A relentless heat wave raging on the West Coast is suspected to have caused at least five deaths as experts urge millions of Americans facing triple-digit temperatures to protect themselves against heat stress — the leading cause of weather-related fatalities in the nation.

The National Weather Service warned that record-breaking heat was expected to scorch the West Coast this week with some areas reaching up to 30 degrees above average. Excessive heat warnings, watches, and heat advisories were in effect for nearly all of Washington state, Oregon, California, Idaho, Nevada, and southwest Arizona. Parts of the South and Northeast were also under heat advisories and expected to last into the week.

Authorities in Multnomah County, Oregon, said four people were suspected to have died from heat since Friday. In Death Valley National Park, a motorcyclist died and another was hospitalized for heat-related illness as temperatures soared to a record-breaking 128 degrees.

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Larry O’Neill, professor at Oregon State University College of Earth, Ocean, and Atmospheric Sciences, told USA TODAY the heat wave was "absolutely out of the norm" from what he has seen in previous years, mostly for how long it has lasted. Extreme heat first reached the Portland area around July 5, according to the weather service, and an excessive heat warning remained in place until Tuesday night.

Persistent dry heat could fuel drought and wildfire conditions in the Pacific Northwest for the rest of the summer, O’Neill said. The heat wave is also earlier than high temperatures usually hit the region.

"We have heat waves that are kind of a natural part of our climate system, but heat waves are now just becoming more intense because of climate change," O'Neill said.

Extreme heat is suspected culprit in four Portland-area deaths

The Multnomah County Medical Examiner’s Office said four deaths since Friday were suspected to be caused by heat as residents faced days of scorching temperatures.

The first heat-related death was reported Friday of an 87-year-old man, county officials said. On Saturday, a 33-year-old man died from a suspected heat-related illness at a Portland hospital. Officials said he was not a Multnomah County resident.

Two additional deaths were reported Sunday of a 64-year-old man and a 75-year-old man in different zip codes.

Officials said it could take weeks or even months to confirm their cause of death. The county declined to provide additional information about the identities of the victims, citing the ongoing investigation.

Multnomah County Health Department spokesperson Sarah Dean told USA TODAY temperatures are forecast to peak Tuesday, and cooler weather is expected to start Wednesday. Dean said the county recorded three heat-related deaths last year, noting the numbers are still provisional.

All of the deaths in 2023 occurred in August and were certified by the medical examiner.

Motorcyclists struck by extreme heat in Death Valley

A group of motorcyclists faced scorching heat Saturday in Death Valley National Park as temperatures soared to a record-breaking 128 degrees, forcing rescue helicopters to stay out of the air.

Six people were making their way through the Badwater Basin portion of the park in California when two of them suffered significant heat-related illness, park representatives said.

One of the riders died due to heat-related illness, officials said. A second was taken to a hospital for treatment of "severe heat illness." Four other members of the group were treated at the scene.

The record-setting heat prevented rescuers from using a helicopter to transport the victims, as they can't safely fly in temperatures above 120 degrees, according to park officials. Saturday's high temperature broke the area's previous daily record of 127 degrees set in 2007.

Park Superintendent Mike Reynolds urged visitors to be vigilant of the serious threat posed by the extreme heat.

"High heat like this can pose real threats to your health," Reynolds said. "While this is a very exciting time to experience potential world record-setting temperatures in Death Valley, we encourage visitors to choose their activities carefully, avoiding prolonged periods of time outside of an air-conditioned vehicle or building when temperatures are this high."

Extreme heat in California:Hundreds of deaths, thousands of injuries, billions of dollars

Heat wave safety tips

The National Weather Service recommends the following steps to keep you and your loved ones safe during a heat wave:

• Check on your friends, family, and neighbors during heat waves.
• Wear light, loose-fitting clothing.
• Drink water often, and don’t wait until you’re thirsty.
• Avoid unnecessary hard work outside or in a building without air conditioning.
• Stay in an air-conditioned area. A/C is the strongest protector from heat-related illness.

The Centers for Disease Control and Prevention also recommends learning the signs of heat-related illness. Heat exhaustion can cause heavy sweating, clammy skin, muscle cramps, weakness, fast or weak pulse, dizziness, headache, nausea, and vomiting.

Signs of heat stroke include extremely high body temperature, hot, dry, or damp skin, strong and fast pulse, dizziness, confusion, and fainting.

Contributing: Brian Day, Victorville Daily Press