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Colorado's History Of July Floods Isn't A Coincidence

Courtesy of Jason Pohl/The Coloradoan
The 1976 flood in Big Thompson Canyon. The rain began falling on July, 30.

The last week of July has seen two of the most severe floods in Colorado’s history - and that’s not a coincidence.

Both floods began at night and both had devastating consequences. On July 28, 1997, the heaviest rain ever recorded in an urban area of the state caused millions of dollars of damage to areas of Fort Collins and killed five people. What became known as the Spring Creek Flood came two days short of the anniversary of the Big Thompson Flood of 1976, when at least 12 inches of rain fell over four hours in the mountains below Estes Park. In the subsequent flooding 143 people died.

“You have to have just the right conditions, and the end of July and early August tends to have a higher potential to produce storms that may cause flooding,” said state climatologist Nolan Doesken.

During that time, the atmosphere in Colorado is warm enough to hold on to the tropical moisture brought north by the monsoon winds. But Doesken said the key ingredient for a storm that could produce a flood is a cold front and upslope winds.

“That [cold front] pushes additional moisture in from the east, and that’s an upslope component which pushes that moisture up against the foothills,” Doesken said. “Then you’ve got a focusing event that can make things particularly interesting.”

Credit CSU Water Center / CSU
Nolan Doesken holds a rain gauge that volunteers use to measure precipitation in their area.

A rain gauge network has been installed since the Spring Creek Flood along the foothills as an early warning system. During the last major flood along the Front Range, in September 2013, many of the gauges were washed away by the deluge and have since been replaced.

Doesken cautions that despite technological advances in weather radar, storm tracking and precipitation estimates, Colorado’s tendency toward highly localized flooding events means that not every area is covered by these systems.

That’s why after the Spring Creek Flood, Doesken developed a simple, reliable rain gauge, trained volunteers to use it and developed a database where users could upload measurements via phone or the web. TheCommunity Collaborative Rain, Hail and Snow Network or CoCoRaHS (pronounced KO-ko-rozz) now has volunteers in all 50 states and is spreading internationally. Should anyone report especially heavy precipitation, an alarm is sent to the appropriate National Weather Service office, who issues watches and warnings.

“Your own experience where you live is the most important thing. If you can share that with others that [information] can sometimes help save lives,” Doesken said. “That’s still the case even though these other technologies seem to be carrying the burden better than ever before.”

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