What is a heat dome? How the weather phenomena is spiking temperatures in Arizona

Arizona is baking as a heat dome settles over the western United States.

Three Arizona counties — Maricopa, Pima and Coconino — were placed under excessive heat warnings until 8 p.m. Friday.

Metro Phoenix was expected to be in the triple digits all week, with high temperatures peaking at a forecasted 114 degrees on Thursday, said Ryan Worley, a meteorologist at the National Weather Service in Phoenix.

"We're gonna see an area of high pressure build over the region over the next few days, and that's really gonna bump up our temperatures," Worley said.

With portions of the Grand Canyon set to hit temperatures between 106-111 degrees, residents and tourists alike were being warned about the high heat and how deadly it can be.

Here's what to know about the heat dome and how it will affect weather in Arizona and the western U.S.

Where is the heat dome located?

Though the entire West is likely to be sweltering, forecasters at the Climate Prediction Center have determined that June 8 to June 18 will be especially hot across the states of Arizona, Colorado, Idaho, Montana, Wyoming, Nevada, New Mexico, Oregon, Utah and and inland California.

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More: A 'heat dome' will boost temps in the Phoenix area this week. What to expect

What is a heat dome?

A heat dome occurs when a persistent region of high pressure traps heat over an area, according to William Gallus, professor of atmospheric science at Iowa State University.

"The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven," Gallus said in an article in The Conversation.

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Persistent heat wave conditions

Forecasters say that as the jet stream nudges its way north in the coming days, the heat will build into the Great Basin. Phoenix's high is expected to reach 112 degrees on Thursday and 113 degrees on Friday, according to AccuWeather.

Experts caution that there is little chance of residents cooling off when the sun sets because the heat will continue overnight.

Heat index chart calculates danger of humidity, heat

The National Weather Service warns that "extreme heat and humidity will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities."

This chart shows how hot it feels when humidity and high temperatures are combined:

Residents are being advised by National Weather Service meteorologists to stay hydrated and avoid direct sunlight.

Doyle Rice contributed to this article.

SOURCE: National Weather Service, Climate Prediction Center, NOAA, AccuWeather and USA TODAY research

This article originally appeared on USA TODAY: What is a heat dome and why is it making Arizona hotter?