PHOENIX — A dangerously hot summer is shaping up in the U.S. West, with heat suspected in dozens of recent deaths, including retirees in Oregon, a motorcyclist in Death Valley, California and a 10-year-old boy who collapsed while hiking with his family on a Phoenix trail.

Heat is the top cause of weather-related fatalities nationwide. But because investigations of suspected heat deaths can take months, and a mishmash of methods is used by counties to count them, it is unknown exactly how many people died in the recent heat wave beginning July 1.

There are indications it was an especially deadly two weeks.

“This is just a harbinger of things to come,” Joellen Russell, a climate scientist at the University of Arizona in Tucson, said Friday. “The floods, droughts, wildfires, heat waves, hurricanes, thunderstorms: We have activated all this extreme weather with the extra carbon dioxide we put into the atmosphere.”

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Where Most Deaths Occurred

Nineteen deaths are being investigated for possible heat-related causes in Northern California’s Santa Clara County, where a heat wave this month pushed temperatures into the low triple digits. The medical examiner’s office reported that four people who died were homeless and nine were older than 65.

At least 16 people are suspected to have died from record high temperatures in Oregon, mostly in the metro Portland area.

There have been nine confirmed heat-related deaths this year in Clark County, Nevada, which encompasses Las Vegas, the county coroner’s office said.

Several recent deaths being investigated in Arizona involved small children, including a 2-year-old girl who was left alone in a hot vehicle outside Tucson and a 4-month-old who died after falling ill while on a boat on Lake Havasu.

How Hot It Has Been

Records for high temperatures have been shattered around the western states this month, with Palm Springs, California, hitting its all-time high of 124 Fahrenheit (51.1 Celsius) on July 5 and Las Vegas registering its all time high of 120 F (48.8 C) on July 7.

Las Vegas baked in a record seven consecutive days of 115 F (46.1 C) or greater during the recent heat wave, nearly doubling the old mark of four consecutive days set in July 2005, the National Weather Service said. The city has seen at least 18 heat records since June 1.

California’s Death Valley saw a high of 129 F (53.8 C) on July 7, tying the daily record set in 2007, according to the National Weather Service. The high in Phoenix hit 115 F (46.1 C) on Wednesday, tying the daily record set in 1958 and 1934.

Portland, Oreg set new daily high records five days in a row through Tuesday, when it hit 104 F (40 C).

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Why the Death Toll Is Uncertain

The toll from the blistering heat is unknown because of varying ways jurisdictions use to calculate such deaths. But some counties in the U.S. Southwest do a better job than most parts of the county.

Pima County, Arizona’s second most populous county and home to Tucson, last year started including heat-related deaths in a new online dashboard. Maricopa County, home to Phoenix, America’s hottest big city, for years has tracked heat-related deaths. Clark County, Nevada, home to Las Vegas, now also identifies deaths in which heat was a contributing factor.

But counting heat-related deaths across most jurisdictions is inconsistent. Death investigations in some places are done by a medical examiner, typically a physician trained in forensic pathology. In other locales, the coroner could be an elected sheriff, such as in Orange County, California. In some small counties in Texas, a justice of the peace might determine cause of death.

Even the numbers from the Centers for Disease Control and Prevention are often several years behind in reporting and offer an incomplete picture because they depend on information from death certificate information, drawn from local, state, tribal and territorial databases.

An Associated Press analysis of CDC data this year found the death certificates of more than 2,300 people who died in the U.S. last summer mention the effects of excessive heat, the highest number in 45 years of records. Doctors, public health experts and meteorologists say that’s just a fraction of the real toll.

Why Confirmation Lags

It can take pathologists months in some cases to determine if a death was heat-related.

Unlike a suicide by hanging or a homicide caused by a bullet in the head, a heat-related death is not always easy to ascertain. It can take weeks, or even months, of toxicological tests to determine if heat was a factor.

There is much ambiguity for investigators to wade through when a body is found in a hot apartment days after a death. Although it may have been very hot when the person was found, it’s impossible to know how hot it was inside a dwelling when the death occurred.

Toxicological tests also can take a long time to determine substance use, such as alcohol or street drugs.

Because of that lag, it wasn’t until this spring that Maricopa County’s Public Health Department was able to issue its final count of 645 heat-related deaths for 2023. The deaths included those in which heat was a secondary factor, such as a heart attack provoked by high temperatures.

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The Forecast

The temperatures in Portland, Oregon, have cooled, but were expected to heat up slightly over the weekend with highs in the low 90s, extending south into Salem and Eugene.

The National Weather Service in Phoenix said an excessive heat warning was forecast to continue through Saturday, with highs to hit 111 F (43.8 C) before falling below 110 F (43.3 C) on Sunday and into next week.

After 10 days under an excessive heat warning, Las Vegas was expected to see slightly cooler weather through the weekend. Still, next week’s highs are forecast to remain higher than normal, ranging from 110 to 112 F (43.3 to 44.4 C), the National Weather Service said.

And summer isn’t over yet.