Phoenix has hit 120 degrees only three times before, and highs are forecast to come precariously close to the city’s all-time record of 122 degrees set during the historic 1990 heat wave.

Top 5 hottest temperatures in Phoenix

122 on June 26, 1990 121 on July 28, 1995 120 on June 25, 1990 119 on June 29, 2013 118 on several days

The culprit is for this heat wave is an enormous, sprawling high-pressure heat dome that will span from roughly San Diego to Kansas City on Monday. One way we determine the intensity of such a heat dome is using a measure known as “geopotential height.” It’s the height in the atmosphere at which 500 millibars of pressure is measured. The higher this pressure level is, the hotter it is, because hot air is less dense than cold air and fills more space.

Sunday and Monday, the 500-millibar geopotential height across a large section of the Southwest is expected to go beyond anything previously observed or “outside climate”, according to a large group of computer model simulations. In other words, the models predict this heat dome will be more extreme than anything observed in this particular climate data set that dates back to 1979.

Forecast models are suggesting that 500 millibar heights could reach 6,000 meters Sunday and Monday. If the forecast proves right, it “would be at historic levels,” said the Weather Service, “rivaling the June 1990 heat wave.”

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Instances of heat domes exceeding this 6,000-meter geopotential height threshold are extraordinarily rare, and have happened only a few times in the Southwest since 1979.

But Ryan Maue, a meteorologist at WeatherBell Analytics, has found these extremely intense heat domes have become more frequent in recent decades in the West. In a dataset he analyzed dating back to 1958, he found almost all of the high-powered heat domes have occurred since 1983 — with the overwhelming majority of them occurring since 1990.

The warming climate creates a background environment more favorable for such extreme events, Maue believes. “I’d surmise that the [6,000-meter] threshold — while an arbitrary big round number — is now more easily exceeded,” he told the Capital Weather Gang in an email.

Maue plotted the trend in both the average and most maximum geopotential height value in the West during June from 1958 to present and the trend is unmistakably upward. Similar trends are seen in July (not shown).