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Did climate change play a role in the 2013 flood?

September 15, 2018

GREELEY, Colo. (AP) — Using dynamic forecasting models, climate scientists like Andreas Prein are looking at how intense rainstorms are changing over the coming decades.

Prein, a climate modeling scientist at the National Center for Atmospheric Research in Boulder, said warmer climates are leading to more frequent heavy rainfalls. Since 1950, the number of three-inch rainfalls in a day has increased about 25 percent across the U.S., according to Chad Gimmestad, senior forecaster for the National Weather Service Office in Boulder. Prein explained that the warmer air holds more moisture, so when rain falls more water is dumped than normal in cooler conditions.

“We have stronger evaporation over the ocean because it’s warmer, and then this moisture is transported over the continent,” Prein said. “It is basically exactly what happened in 2013 during the Colorado flood.”

Looking at the regional trends, Gimmestad said the heaviest two rainfalls expected in a year have increased about 10 to 20 percent in the western U.S. In Colorado, where there were once a two-and-a-quarter inch rainfalls, forecasters see two-and-a-half inch rainfalls, he said. While that increase isn’t as large as it is in the eastern U.S., where the heaviest rainfalls in a year have increased from 30 to 60 percent, it still has a significant effect.

“That’s when your flood occurs, so you just made your flood 10 percent bigger,” Gimmestad said.

Because of the Front Range’s location on the slope of the mountains and at the start of the plains, the area is no stranger to big floods. In 1965, a sustained thunderstorm caused the largest flood of record on the South Platte River and the costliest flood in Colorado history, accounting for inflation.

“Every drop of water between South Park and the Wyoming border winds up going past the Platte and Poudre confluence just east of Greeley, so all the rain that falls over that huge area ends up going right by Greeley,” Gimmestad said.

Prein said the Front Range’s major flood events are often related to a stream of moisture from the Gulf of Mexico from the south traveling north along the mountains and turning west to create upslope flows. As the mountains push the moist air upwards, the heavy rainfalls are practically anchored by the mountains. Gimmestad, who commuted from Greeley to Boulder during the floods, said forecasters knew something big could happen from the weather patterns in 2013, but seeing it was something else. Gimmestad drove to Boulder on the night of Sept. 12, the second night of the heaviest rain during the flood. After about three hours of driving around, he found the one bridge still above the water over Boulder Creek. By the time he drove back to Greeley the morning of Sept. 13, he had to go out to Kersey because everything upstream on the Platte had flooded.

“You can say, ’20 inches of rain,′ but when you actually put it on the ground and see what happens, you really can’t imagine it,” he said.

Laura Read, a water resources engineer at the National Center for Atmospheric Research, said the heavy rainfalls led to sustained flooding because of the Front Range’s relatively flat topography next to the steep gradient of the foothills. Residents in areas like Milliken didn’t see the flooding go down for as long as two weeks because the flood brought the groundwater table up, she explained. As the water came up from the ground from the 2013 flood, it caused additional damages to structures.

Despite the Federal Emergency Management Agency’s periodic mapping of floodplains, Read said rapid development, such as the recent growth in Weld County, can have major effects on the ability of the area to absorb water. Without careful planning, impervious concrete and engineered channels can increase the risk of flooding.

“When you have a static map trying to communicate (flood risk), it’s not quite matching up with the reality that if somebody builds a big development right on the river, or near the river, your flood risk just changed and nobody came out and reassessed that,” Read said.

Having worked at the National Weather Service office in Boulder since 1994, Gimmestad saw 10- or 12-inch rains annually somewhere in Colorado, but the 2013 event lasted longer and covered a larger area than normal. A similar rainfall in 1938 farther south, along with two rainfalls in the late 19th century, show the Front Range might expect rainfalls like this about four times every 150 years, Gimmestad said.

“These are things that happen periodically,” he said. “But as the climate is gradually changing, the frequency or magnitude of that event is also changing.”

Brad Udall, senior water and climate research scientist at Colorado State University’s Colorado Water Institute, said scientists of just a few years ago were asking the wrong question about climate change’s effect on extreme weather events. Where scientists before framed the conversation about whether or not climate change caused extreme weather, they’re now looking at how it impacts these cyclical events.

“Climate change may not have caused them, but they add to their intensity. It adds to their frequency. It can add to their duration,” Udall said.

Scientists are still hashing out the degree to which climate change impacted the 2013 flood, Udall said. Heavier rainfalls, drier soils and growing risk for wildfires — which leads to more erosion, Read explained — are expected with the warming climates, experts said. Though there’s not yet enough data to definitively say whether these conditions will lead to more events like the 2013 flood, Udall said climate change is creating unprecedented weather events.

“We’re now in an era, because of climate change, where all the sudden our predictability has gone away,” he said. “We need to be prepared for really bad outcomes.”

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Information from: The Tribune of Greeley, Co, http://greeleytribune.com

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