PHOENIX (CN) — It’s been nearly a month since Phoenix Valley residents have seen a day that didn’t eclipse 110 degrees Fahrenheit.
The metro set a national record last week for the longest stretch of 110-plus-degree days in any major city in the United States — 19 days — and has been under excessive heat warnings since July 2. On Monday that record was extended to 25 days since June 29, when the high capped at 108 degrees.
Sunday marked the seventh consecutive day at at least 115 degrees, and while the National Weather Service forecasts a high of only 112 Monday, Austin Jameson, a meteorologist in the Weather Service’s Phoenix office, said the temperature may creep up to 115 degrees at Sky Harbor International Airport, where a lack of trees and other plants allows for slightly higher temperatures.
Maricopa County has confirmed 18 heat-related deaths since April, as of July 15, and is investigating 69 more.
Nights have been no reprieve from the scorching weather.
Temperatures haven’t dipped below 90 degrees for 15 straight days — another record — Jameson said.
Kathy Jacobs, director of the University of Arizona’s Center for Climate Adaptation Science and Solutions, laid out two factors contributing to the heat wave.
“The first is that the monsoon is significantly delayed this year, and the monsoon historically has helped to cool things off,” she said. “The second thing that’s happening is this regional heat dome that has built up in part because of the fact that the monsoon has not moved in.”
Monsoon season in Arizona spans June 15 to September 30 when shifting wind patterns bring humidity from the Gulf of California and other tropical regions, adding enough moisture to the air to trigger thunderstorms and heavy rains.
Rains typically don’t fall in Phoenix until the beginning of July, though, and an unusually cool June hindered the monsoon’s development, Jameson said.
Because the monsoon formed so late, predicted by a strong El Nino winter other patterns formed late as well, including the high pressure system keeping hot air floating over the state.
“Now the pressure’s been taking charge,” Jameson said, though there’s “no clear cut reason” why the system still hasn’t moved out.
Jacobs said it’s partially because of the weak monsoon.
“It’s all related to pressure,” she said. “If the monsoon’s strong enough, then it can push the high pressure dome out.”
Now at 124 days without measurable rain, the Phoenix Valley is experiencing its fifth-longest dry spell in history.
“It’s incredibly hard on the natural environment as well as on people of course,” Jacobs said. “There are many animals and plants that when they cross a certain temperature threshold they can’t survive, and it’s always worse when it’s both a drought and a heat wave. It’s sort of a double whammy.”
Without monsoon rains, fire season rages on. More than 30 fires are burning across the state, including two that have been contained east of Phoenix, one in Scottsdale and one in the Tonto National Forest.
Started by lightning on June 6, the Ridge Fire has burned more than 10,000 acres in the Kaibab National Forest, about 14 miles south of Grand Canyon National Park. The U.S. Forest Service has contained and is actively managing the fire.
While regional weather patterns influence things like heat and drought, Jacobs emphasized the role of global climate change on the conditions in Arizona.
“Climate change is obviously contributing to the high temperatures,” she said.
The average global temperature reached 62.9 degrees Fahrenheit on July 4, which is the highest it's been since the U.S. National Centers for Environmental Prediction began recording data in 1979.
Jacobs pointed to Climate Central’s Climate Shift Index, which assigns scores to cities’s weather events based on how likely they would occur without the presence of human-caused climate change. The index suggests that current conditions in Phoenix are two times more likely to occur now than they would have been before humans began altering the climate. Tucson’s conditions are three times more likely.
While most parts of the state remain dry, monsoon season reached Tucson last week, dropping brief but heavy rains on the city a few times since July 17. Carl Zerniglia, a meteorologist in the National Weather Service’s Tucson office, said the winds have knocked down trees and powerlines, and even damaged some structures, but no flood damage has been recorded.
The rain may have helped Tucson snap its record-tying streak of eight 110-plus-degree days on Saturday, but the city is now at 39 straight 100-plus-degree days, and should remain above 100 degrees for at least the next four days, Zerniglia said. From July 16 to 22, every day either tied or broke a daily record.
While it hasn’t quite abated the heat, Jacobs said the rainfall in Tucson is a “very good sign” for Phoenix and the rest of the state.
There’s a 10-20% chance of rain in the Valley right now, but those odds increase to as much as 40% going into the weekend, Jameson said. Temperatures should drop to only 108 degrees by next Monday.
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