes in the 100- to 106-degree range.

Miami has faced a heat index of 104 degrees or higher for the past eight days. Monday logged a heat index of 110 degrees. The city has also seen a heat index of 100 or higher <a href="https://twitter.com/BMcNoldy/status/1678450271666716673" target=_blank>for nearly a month in a row</a>.

A lobe of the Saharan Air Layer (SAL), a hot, dry slice of air several thousand feet above the ground, had been draped over South Florida during the past few days. That had allowed sunshine to pour down while also suppressing thunderstorm chance. Thunderstorms may resume in the coming days as the SAL thins/moves away, which may help cool temperatures during the afternoons.

3. What dangers could the heat waves bring?

The National Weather Service in Phoenix is warning of “Major to Extreme Heat Risk.” Vulnerable populations will be susceptible to an “increase in heat related illnesses, including heat cramps, heat exhaustion, and heat stroke. Heat stroke can lead to death.”

A large number of locations will be dealing with hot overnight lows. Dallas-Fort Worth will struggle to fall below 80 degrees, possibly for the remainder of the week, each night. Warm overnight lows in particular are linked to excess mortality, or premature deaths, for those without access to cool locations. Public cooling shelters are frequently opened during the daytime, but fewer are available at night. This is of greatest concern for unhoused and/or vulnerable populations, like the elderly.

Both dry heat and humid heat can pose different problems.

During a dry heat, the dry atmosphere immediately saps moisture, i.e. sweat, off the skin. That can dehydrate a person before they even realize they’re dehydrated. That can quickly become deadly, especially for hikers and those participating in outdoor recreation. (Also note that low elevations, like river valleys, are usually the hottest.)

During a moist heat, higher moisture content in the atmosphere means the air is already partially saturated, and can’t evaporate more moisture off the skin. That means our sweat doesn’t go anywhere. Evaporative cooling is the whole purpose behind why humans sweat; if that can’t happen, we can’t cool ourselves.

The Post is tracking dangerous heat waves across the United States. <a href="https://www.washingtonpost.com/weather/interactive/2023/heat-waves-map-us-tracker/?itid=cb_box_QV2IPSUWP5B2BAUDD5WTDG46AY_1&itid=lk_inline_manual_40" target=_blank>Look up your city to see extreme heat risks</a> near you.

In recent days, the Earth has reached its highest average temperatures in recorded history, and <a href="https://www.washingtonpost.com/weather/2023/07/08/earth-hottest-years-thousands-climate/?itid=lk_inline_manual_41" target=_blank>here’s how we know</a> that’s the case.

What is a heat dome? <a href="https://www.washingtonpost.com/weather/2021/07/10/heat-dome-heat-wave-faq/?itid=lk_inline_manual_42" target=_blank>Understand the science</a> and how drought and climate change make them worse.

Kate Selig contributed to this report.

Harvard University, B.A. Atmospheric Sciences

Matthew Cappucci is a meteorologist for Capital Weather Gang. He earned a B.A. in atmospheric sciences from Harvard University in 2019, and has contributed to The Washington Post since he was 18. He is an avid storm chaser and adventurer, and covers all types of weather, climate science, and astronomy.

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