Last April, the lead forecaster for the National Weather Service in Phoenix told a panel of drought experts that while there was a good chance Arizona’s approaching summer would be a hot one, there was at least a decent chance of normal monsoon-related precipitation in the region.
Regardless, said NWS forecaster Mark O’Malley, it couldn’t be worse than the near-record dry “non-soon” of last year.
“Well up to this point, it's been far worse,” said O’Malley to Arizona Water News recently.
“Add in the record heat, and drought is quickly becoming an issue for the state.”
In April 2019, O’Malley and other forecasters correctly predicted that cooler-than-average sea surface temperatures (SSTs) would keep hot, moist air masses from building up in northern Mexico. That would prompt a drier-than-normal 2019 summer monsoon season in Arizona.
How right they were. By late August 2019, for example, Tucson had received just half the monsoon rainfall it had gotten by the same time the previous year.
By April 2020, forecasters were seeing a mixture of SSTs. They detected both cooler-than-average and warmer-than-average SSTs – the latter of which would help encourage those billowing mountains of airborne moisture to surge into Arizona, producing our once-familiar summer storms.
As it turned out, though, neither the cooler nor the warmer SSTs appear to be to blame for the 2020 “non-soon,” which has contributed to record July heat. Rather, this year’s moisture-thief is a high-pressure dome that settled remorselessly atop the Southwest throughout the summer months so far.
“Typically, high pressure will oscillate from northern Mexico into the Four Corners and even northern (Utah and Colorado) through the summer,” said O’Malley. That northward expansion of high pressure allows the thicker moisture to surge north periodically into Arizona.
But this year? “Persistent low pressure along the West Coast has kept the high situated more often in southern Arizona and New Mexico,” said the NWS forecaster. As a result, “the best moisture has been trapped in northern Mexico.”
“With this high pressure sitting almost directly over us and the reduced moisture, we have been extremely hot,” noted O’Malley. “July 2020 was the hottest month ever in Phoenix and Tucson.”
If record and near-record dry heat is your thing, you’re in luck: There likely is more of the same on the way.
All signs point towards a continuation of this heat for at least the next month, and, likely, into September. The odds for August precipitation are tilted towards drier than average, he said, with more of a trend towards equal chances of above, below, or near-normal rainfall in September.
Oh, and it gets better still.
The prospects for drier-than-average conditions this winter are gaining as well. The chances for a “La Niña” condition emerging during the autumn and lasting through the winter are becoming much better.
“There's currently better than a 50 percent chance of this occurring,” said the forecaster. “Unfortunately for Arizona, La Niña winters are typically not good for producing a lot of rain and snow. In fact, some of our worst years are La Niña winters.
“The odds are pointing towards this type of outcome this winter.”