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John Wheeler, WDAY, Published September 22 2013

Weather Talk: Rainstorm that caused Colorado flooding a ‘black swan event’

The rainfall that produced terrible flooding in Colorado one week ago falls into the category known among climatologists as a “black swan event.” It was a rainstorm so unusual that it is impossible to compare to any recorded storm in the area.

Boulder, Colo., is a flood-prone city. It is located at a point at which Boulder Creek flows out of the Rockies through Boulder Canyon. Heavy rainfall or melting snow coming down that canyon can be expected to cause some flooding concerns in Boulder. But this flood was remarkable. Three-day rainfalls of 13 to 17 inches exceeded all recorded monthly totals for any month with records dating back to 1897.

The unusual rain was caused by a perfect set of conditions: ample atmospheric moisture, a stalled low-pressure area and the orographic effect of this moisture-laden air being pulled upslope against the mountains by the persistent, perfectly placed low.


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