NEW YORK (NYTIMES) - Dangerous and potentially deadly heat will settle over the southwestern United States through much of the weekend, with temperatures in some locations expected to break records and exceed 100 degrees Fahrenheit (37.9 degrees Celsius).
Nearly 38 million people from California to South Texas are under some sort of heat-related alert through at least part of the weekend, the National Weather Service said.
A heat wave is defined as a period of abnormally and uncomfortably hot and unusually humid weather that lasts for two or more days.
"Please protect yourself," the weather service office in Phoenix warned residents, while the office in Sacramento, California, said that the heat would affect everyone, not just people most sensitive to heat risk.
Meteorologists in San Diego advised residents to learn the signs of heat exhaustion and heat stroke.
An excessive heat warning was in effect through Sunday night for the San Diego area, where temperatures were forecast to reach up to 115 degrees.
Similar sweltering conditions were expected around the Grand Canyon and other parts of central and southwest Arizona.
Las Vegas, a city used to soaring temperatures, could reach 109 degrees.
Some of the most extreme heat is predicted in Death Valley, along the California-Nevada border, where the mercury could rise to 120 degrees.
A heat advisory was in effect through Saturday for a large swathe of south central California and western Nevada.
Temperatures up to 102 degrees were expected around the Los Angeles area and up to 106 degrees in the San Joaquin Valley.
The weather service in Reno, Nevada, said temperatures were forecast to max out around 100 degrees Friday and that the potentially record-setting highs were unusually early in the summer season.
On average, Reno doesn't usually hit the 100-degree mark until about July 10, meteorologists said.
Parts of Utah and New Mexico were also under the advisory, as well as a portion of South Texas, where temperatures were forecast to be 100-105 degrees Friday.
In a report issued last month, the National Oceanic and Atmospheric Administration said that above-normal temperatures were likely across almost all of the lower 48 states in June, July and August, except for small areas in the Pacific Northwest and Northern Plains.
In addition to high temperatures, the agency expected lower-than-normal precipitation across the West, which continues to face a gripping drought.
This is the first heat event of the summer season, meteorologists said, adding that many people have not yet become acclimatised to heat and may be more affected than normal by high temperatures.
Forecasters said that now was a good time to ensure that cooling systems were in good working order. They also said to stay in air-conditioned rooms and reminded residents that children and pets should never be left alone in vehicles.
As ever, staying hydrated during heat events is key. Drink water more than normal and avoid alcoholic, sugary or caffeinated drinks, they said.
Depending on the location, most heat-related alerts will expire by Saturday evening or Sunday evening.
Recent heat waves have been deadly.
Late last month, blazing heat and humidity tied or broke heat records in cities from Texas to Massachusetts. And last summer, record-breaking heat over the Pacific Northwest led to the deaths of hundreds people and jeopardised the health of labourers in fields and warehouses.
The deadly weather event would have been all but impossible without climate change, according to a team of researchers.