CU-Boulder Scientist Says Monsoon Season Has Begun And Came Earlier This Year

July 18, 2002

Despite differing opinions on the topic, Colorado's monsoon season is already underway, according to Klaus Wolter, an atmospheric scientist at the University of Colorado at Boulder. 

In fact, Wolter believes the Southwest monsoon began early this year.

The monsoon season, which typically starts around the middle of July in Colorado, actually began at the beginning of the month, said Wolter, a researcher with the Climate Diagnostics Center, part of the Cooperative Institute for Research in Environmental Sciences at CU-Boulder.

"The monsoon season started a little late for Arizona, where the moisture came in about July 10," Wolter said. "For Colorado, however, the big humidity increase happened around July 4, along with a fair number of thunderstorms both in the mountains and just to the east of Boulder, so in my book we had an early onset."

The difference of opinions about whether the annual monsoon season is underway stems from the fact that scientists use different methods to define monsoon moisture, Wolter said. One method looks at average humidity over a period of days as the measure for determining monsoon onset while another method focuses on total rainfall amounts, he said.

For the purest monsoon definition, some scientists require that the moisture comes directly from Mexico.

"In Arizona, for example, they use a three-day running average of humidity as a measure, and by that measure we qualified by July 6," Wolter said. "If you include actual rainfall as a measure of the monsoon onset, quite a few stations both east and west of Boulder have already exceeded that threshold, but Boulder and Fort Collins have not."

This year's persistent drought, however, may be hampering the region's sometimes significant monsoon rainfall. And while he believes the monsoon season is underway in Colorado, Wolter emphasized that the state has a long way to go before it is out of the current drought.

"If the peak monsoon period around late July or early August delivers above normal moisture, we could start talking about a reduction," of drought conditions, he said. "However, it will probably take more than one winter of above average snowfall in the mountains to really get rid of this drought."

In a normal, or non-drought, weather year, the Southwest monsoon typically begins in Arizona, New Mexico and Colorado in early to mid-July and runs through mid-August to early September. Monsoon weather can extend further north as well to Idaho, Montana and Wyoming.

The monsoon begins when the clockwise flow of winds around high atmospheric pressure in the central U.S. brings a deep layer of humid air northward. The moisture in the air forms scattered clouds early in the day. As evening approaches, the thunderstorms intensify with lightning and sometimes heavy rain.

The thunderstorms can create flash floods in streams and in streambeds that are dry much of the year. The storms also account for much of the West's summertime moisture.

Rainfall amounts during a typical monsoon storm can range from brief showers measuring only a trace of moisture to heavy rains of several inches that cause localized flooding, Wolter said. For the season as a whole, favored regions such as the San Juans and the Southern Front Range typically receive close to 10 inches of moisture, while most other locales in Colorado average half of that, he said. 

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