The season, which can bring beneficial rain to the parched desert, began Tuesday. Its onset wasn’t always this straightforward.

A monsoon storm produces a forked lightning bolt at Saguaro National Park in southern Arizona on July 23, 2012. (Pete Gregoire/NOAA/Flickr) 

By  Mike BranomJune 17, 2021 at 12:28 p.m. MDT14

Monsoon Awareness Week kicked off across the Southwest on Sunday, while monsoon season itself launched Tuesday. Amid an exceptionally bad drought, people will be paying close attention to the monsoon this summer, hoping it can produce much-needed rainfall.

Driven by temperatures contrasts between the hot desert and adjacent chilly ocean waters, the summer monsoon in a typical year generates nearly half of Arizona and New Mexico’s rainfall.

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But the monsoon, for all of its benefits, is also a source of hazardous weather. Awareness Week is dedicated to cautioning the public about the monsoon’s potentially deadly threats: torrential cloudbursts, blowing dust, lightning strikes. As summertime rolls on, the potential can turn kinetic: flash floodingzero-visibility conditionswildfires.

Thanks to a decision made years ago by the National Weather Service, the focus of forecasters, emergency officials and the media can be about safety alone, as this monsoon season commences.

The term “monsoon” describes a seasonal wind shift that brings moisture to an ordinarily parched area. Low- to mid-level winds take on a southerly component, drawing ocean moisture northward. This allows for isolated thunderstorms to form during the summertime. While sporadic, they can be intense.

For decades, the monsoon’s onset was determined by a meteorological tripwire: three consecutive days with a dew point of at least 55 degrees in Phoenix, evidence of the usually dry desert skies moistening up.

The dew point is an indicator of the amount of moisture in the air and, when it’s above 55 degrees in desert environments, that’s considered humid. Over time, the Weather Service learned people found this standard to be confusing and misleading, prompting officials’ concern that public safety suffered.

Some 15 years ago, a push began within the Weather Service to scrap the dew point definition. Despite grumbling, both externally and internally, the Weather Service forged ahead with what we have now: monsoon season, which runs from June 15 through September.

Phoenix television meteorologist Amber Sullins of ABC15 praised the simplicity.

“Before, when we were tracking dew points, we’d have a couple of days where we met the threshold then dropped below on the third day,” Sullins told Capital Weather Gang. “We’d have to keep resetting the three-day clock over and over — and then the messaging gets really complicated.”

If monsoon season sounds much like Atlantic hurricane season, with start and end dates determined by the calendar and not meteorology, that’s the point, according to Sullins and others.

“Now we can start our messaging at the same time every year, and even if no thunderstorms start then, at least people are prepared,” Sullins said

Adding emphasis was Allen Clark, director of the Arizona Division of Emergency Management. He believes repetition of the warnings to the public is what makes cautionary messages sink in.

“If you’re relying on the monsoon season to start, based on dew points, you may miss a year,” Clark said. “As you’ve seen over the past couple of years, we really haven’t had busy seasons. By defining a period of time, it doesn’t matter what the season is, because it allows us to educate regardless.”

Over the previous two summers, according to data provided by acting state climatologist Nancy J. Selover, Phoenix’s official monsoon rainfall totaled but 1.43 inches — a “non-soon” when measured against an annual average (compiled from records starting in 1948) of 2.61 inches.

The 3-of-55 rule has its genesis in a 1963 paper, said Paul Iñiguez, science and operations office at the Phoenix Weather Service office, with the three other offices in Arizona adopting it about a decade later.

In an era without actionable data from satellites and radar, Iñiguez said, the dew point near the ground was used as a stand-in for determining whether winds at the middle and upper levels of the atmosphere had shifted, from the west to the south and southeast, bringing up humid air from the gulfs of California and Mexico.

But it didn’t take long before this signal of adequate moisture began springing leaks.

For one, 55 degrees didn’t hold up everywhere. Tucson, at a higher elevation than Phoenix and located closer to the moisture surge creeping up from Mexico, found it needed to set the triggering dew point at 54. In Douglas, even higher and in the state’s far southeast, 53-for-3 worked. Albuquerque, at one of the highest elevations of any major American city, began tracking at 47 degrees.

Obviously, this meant the monsoon lacked a uniform start across the region. For Douglas and southwestern New Mexico, the average onset date is July 3. Three days later is Phoenix’s mean start date, with Albuquerque’s following on July 9. At the extremes, the season could begin as early as mid-June or as late as the final week of July.

Also, while the dew point was okay for determining the monsoon’s start, there was nothing to mark the end. Usually, the monsoon was declared over only after a few days of evidence piled up that the winds had shifted back to the west. But even that was educated guesswork.

Finally, floating starts and vague endings added a layer of frustration when undertaking climatological comparisons spanning years.


When it was announced in March 2008 by NWS-Phoenix’s meteorologist-in-charge, Tony Haffer, that the coming summer would be the first with a monsoon season, he wasted few words in explaining why the change was needed.

Haffer, now retired and living in Eloy, about halfway between Phoenix and Tucson, remembers pushback to the idea and how “it took a lot of convincing the dunderheads — like me.” His opposition, he recalled, was based in how the traditional method brought a “semi-scientific” way to measure, with some precision, the monsoon’s arrival.

Doing the convincing was the Weather Service’s Tucson office, led by meteorologist-in-charge Glen Sampson. As he saw it, what was needed was an effective way of looking forward and cautioning the public about the monsoon’s dangerous weather “rather than, ‘Here’s something that happened.’ ”

But what finally convinced Haffer was a reminder of who he worked for: “The real purpose of the Weather Service is service — hello?!”

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