It’s a raucous meteorological symphony that’s dazzled and drenched this summer. Known as the Southwest monsoon, it largely failed to materialize last summer. Perhaps making up for lost time, it’s cranking this year.
Strictly speaking, a monsoon is a seasonal wind shift, but when it arrives, it’s like flipping a switch. The ordinarily arid desert landscape quickly becomes a minefield of thunderstorms, with an abrupt influx of humidity arriving as the jet stream retreats north. High pressure to the east and southeast also aids moisture inflow to the region, as a tropical air mass gets shoved in from the south.
Last year’s monsoon barely showed up and only after considerable delay, but the fierceness of the storms didn’t make up for their tardiness.
On July 25, 2020, a rain shower in Phoenix ended a 103-day dry streak. Several tornadoes even touched down in the Desert Southwest, whirling ominously in an area accustomed to tumbleweeds and dust devils.
This year’s storms have been a seemingly relentless cacophony of meteorological ferocity and chaos, interspersed by more delicate, visually compelling moments. As meteorologists worked to keep abreast of the unstable conditions, photographers seized the opportunity to snap stunning shots.
“I love the lightning, which really piqued my interest,” said Mike Olbinski, a well-known storm chaser and photographer who lives in Phoenix and follows the atmosphere’s most spectacular storms.
Olbinski continued: “As for my favorite thing to shoot, I love a dust storm … it’s so dynamic. Lightning also is, if you can get it anywhere, with our landscape, cactus, mountains … it’s kind of special.”
Behind the monsoon
During most of the year, upper-level winds over the Desert Southwest blow from west to east. That keeps the area dry and sunny, starved of the moisture needed to generate storms. Once the strong upper-atmospheric winds relax and high pressure builds over the central United States, though, a more southerly flow can sneak in at the surface, bringing a humidity increase.
That added moisture actually cools temperatures back several degrees from the peak, which often comes before the monsoon begins, but it’s still blisteringly hot. It also allows for pop-up storms to appear about the area.
“We use a seasonal definition across the Southwest,” said Paul Iñiguez, a meteorologist at the National Weather Service in Phoenix. “It’s part of a much larger pattern across a chunk of the Southwest and Mexico [between about] June 15 and September 30.”
These dates encapsulate the entire monsoonal feature that drives storminess over the Western United States. Specific dates vary from one local region to another. The monsoon ordinarily kicks in at the start of July in Phoenix, for example.
“You kind of get ebbs and flows of moisture as these air masses are kind of moving around,” Iñiguez said. “You’ve got a more humid air mass coming up and out of Mexico, with repeated thunderstorms, repeated convection.”
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Last year featured a slow start to the season for much of greater Phoenix, a city that sees roughly three inches of rain annually during monsoon season. That may not sound like much, but the city of 1.7 million averages only eight inches a year. That means monsoon season makes up nearly 40 percent of the city’s yearly rainfall.
“On average, first week of July, with a lot of spread, is when we get those first rains,” Iñiguez said during a 2020 interview.
Last summer, Phoenix didn’t pick up any measurable rainfall until July 24. The delayed monsoon probably played a role in the city achieving its hottest July and summer on record. Last year’s average July temperature was a whopping 98.9 degrees; this July, riddled with clouds, was about five degrees cooler.
Both July and August 2021 have featured double the average amount of rain that falls in the city, a stark reversal from last year, during which hardly an inch fell between the two months.
Despite how heavily the rains come down, much of it evades collection.
“It’s not a big contributor to water resources,” Iñiguez said. “A lot of it gets recycled or goes into evapotranspiration. A little farther south and east into Arizona, in the far southeastern part of the state, 70 percent of their annual rainfall comes down this time of year.”
Ken Drozd, warning coordination meteorologist at the National Weather Service in Tucson, concurred.
“We like to get the winter rains and mountain snows, because [they] will actually soak in more,” Drozd said. “A lot of [monsoon rainfall] runs off, though some of it will certainly soak in and replenish [groundwater.]”
It’s not uncommon for rainfall rates in the core of monsoonal thunderstorms to exceed three inches per hour, overwhelming soil’s ability to absorb water.
“I’ve even seen 10-inch-per-hour rain rates,” Iñiguez said. “In the summer, we get into our more unique hazards … the topography, steep terrain, [lack of] topsoil, hydrophobic surfaces, [and] runoff” can occasionally yield debris flows after the wildfire season, Iñiguez said.
In fact, it’s the monsoon that usually brings about the end to wildfire season in the first place. Phoenix, for instance, sees its temperatures decrease slightly in midsummer as humidity ticks upward and sporadic rains arrive. July is the city’s wettest month, in stark contrast to June, which is often bone dry in Phoenix, averaging just 0.02 inches of rainfall for the entire month.
A rapid ‘greenup’ of the landscape
To show the effect the recent welcome rains have had, the National Weather Service in Phoenix posted a comparison map juxtaposing last year’s parched landscape against the verdure that’s blossomed this summer.
The maps show the relative “greenness,” or health, of the vegetation using the Normalized Difference Vegetation Index (NDVI), derived from data collected by NASA’s Terra satellite. NDVI maps offer the ability to contrast last year’s laggard monsoon against this year’s hyperactive one, and the results are night and day from a vegetation standpoint.
More than just rain
It’s not just heavy, localized rain and lightning that accompanies the monsoon. Dry air near the surface allows pockets of rain-cooled air to accelerate out of storms toward the ground, fanning outward ahead of the rain. Microbursts, both wet and dry, are a frequent byproduct of thunderstorms.
Sometimes, the exhaust-like outflow from thunderstorms kicks up enough dust to generate a large dust storm — technically known as a haboob. Oddly enough, much of the dust doesn’t stem from the fact that it’s a desert landscape. It actually has to do with land use that loosens the top soil, allowing it to fly free in strong winds.
“A lot of the dust we’re seeing comes from farms between Phoenix and Tucson,” Iñiguez explained. “There’s a lot of disturbed land, [it’s been] sitting there for quite a while, and can be a really big source for dust generation.”
Haboobs, particularly those paired with vibrant sunsets that bathe the stormy skies in delicate hues of Technicolor, are among the most breathtaking sights in the Desert Southwest. Olbinski says the July 5, 2011, dust storm that swallowed downtown Phoenix has been the highlight of his storm chases in the Southwest.
“It was this apocalyptic end-of-the-world scene,” Olbinski recalled. “Visibility down to probably three feet, the sun was already down. … I could see city lights and [the] freeway below me, little light on the parking garage, all glowing in the dust. But that was it. Then pitch dark … nighttime.”
The cool air that exits thunderstorms can propagate for miles, sometimes colliding with other gust fronts and generating new storms, seemingly at random. That makes forecasting during the monsoon quite difficult.
“It’s interesting to monitor day-to-day how the atmosphere will respond,” Drozd said.
Severe weather hazards
Because the upper-atmospheric winds are relaxed during monsoon season in the Southwest, it’s tough to produce rotating thunderstorms. But on July 24, 2020, one tornado did snake its way down from roiling thunderheads near Kingman, Ariz., surprising drivers on Interstate 40.
Two other “landspout” tornadoes spun up elsewhere in the state beneath other storms. Landspouts can form under any kind of convective cloud and are typically not related to storm-level rotation, but rather stretching vorticity nearer unstable air toward the ground.
This year has been quieter from a tornado standpoint across the region, but wind and hail have still battered parts of the Southwest.
Perhaps the most spectacular staple of the Southwest monsoon? Lightning. Sure, it’s technically the same as lightning anywhere else, but there is one key factor that makes it magnificent: incredibly high cloud bases.
Because the Desert Southwest even at its moistest is still comparatively dry, clouds form much higher up — especially during the start of the monsoon season. It’s not uncommon for the base of thunderstorms to be more than a mile above the ground.
“Many times if you’re in the valley, you can have cloud bases 7,000 to 10,000 feet above valley floors, typically earlier in the monsoon,” Drozd said. “[Thunderstorms] can have a pretty high cloud base, you can see them from a ways away, and you can get some pretty clear shots. Plus we receive a tremendous amount of lightning during that two- to three-month period.”
Approximately 1.2 million cloud-to-ground strikes occurred between June and September last year in western Texas, New Mexico and Arizona, a record minimum; the previous lowest total was about 1.9 million strikes for a monsoon season. Records date back only to the mid-1990s, but the data highlights a significant anomaly.
This year, strikes are way up in places like Arizona but still close to the pre-2020 average. We’ll have to await the final tally, although we’ll probably see similarities across the region as this monsoon wraps up over the next few weeks.