3-day inundation a 1 in 1,000 year event

  • Article by: Andrew Freedman
  • Climate Central
  • September 14, 2013 - 7:08 PM

The Boulder, Colo., area is reeling after being inundated by record rainfall, with more than half a year’s worth of rain falling over the past three days. During those three days, 24-hour rainfall totals of between 8 and 10 inches across much of the Boulder area were enough to qualify this storm as a 1 in 1,000 year event, meaning that it has a 0.1 percent chance of occurring in a given year.

At least four people have been confirmed dead so far with many more still missing, and thousands have been evacuated from their homes and businesses. All along Colorado’s Front Range from Denver northward to Boulder and in nearby areas, small creeks have been transformed into raging rivers, and surges of water, mud and debris have blasted their way through canyons, at one point trapping a firefighter on a treetop before he was rescued. Numerous records have been smashed, including the all-time 24-hour rainfall record in Boulder.

“This is clearly going to be a historic event,” National Weather Service Director Louis Uccellini said in an interview. “The true magnitude is really just becoming obvious now.”

Uccellini said the Weather Service has initiated a review of its performance leading up to and during the event. Although the potential for heavy rainfall was in the agency’s forecasts a week in advance, he said, “Clearly the magnitude of the rainfall and the repetitiveness of it in some critical areas was not pinpointed” well ahead of time. Uccellini said that this event will be the new historical high water mark for many area rivers and streams. In a technical discussion on Thursday, the NWS described the rainfall amounts as “biblical.”

On average, Boulder gets about 1.7 inches of rain during September, based on the 1981-2010 average. So far this month, Boulder has received 12.3 inches of rain. This smashes the record for the wettest month ever in Boulder, which was set in May 1995 when 9.59 inches of precipitation fell — and September isn’t even half over. The average yearly rainfall in Boulder is 20.68 inches. This means that Boulder picked up well over half its annual precipitation in just a couple of days.

This comes on the heels of a summer when Boulder experienced a moderate drought, according to the U.S. Drought Monitor. This summer also featured the Colorado’s most destructive wildfire on record.

Colorado has a long and tragic history of flash flood events, most notably the Big Thomson Canyon Flood in 1976, which resulted in 139 deaths after a slow-moving thunderstorm dumped a foot of rain in just four and a half hours, causing a massive wall of water to blast through the canyon. According to the NWS forecast office in Denver/Boulder, the river stage at the North Fork of the Big Thompson river so far has exceeded the Big Thompson Flood of 1976 by more than one foot.

It will take climate scientists many months to complete studies into whether man-made global warming made the Boulder flood more likely to occur, but the amount by which this event has exceeded past events suggests that man-made warming may have played some role by making the event worse than it would have otherwise been.

Considering that this flood event occurred in the back yard of some of the world’s top climate researchers, it is likely that this event will be closely researched.

Boulder is home to several major weather and climate research institutions, including the National Center for Atmospheric Research and the Earth System Research Laboratory, both of which were forced to close due to flooding.

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