A truly ferocious and exceptional event. That is how Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, describes the storm that pummelled his state last week.
"This was a once-in-1000-year rainfall," he says, meaning that the storm was of such an intensity and duration that it had a 1-in-1000 chance of occurring in any given year in Colorado.
The rains and subsequent floods have so far killed eight people, displaced 11,750 and destroyed close to 18,000 homes. The city of Boulder received a year's rainfall in less than a week, says Daniel Leszcynski at the National Oceanic and Atmospheric Administration.
That huge volume was due in part to a lingering heatwave that for months blocked tropical moisture from the Gulf of Mexico from reaching the Rocky Mountains, he says. When that heatwave began to move east last week, weak winds allowed the growing storm system to sit above the Colorado peaks for days.
Fire and flood
Once that deluge hit the ground, more trouble awaited. Because of Colorado's mountainous terrain, the region is flood-prone anyway but recent wildfires exacerbated things near Boulder and Fort Collins, two areas hardest hit by floodwaters. The fires had cleared land of vegetation that would normally absorb rainwater, says Trenberth.
Urban areas were also hit hard because of their abundance of impenetrable surfaces, says Matthew Kelsch from the National Center for Atmospheric Research. "Cities have drainage systems designed to move water off streets and into streams as quickly as possible," he says.
Though natural disasters are difficult to attribute to climate change, Trenberth says that the 1 ˚C rise in ocean temperature since the 1970s accounts for 5 per cent more moisture in today's atmosphere. That's enough to invigorate already powerful storms such as last week's, he says. "There's natural variability to these events, but maybe there was a little more rain because of climate change," he says. "With weather, small differences can actually result in big effects in terms of damage done."
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