Dangerous and potentially deadly heat will settle over the Southwestern United States for the next several days, with temperatures in some locations expected to break records and exceed 110 degrees.

More than 22 million people in California, Nevada and Arizona 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 told its residents, while the office in Sacramento warned that the heat would impact 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.

Here’s what you should know.

What’s the forecast?

It’s going to be dry and very hot. An excessive heat warning was in effect through Sunday night for the San Diego area, where temperatures were forecast to reach 117 degrees. Similar sweltering conditions were expected around the Grand Canyon and other parts of central Arizona, including Flagstaff. Las Vegas, a city used to soaring temperatures, could reach 110 degrees. Some of the most extreme heat is predicted in Death Valley, along the California-Nevada border, where the mercury could rise to 121 degrees.

An excessive heat watch was in effect through Saturday for a large swath of California from Sacramento to Los Angeles. Temperatures up to 106 degrees were expected in the San Joaquin Valley and the lower Sierra Nevada foothills, while highs around the Bay Area could peak at 102 degrees. In Los Angeles, temperatures may max out at 105 degrees.

A small section of Northern California and western Nevada were under a heat advisory until at least Friday. Temperatures around the region were expected to top out around 100 degrees.

How bad will it get?

Don’t expect the heat to subside anytime soon. This is the beginning of a potentially scorching summer.

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.

What precautions should I take?

This is the first heat event of the summer season, meteorologists said, adding that many people have not yet become acclimatized 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 dehydrating alcoholic, sugary or caffeinated drinks.

What is the heat index?

The heat index is a measure of how hot it really feels outside, when humidity and other factors are considered along with the temperature, according to Kimberly McMahon, a meteorologist with the National Weather Service.

While the heat index is routinely used to provide a more accurate measure of what it feels like outside, meteorologists also use it to indicate exactly how much heat the human body can tolerate. Dizziness, thirst and heavy sweating are signs of heat exhaustion, according to the Weather Service. The signs of heat stroke are more serious and can include confusion and unconsciousness, in which case call 911 and move the person to a cooler area.

When will it end?

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 jeopardized the health of laborers in fields and warehouses.

The deadly weather event would have been all but impossible without climate change, according to a team of researchers.