Colorado floods triggered by convergence of geography and climate, experts say

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By John Roach

The torrent of water that gushed over and down the Rocky Mountains late last week resulted from a fateful confluence of geography and weather. While the deluge is unprecedented in the historic record, it may offer a window onto the new normal as the planet continues to warm.

The exact role of global climate change in the deluge is uncertain, but it certainly played a part, according to climate, weather and policy experts.

As of Tuesday, more than 17 inches of rain had fallen since Sept. 12 in Boulder, Colo. The soaking, described as "biblical" by the National Weather Service, left at least eight people dead with hundreds more still missing and rendered untold millions of dollars in property damage. 

Cause of the storm
The flooding resulted from a fetch of tropical moisture drawn north from Mexico by a weak, but large, upper-level low-pressure system that was blown up into the Rocky Mountains by a persistent southeasterly airflow, according to the National Oceanic and Atmospheric Administration.

"The moisture-laden air was thus moved upwards and forced" to turn to rain, "most prolifically" in the area between Interstate 25 and the Continental Divide, Martin Hoerling, a research meteorologist who studies climate variability at the administration's Earth System Research Laboratory in Boulder, explained to NBC News in an email.

Earlier in the summer, the region is prone to North American monsoon-type rains that come when heat in the Southwest draws up moisture from the tropics and causes thunderstorm activity in Arizona, Utah and Colorado, noted Bob Henson, a meteorologist and writer with the University Corporation for Atmospheric Research in Boulder.

But last week's rains came during what is typically Boulder's driest month and were unyielding in intensity — unlike the scattered thunderstorms of monsoon season. "What was unusual was that it lasted so long and produced heavy rain over such a large area," he told NBC News.

As the week wore on, "the moisture kept coming, and the rains kept falling, anchored geographically by the terrain," Hoerling added.

"Boulders running down rivers"
The foothills of the Rocky Mountains where the idyllic college town of Boulder rests are drained by a veiny network of streams and creeks that were turned into raging rivers as the rains fell.

"It was like living right next to a jet engine ... you could hear the boulders running down rivers at night, which was very eerie, creepy and disturbing," Bradley Udall, an expert on the impacts of climate change on water resources at the University of Colorado Law School in Boulder, told NBC News. "I couldn't go to sleep."

Udall has lived in Boulder off and on since the 1970s and not once seen anything close to the deluge of last week. 

The 1976 flash flood in Big Thompson canyon between Estes Park and Loveland — a seemingly similar event which killed more than 100 people — was caused by a highly localized hovering thunderstorm that dumped more than 10 inches in four hours, he noted. The 1976 storm was over almost as quickly as it had begun, while the torrents of the past week kept up for days.

"If you would have told me a week ago that we could get 15 inches of rain here in a week, I would have laughed at you," Udall said. "There is just no analog for this type of storm for this time of year."

Climate change's fingerprint
The question then is what's behind the unusual confluence of factors that caused the unprecedented event, which Henson said is the type of flooding expected only about once every thousand years. 

According to Hoerling, climate change was likely responsible for "about 3 to 5 percent" of the water vapor in the fetch of tropical moisture, given that a warmer atmosphere can hold more water and published studies indicate "a few percent increase in water vapor to date."

The rest, he said, is due to atmospheric circulation patterns that appear inconsistent with models used to simulate the global climate, including those being examined for the upcoming report from the Intergovernmental Panel on Climate Change. Those models project a slight decline in summertime rainfall, he said.

The caveat is a 2012 report from the panel that indicates heavy downpours may increase in intensity or frequency "'in the 21st century over many areas of the globe,'" Hoerling said. "It is unclear how, if at all, that assessment pertains to the Front Range, however."

According to Udall, to link the Colorado flooding to climate change is an invitation to criticism from some people in the climate science community, but "if anybody wants to tell me that climate change is not at least partially at work here, I'm going to tell them that they are (in denial)."

Determining whether the contribution is a few percent or 99 percent, he added, is beside the point and perhaps impossible with current modeling ability. That's the uncertain nature of climate science, he explained. But "some things are uncertain and scary enough that you actually do have to act."

Udall has spent much of the past decade lecturing on the threat of drought that climate change poses to the southwestern U.S. while downplaying the risk of floods, he noted. But climate models do indicate occasional high flow years on the Colorado River, which make sense given the increase in water vapor and high mountains to squeeze out the water, he said.

"What the scientists tell us is that these big flood events are probably more likely to occur in areas that are wetter, but that doesn't mean that we won't get them in these drier areas as well," he said. "It fits perfectly with the more extreme events on both sides of the drought-flood continuum."

John Roach is a contributing writer for NBC News. To learn more about him, visit his website