Although the National Weather Service measures the monsoon season as desert rainfall between June 15 and the end of September, that didn’t prevent a rainstorm last week from feeling particularly monsoonal. On Tuesday, Oct. 5, Tucson and the Catalina Foothills were placed under a severe thunderstorm warning as a storm system rolled through Southern Arizona.
The storm brought nearly a quarter inch of rain and lightning to parts of the Tucson area, as well as gusts up to 50 mph. Further out, towns like Willcox and Bowie were even hit with hail. In total, more than 50,000 lightning flashes were recorded across Arizona due to the storm.
Although this storm had the strength and symptoms of a monsoon, it was not included in the NWS monsoon statistics because it fell just outside of monsoon dates. Had it been included, it may have produced enough rainfall to bump this year’s monsoon up to the second rainiest ever recorded. But as it stands, 2021 is the third rainiest monsoon in Tucson’s history since weather data is available dating back to 1895. From June 15 through September of this year, NWS measured that the Tucson area saw 12.79 inches of rain. This is only behind 1955’s monsoon which saw 13.08 inches, and 1964’s monsoon which was 13.84 inches.
On an average year, Tucson only sees 5.69 inches of monsoon rain. This July alone, Tucson was hit with more than 8 inches of rain—the rainiest July ever recorded, and the only month in Tucson’s recorded history to experience more than 8 inches of rain (as measured from Tucson International Airport).
This was quite a change from last year’s monsoon, which was only .03 inches away from ranking as the driest monsoon ever recorded. The only other times a top 10 wettest and top 10 driest monsoon fell back-to-back were in 1918 to 1919, and 1989 to 1990. But neither of those included a Top 3 rainiest and Top 3 driest monsoon back-to-back like we just saw.
The active monsoon season improved drought conditions throughout Arizona, particularly along the borderlands. As of June 15, the majority of Pima County was categorized as D4, or “exceptional drought,” the National Oceanic and Atmospheric Administration’s most severe drought label. During the heavy monsoon, no part of Pima County measured above D1, or moderate drought, with some corners of the county not even reaching drought conditions at all.
“While this will help with the ongoing drought in the southwest, in many regions the precipitation deficit has been building for a long time,” wrote Emily Becker, a scientist who works with NOAA, in an article about the North American monsoon. “The current drought outlooks expect that the drought in Arizona and New Mexico will improve in the short term, but persist.”
The monsoon brought a similar story to many other Arizona counties. As of June 15, parts of 14 out of 15 Arizona counties measured extreme drought—only Yuma County was free of the label, and it still had mostly moderate drought conditions. But by September, no counties had D4 “exceptional drought” conditions, and only five counties had D3 “extreme drought” conditions.
Looking forward, the NWS expects a drier- and cooler-than-average winter due to La Niña conditions. Forecasters use a complex flowchart to determine if a season is truly experiencing La Niña conditions, but ultimately estimate this winter has a roughly 75% chance of reaching it.
The greater Tucson area may already be experiencing the initial effects of the season. On Thursday, Oct. 7, NWS announced that we will see temperatures “well below normal” this week, as well as breezy conditions due to a pair of weather systems moving through the Southwest. These may even result in the valleys east and south of Tucson experiencing freezing temperatures at night.
Sounds like autumn to us!