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.

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.

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."
 
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
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.

 http://www.nbcnews.com/science/colorado-floods-triggered-convergence-geography-climate-experts-say-4B11180290