The North American monsoon, the annual wind shift that brings our summer rainy season, is both dependable and unpredictable.
In most years it arrives on time in the first week of July and reliably delivers half of the region's annual rain, about 6 inches of it, in two months time.
It is unpredictable, because its arrival and intensity depend on an array of factors, from sea-surface temperatures in the Pacific Ocean to land warming in the Great Basin to, possibly, the aroma given off by pine needles in Southwestern forests.
The good news for this year is that some of the major factors known to influence the monsoon point to an early and productive one.
We've had a fairly dry winter and, in most cases, that is followed by an early monsoon start. We have to dry out and warm up in order to produce the updraft that makes way for cooler, moister air from the mountains of Mexico.
It's currently dry and hot to the north of us. That makes it easier for the high pressure system now parked over us, and delivering 100-degree-plus days, to move north as well, creating the vacuum that brings moist air from the Gulf of Mexico and Mexico's Sierra Madre.
Those conditions are similar to last year, when the area received its saturating moisture on June 24, said Christopher Castro, an associate professor of atmospheric sciences at the University of Arizona.
Castro predicts an early start - July 4 or before.
The National Weather Service still gives equal chances for an early, late or on-time monsoon. But local forecasters have begun hedging their bets - noting the same conditions mentioned by Castro.
The bad news for coming years is that many factors that contribute to the seasonal storms are changing, and recent research into monsoon history showed it has not always been so reliable. The tree-ring record shows strings of years in which dry winters teamed with dry summers.
A group of scientists at the UA, including Castro, is working with colleagues in New Mexico and Utah to refine the computer models used to predict rainfall under varying conditions. They're linking the historical record from tree rings to known historical weather records so they can add in other factors, such as sea temperatures, atmospheric conditions and moisture from vegetation.
Russell Monson, a UA professor of natural resources who heads the $3.6 million, five-year study, said a key subject of the investigation will be the effects of climate change on the monsoon and the vegetation of the area, including invasive grasses.
Even more erratic
The monsoon, already difficult to predict, will become more erratic under changing conditions, Monson said.
Monson, Castro and other UA researchers are not ready to buy into recent predictions that the monsoon's rainfall will shift to an August-September phenomenon as global and regional temperatures increase.
That computer model, proposed by researchers at Columbia University's Lamont Doherty Earth Observatory, predicts that shift will occur by the end of the century. The findings were published in March in the Journal of Geophysical Research.
Castro said the study uses climate models that are too large in scale.
The monsoon is, at its core, a local phenomenon. The regional wind shift makes moisture available, but individual storms are created in convective processes on individual mountain ranges and land forms across the Southwest.
The current investigation will resolve weather patterns at a much finer scale, said Monson. He is an ecologist, hired through an initiative of the UA's Institute of the Environment to foster collaborative research across the UA's many specialties.
His previous research in Colorado examined a link between the aromatic compounds emitted by pine needles and cloud formation. That continuing study indicates these terpenes, which leak out more quickly on hot days, "could have a significant effect on producing and thickening clouds and producing wetter storms," Monson said.
The growth of invasive grasses such as buffelgrass could also have a dramatic effect on the monsoon and the fire cycle in the Southwest, he said.
"Some of these invasive grasses have a unique physiology," he said. "They are summer active grasses that evolved in Africa near the equator. They like warm, moist summers."
The study will examine the links between the vegetation and the atmosphere. "We are trying to ecologically tease out these interactions," he said.
Look to arrival date
No two monsoons are alike, said Zackry Guido, associate staff scientist with the UA's Institute of the Environment.
"There is always a monsoon, but the character of the monsoon matters. It's not just the rain gauge at the Tucson International Airport," he said.
The arrival date is a crucial factor in the wildfire season, he said. Blazes erupt with more frequency at the dry end of June. Lightning in early monsoon storms provides an ignition source. Breaks in the monsoon can allow vegetation to dry, creating a second fire season.
Public safety, meanwhile, is threatened more by heavy, intense storms that flood streets and washes, than it is by a series of smaller storms.
"Where it rains matters, when it rains matters, and how it rains matters," Guido said.
On StarNet: We're live-blogging the monsoon at live.azstarnet.com. Tweet your experiences and send photos via Instagram using the #tucsonmonsoon hashtag.
• Tucson's average monsoon rainfall: 6.06 inches.
• Wettest summer: 13.84 inches in 1964
• Driest summer: 1.59 inches in 1924
• For detailed info on the monsoon and its dangers, go online to weather.gov/tucson or visit the National Weather Service Tucson Facebook page.
Source: National Weather Service
Average monsoon start dates in Tucson by decade
1950s - July 1
1960s - July 2
1970s - July 2
1980s - July 6
1990s - July 3
2000s - July 4
Earliest: June 17, 2000
Latest: July 7, 1987
Source: National Weather Service
A week of weather hazards
The National Weather Service focuses on a different weather danger each day this week in preparation for the monsoon season June 15 to Sept. 30.
It will also host a Monsoon Safety Awareness Fair on Thursday at Park Place mall from 10 a.m. to 2 p.m. with safety experts and exhibits.
• Monday - Flash floods: Flooding in our rivers and washes is the No. 1 cause of thunderstorm-related deaths in Arizona, the National Weather Service says. Do not drive into a flooded wash. Almost all flood deaths occur in vehicles.
• Tuesday - Lightning: Arizona is 27th among states in the number of lightning strikes per year, but almost all of them occur during the thunderstorms of the summer monsoon. Lightning can strike up to 10 miles from a storm. If you can hear the thunder, it is close enough to seek shelter in a building or vehicle.
• Wednesday - Downburst winds: Downbursts from thunderstorms can create straight-line winds up to 100 mph and produce damage comparable to a tornado.
• Thursday - Dust storms: Early in the rainy season, thunderstorm downbursts can generate areas of blowing dust that make driving extremely hazardous. Do not drive into the dust. If you are on the road and lose visibility, pull off the roadway as far as possible, turn off your lights, take your foot off the break pedal, and wait it out.
• Friday - Heat: Excessive heat is the nation's No. 1 weather-related killer. Restrict activity in Arizona's midday heat. If you must be outside, wear loose, light clothing and a hat. Drink lots of water. Never leave children or pets in a vehicle. If you observe signs of heatstroke in yourself or others, call 911.
Contact reporter Tom Beal at email@example.com or 573-4158.