Monsoon weather, as dependable each Valley summer as the sweltering heat, is one of the least understood climate systems in the United States.
Thunderstorms of the monsoon sometimes strike like random artillery fire, their strong wind gusts toppling power poles, peeling back roofs and laying waste to mobile homes.
They also are things of beauty, arriving around the first week of July each year to fill the sky with towering cumulus clouds, brilliant lightning flashes or Martian walls of orange dust. Perhaps most importantly, the critical water they bring to the Sonoran Desert helps make the area one of the most biologically diverse places in North America.
Yet while scrutiny of El Niño and other large-scale phenomena has given experts a stronger grasp on winter weather patterns, the Southwest's summer climate remains somewhat mysterious.
“There's still a lot of debate on Arizona monsoon thunderstorms,” said Randy Cerveny, an associate professor of geography at Arizona State University. “We only really started studying them in the late '80s.”
Climatologists often find it difficult to predict the effects of the monsoon season, which runs roughly from July to September. Cerveny said this summer's rain is likely to be near-normal — about 2 to 3 inches. Others anticipate the monsoon may skew slightly wetter than average.
Exactly how the storms are generated and why they are so unpredictable is still unclear, Cerveny said.
What is known is that a seasonal shift from westerly to southerly winds across Arizona and New Mexico brings moisture north from the Gulfs of California and Mexico, and the Pacific Ocean.
The monsoon pattern has a dramatic effect in Mexico, causing a long rainy season, and the southwest United States takes advantage of the northern edge of the relatively wet air mass. The moisture and heat combine to produce thunderheads. But other, more complicated, factors contribute to the unique patterns seen in Arizona's storms.
One area of current study is a thrusting of moist air that gets funneled up the Colorado River and causes high humidity in and around Yuma, Cerveny said. Experts now speculate that this “Gulf surge” may be crucial to generating thunderstorms in the Valley.
Other unexplained features include why the storms come later or earlier each year, why some years are wetter or drier than others, and also the curious “spottiness” of the storms, which can deluge the streets of Scottsdale or Tempe while leaving west Phoenix bone-dry. Next year, a group of scientists from across the country is scheduled to study monsoon storms in Arizona in more detail.
A few years ago, one ASU study of storm patterns in the Valley showed “a lot of variability” in where monsoon rain falls from decade to decade, Cerveny said.
“In the last few years it has been in the east-southeast part of the Valley,” Cerveny said. “Frankly, we don't know why.”
Scientists even debate about the official “start” of the monsoon, though a push to set standard dates for the season three years ago did not catch on. In the late 1960s, two Phoenix-area weather experts devised the system still in use today, which decrees that the monsoon season begins on the first of three consecutive days that the dew point is 55 degrees or more. Over the last century, the average start date has been July 7.
That definition, however, does not foretell when the “heavy stuff” is going to arrive. The humidity and first few storms of the season may lack strong rainfall, or strike hard and then dry up for weeks afterward.
From the perspective of wildland fire officials, the beginning of monsoon season can be a stressful time. Jim Payne of the U.S. Forest Service, who was in the Tucson area helping with the Aspen fire last week, said a light rain would only hamper efforts by dousing the “backfires” that are used to burn up fuel and block the main fire's advance.
Dry lightning storms, common during monsoon season, can spark troublesome fires in remote areas, he said.
On the other hand, a big rain would suppress any existing fire. And the higher humidity that occurs later in the season prevents fires from spreading as much, he said.
“Right now, we would like the monsoon to come in and get fully established,” Payne said.
Hector Vasquez, a meteorologist with the National Weather Service in Phoenix, said on Thursday that moisture is “piling up” over Mexico but remains well south of the state.
It would only take “a little tweak in the wind” to bring monsoon weather here, he said. “It's kind of hard to forecast.”
Though the monsoon storms don't do much for the overall drought problem — soaking winter rains and thick snowpack on the mountains are needed to help with that — many desert creatures depend on it for survival.
Randy Babb, a state Game and Fish Department biologist, said some desert plants and animals, from saguaros to toads, rely exclusively on summer rain to propagate themselves.
The appreciation of monsoon weather from aficionados such as Babb is not far behind.
“The summer rains are just magical,” Babb said. “There's nothing more I love than standing out on a saguaro-studded hillside, watching the lightning, and the smell of rain in the desert.”