Whither the monsoon? ~ Telluride Daily Planet

‘A fickle phenomenon to predict’


Monsoonal rains over New Mexico. (Photo courtesy of Leaflet/Wikipedia)

Drenching seasonal rains: We anticipate them each summer in the San Juans, and when the annual soaking moisture doesn’t arrive, we feel misled.

Why bother calling it a monsoon if it doesn’t deliver?

This year, the big rains have failed to arrive, at least for any extended period. Chris Cuoco, a meteorologist at the National Weather Service in Grand Junction, said that that is perfectly normal, as far as “monsoons” go.

Monsoon season, he pointed out, arrives right on schedule every year. “The technical definition,” he added, “is just a change in the pattern of the wind.”

It probably doesn’t help that we associate the word “monsoon” with the annual heavy rainfall that sweeps in across the Asian continent, drenching travelers to India and thwarting summiteers in the Himalaya. But the annual global summertime winds — and rains that follow — are not the same for the so-called North American Monsoon as they are in Asia, or Africa, or Australia, or Europe (whether the winds are known as the Westerlies).


“In the Southwest, we see precipitation from monsoonal winds when pressure systems draw north, from the high elevations of western Mexico,” Cuoco said. “The monsoon in Tibet pushes moisture up from India. The same thing happens in America, but not to the same degree; our system is set up to pump moisture from the Gulf of California and the Gulf of Mexico to the mountains of west Mexico. It’s just not as strong or consistent as it is in Tibet.”

Nor is it aimed directly at Colorado: “Over time, it moves up here,” Cuoco said. “But the pressure systems have to set up just right. Usually, that means that high pressure is centered over West Texas — the Big Bend area — with pressure rotating around and into New Mexico, Arizona and Utah.” In this sense, Colorado is an almost incidental beneficiary of the resultant moisture, in other words. The “core” regions of the North American monsoon, according to the Wikipedia listing, include “Arizona, New Mexico, Sonora, Chihuahua, Sinaloa and Durango” (the state in Mexico, not the town in Colorado). “Some years, these pressure systems just don’t quite set up to cause consistent precipitation,” Cuoco summed up. When that occurs, “People will say we had a non-monsoon year. Yet the monsoon always happens. It just varies, based on how long, how consistent (and) the exact setup of the pressure systems. And that effects how much rain we get from the monsoonal flow. It’s never constant; it always goes in phases. It’s a fickle phenomenon to predict.”

And, it isn’t over. “Our monsoon season usually runs from mid-July through mid-September,” Cuoco pointed out. In other words, there is almost a month left of it. So even if the outlook for the next seven days is for more dry conditions, big rains could still come. What’s more, over the next 90 days, “We have a higher-than-normal chance of precipitation,” Cuoco said. “In the mountains of Colorado, there’s a 40 percent chance of more precipitation than average, and along the border with Utah,” where there are even fewer conifers and ever more desert, “there’s a 33 percent chance of higher-than-normal precipitation.”

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