Heavy rain that battered the tightening city of Denver Sunday night drained 1.76 inches of water in 26 minutes — exceeding norms in a flood-prone region, state and federal weather forecasters concluded Monday when they reviewed the measurements.
Intense rain showers fit patterns associated with global warming – because warmer air can hold more water and has more clout when atmospheric currents create storms.
“You have more dry spells, but when it rains, the rain is more intense. That’s the general picture of what a warming climate is doing to the water cycle,” said Colorado State University state climatologist Russ Schumacher.
Flooding is increasingly plaguing concrete-heavy cities, including Denver, where city drainage officials are advocating a shift to “green infrastructure” that could protect residents. For decades, developers in Denver have poured increasing amounts of concrete to cover natural terrain and create increased potential for water accumulation on surfaces.
Weather monitoring stations, operated by Denver’s Mile High Flood District east of downtown near the zoo, measured surges of 1.42 inches in 20 minutes, 1.3 inches in 15 minutes and 1.76 inches in 26 minutes Sunday night , forecasters from the National Weather Service said.
“Certainly remarkable, very intense rain, no question,” said Schumacher after checking those numbers.
Climate researchers have long expected heavier rain. For every 1 degree Celsius (1.8 degrees Fahrenheit) increase in temperature, the air’s water vapor capacity increases by 7%. When atmospheric currents carry clouds over a region, more rain falls. A trend has been documented in the Northeast and Midwest that “the heaviest rains are getting heavier,” Schumacher said.
In the Southwest, rainfall overall has declined during a prolonged drought since 2000, although Colorado has faced heavy rains like the 2013 storms that led to historically extreme flooding.
“We have a history of very bad flooding here along the Front Range. Even considering the types of storms we’ve experienced in the past, having infrastructure robust enough to withstand the rain would be a major challenge. But the trend is likely to be seen here in the future, with short-term intense rains becoming more intense. This further increases the infrastructural challenges.”
How much water collects and inundates an area depends on where the rain falls and the type of terrain, National Weather Service meteorologist Dave Barjenbruch said, describing Sunday night’s rains in northeast Denver as a “fairly significant” storm.
“Say you are in a forest. There is a lot of protection and water penetrates the ground easily,” says Barjenbruch. A cornfield or open area could soak up sudden heavy rain.
But in densely packed cities, “the combinations of concrete, asphalt and hard-packed dry soil tend to repel water,” he said, comparing Denver to wildfire scars in western Colorado, where burned soil makes landscapes impervious to falling rainwater.
“You’ll get more drain.”
Holly Piza, research and development director for Denver’s Mile High Flood District, on Monday acknowledged the challenges and pointed to the benefits of the natural terrain.
“Paving green spaces leads to increased stormwater runoff and flooding,” Piza said. “The Mile High Flood District recommends good planning and solutions that leverage green infrastructure, such as B. moving paved areas to permeable areas and keeping streams open – not cased.”