Climate experts warn Arizona’s monsoon season is changing

PHOENIX — Arizona’s monsoon season is consistently inconsistent.

Variability year in and year out between dry versus wet summers is not strange for the Sonoran Desert.

However, Arizona State Climatologist Erinanne Saffell said there has been a trend over the past 25 years.

After an “exceptionally wet” 1993 across the state, rainy seasons started to become rarer.

“Then in 1994, it started getting dry, and you don’t look at it until you get another dry year and then another dry year and then another drought year,” Saffell said. “So we’ve had a long-term drought status since about the mid-90s. We’re just not getting that same amount of precipitation according to our average amount, and so we run a deficit.”

Average precipitation in Arizona during monsoon months is 5.3 inches, according to the National Oceanic and Atmospheric Administration. Looking at five-year periods since 2000, only 2010-14 resulted in above-average rainfall.

In Phoenix, where the average is 4.23 inches according to the National Weather Service, there were two wet summers from 2000-2009 and three of four monsoon seasons from 2017-2020 were drier than normal.

The city’s monsoon precipitation hit a record low in 2020 when 1 inch of rain fell.

“What I usually try to bring forward is that we’ve had a period on record of 25-26 years and we’ve had only maybe six or seven wet years,” Saffell said. “We have to make up for that mound of deficit. And so that’s going to take a little bit longer than one or two years of really wet years, wet winters and wet summers.”

More than half of southern Arizona’s annual rainfall comes during the summer months, according to the NOAA.

It’s a vital phenomenon for the ecosystem, as Katharine Jacobs, the director for the Center for Climate Adaptation Science and Solutions at the University of Arizona, pointed out that summer is when streams, rivers, trees and wildlife need the rain most.

She also said monsoons depress the need for water agriculturally and municipally, which eases the burden on the state’s lowering water supply.

Last year, Arizona’s monsoon season involved 4.20 inches falling at Phoenix Sky Harbor, the wettest summer since 2014.

“We did experience some short term drought mitigation last year … It alleviated some of that really extreme and exceptional drought,” Saffell said. “We had a good wet December and then everything else was fairly dry across the state. So we are able to get some short-term relief but when we’re looking at that long-term drought aspect, year after year after year we’re not getting as much precipitation as we should.”

Just about halfway through #Monsoon2022 and not unusual to see a wide disparity in rainfall. Much of AZ is doing very well; however as is common during monsoon season, there are still a few pockets lagging behind normal. Good opportunity to add more later this week. #azwx pic.twitter.com/i0l4gn0P9b

— NWS Phoenix (@NWSPhoenix) August 2, 2022

Arizona and the American southwest are getting hotter and drier as a result of climate change, instructed Jacobs. The state’s temperature has risen 2.5 degrees Fahrenheit since 1900, according to the NOAA.

Jacobs said the hotter weather caused largely by greenhouse gas emissions leads to more water evaporating from the soil and plants, amplifying the drought and contributing to weather extremes.

That doesn’t only include heat, but storms when they occur.

“We will continue to have variability just as we have in the past, but it’s variability around a long-term trend, rather than just variability around something normal,” Jacobs said.

“We’ll have higher peaks in terms of temperatures, we may even have higher peaks in terms of rainfall, but on average, it is getting drier and that’s driven in part by the temperature.”

There are fewer rainy seasons, but the ones Arizona gets have brought a lot of precipitation. Last year provided the second-most rainy days on record in Phoenix. In 2014, the city received 6.34 inches, its most since 1984.

Those higher peaks in rain bring other environmental dangers.

Dr. Arjun Heimsath, a professor at the School of Earth and Space Exploration at Arizona State University, said that soil becomes hydrophobic as the heat dries it out. This leads to the soil being less able to absorb water during rain storms, and thus the water flows over the surface. It creates a runway as opposed to a brake, which creates flash flood concerns.

“When you do get a significant monsoon storm or something, it’s just going to lead to more extreme flood events and more extreme kinds of overland flow and erosion,” Heimsath said.

“When you lose the roots, the soil doesn’t have as much cohesive strength and then you bring in a big storm and the soil is more susceptible to washing away.”

The Valley experienced major flooding during a 4.0-inch rain storm in 2014 — which was exacerbated by hurricanes to the south.

WRO East: An active monsoon storm pattern with elevated rain chances will remain throughout the week. Due to the higher moisture levels and potential cloud cover, temperatures during the next several days will be near to slightly below the seasonal average. #AZwx pic.twitter.com/ky40sxxm5e

— NWS Phoenix (@NWSPhoenix) August 10, 2022

The hotter weather’s impact also include melting of the snowpack north in the state, which the NOAA said will lead to heavier rainfalls than snowfalls which depreciates water supplies. Lake Mead’s and Powell’s water levels continue to fall, and there have been fewer rainy summers to combat the heat.

Saffell said it is very difficult to forecast the monsoon season and impossible to do so accurately for future years.

What is clear is that more extremes are expected.