Lightning strikes on Tucson’s west side during an electric evening monsoon on Aug. 31.
The 2015 monsoon was hit-and-miss in Tucson.
Some areas of town were hit hard and often by damaging winds and flooding downpours, while others missed most of the moisture.
Monsoon rainfall totals were officially above normal this year, according to the National Weather Service rain gauge at Tucson International Airport, but that was not the case across much of western Tucson.
It was all a bit of a “mess,” said Mike Crimmins of the University of Arizona’s CLIMAS report.
“Some places got a couple inches below normal, while others received several inches above normal,” said Crimmins, a climate science specialist for the Arizona Cooperative Extension.
Monsoon totals ranged from less than 3 inches west of town to 31.34 inches atop Mount Lemmon, with midtown readings hovering around 5 inches.
The monsoon season recognized by the National Weather Service in Tucson is the 108-day period from June 15 to Sept. 30. It’s characterized by a seasonal shift in winds that brings increased thunderstorm activity and torrential rains to Southeastern Arizona and throughout the state.
Moisture from tropical storm remnants is also a driving factor in increased rainfall totals during the summer.
“There is no such thing as a typical monsoon,” said Mike Leuthold of the University of Arizona’s Atmospheric Sciences Department. This year, for example, Southeast Arizona experienced earlier and more frequent rain from tropical storms in the Pacific.
Phoenix recorded half an inch more rain than normal. The mountains surrounding Tucson were drenched with 30 to 40 percent more rain than the previous year, said Crimmins.
Crimmins gives Tucson’s monsoon a grade of “C+” — somewhat wetter than average, but not extremely so.
It was humid, even when it didn’t rain. Clouds towered over the Rincons and Catalinas most days, signifying a moist and unstable atmosphere.
The majority of the monsoon thunderstorms stayed atop the mountains, though on some days the bubbling cumulonimbus towers escaped to bring strong winds and heavy rains to the valley.
Monsoon storms “can live and die in a matter of minutes,” said Crimmins.
When the monsoon storms did pass over Tucson, much of the rainfall was heavy, but short-lived.
The strengthening El Niño in the Pacific was responsible for an active tropical season. Several tropical systems delivered soaking rains to Southern Arizona throughout the summer.
Early surges of tropical moisture from Hurricane Blanca gave the monsoon an early start in June.
Relatively high dew-point temperatures suppressed wildfire risk across Southwest Arizona, dropping early rains across the higher terrain of Arizona’s Sky Mountain ranges.
Thunderstorms began exploding across Southern Arizona by mid-June, and arrived in Tucson two weeks ahead of the average start date of July 5.
Mount Lemmon was drenched with close to 1.5 inches of rain by one of the first monsoon thunderstorms of the season on June 24. The storm also ignited a fire on Mount Fagan in the Santa Rita Mountains, which was quickly extinguished by rainfall.
By the end of June Tucson was ahead of its average precipitation totals for the season. That was the story across much of Arizona, which experienced its second-wettest June on record.
Officially, June 2015 was the wettest since 2001 with a total rainfall amount of 0.35 inches. Other parts of Tucson saw anywhere from 0.03 inches in midtown to 1.4 inches in the Tanque Verde area, and over 6 inches on Mount Lemmon.
In mid-July, the monsoon weakened slightly across Southeastern Arizona, but highly localized thunderstorms still increased rainfall totals across Tucson.
The heaviest rain of this season here was reported at the airport, totaling 1.49 inches on July 28. Storms downed power lines and trapped two people in their cars on South Julian Drive, near the airport.
The monsoon seemed to take a vacation from early to mid-August for the majority of Tucson, but areas to the north, including Oro Valley, received several inches of rain.
By late August the monsoon was back on track as severe monsoon thunderstorms rolled across Southeast Arizona. A mother and child were rescued from a flooded road in southeast Tucson on Aug. 24.
The following day, winds downed power lines in Marana, trapping six vehicles on Twin Peaks Road. Tucson inched closer to a “normal” amount of monsoon precipitation.
September brought two additional surges of moisture from the tropics. Hurricane Linda brought rain early in September while the remnants of a weak tropical depression brought soaking rains on Sept. 22.
This final gush pushed the airport gauge above normal.
But many areas were actually below normal. Leuthold said the “precipitation distribution during the 2015 monsoon was all over the place.”
“Some areas just happened to get lucky,” he said.
Looking ahead, the Climate Prediction Center continues to forecast a high chance for above-normal precipitation for the winter months.
That could make for some interesting winter weather.