Two men carry bottled water down a street in Lyons, Colo. Access to the small mountain town was cut off after bridges were destroyed by flash flooding.
DENVER — Except for the Big Thompson fly fishermen and tubers lolling down Boulder Creek, most residents of the Colorado Front Range usually pay little mind to the small rivers that trickle by on their way from the mountains to the plains.
Until this week, when more than a foot of rain from a storm system hung up on the Rocky Mountains supercharged those streams and others with a deadly force that left vast corridors of destruction stretching from the foothills to the farmland of the plains.
“The water came over and it was 2, 3 feet deep and broke our doors down,” said Jack Hammond, who left his home in the foothills west of Lyons for a Fort Collins shelter with his wife, their daughter and their dog.
Dams along a chain of five small reservoirs failed upstream of Hammond’s home on a Little Thompson tributary as the rain picked up Wednesday. As the family huddled upstairs, water downstairs toppled their refrigerator and dumped 6 inches of mud. Finally, on Friday, a Colorado National Guard helicopter hoisted them and their young German shepherd to safety.
In semi-arid Colorado, the problem is usually too little rain that leads to drought and wildfires. But when the skies open up, the potential for the biggest drenching lies along the Front Range, where the eastern foothills meet the plains and most of the state’s population lives.
Under the right circumstances, a moist air mass can hit the foothills and get stuck. Moisture turns into rain — in the winter, snow — and just keeps falling. That’s what happened last week, when a storm system parked itself over a big swath of the state with so much embedded vapor that forecasters described it as tropical.
Colorado is far from tropical moisture sources, meaning those factors come together rarely, perhaps once every 10 to 15 years, said Nolan Doesken, the state climatologist at Colorado State University.
It’s difficult to predict monster storms, meteorologists say, and early computer models forecast 3 inches of precipitation would fall from the statewide system. That amount alone would be “a big deal” for dry Colorado — a big fraction of its annual rainfall, Doesken said.
“That far in advance, you couldn’t say it was going to be Boulder Creek, St. Vrain, Poudre,” he said, naming several flooded waterways. “You just knew there would be ingredients in place for substantial precipitation for the parts of the state that would be vulnerable.”
But then all that moisture bumped against a sprawling, counter-clockwise wind pattern that pushed it repeatedly up the slopes of the foothills. It condensed, fell as rain, and then the cycle repeated.
In Boulder alone, the system dropped a record 9 inches between Wednesday and Thursday evenings, smashing the old record of 4.3 inches. By week’s end, Boulder County had as much as 15 inches in some areas.
When the rains come, the mountain topography can turn it into disaster. All that water is channeled into a handful of narrow, steep canyons that concentrate the rushing runoff, giving it the strength to push cars, boulders and even buildings out of the way.
It’s happened before several times in the state’s history, including a 1965 flood in Denver that killed between 20 and 30 people. In 1976, the state’s deadliest flood killed 144 people in one of the areas devastated by the latest flooding, Big Thompson Canyon.
That deadly flood was the result of an hours-long deluge, not one that lasted for days. After that flood, signs were installed on the canyon leading to Rocky Mountain National Park warning drivers to “Climb to Safety” in case of flooding.
Jan Guswel, one of about 100 residents stranded in Poudre Park along the Poudre Canyon, knows Colorado’s extremes well.
Last year, they were evacuated for several weeks because of wildfire. Now, it’s flood damage on the road through the canyon, their children unable to get to school and cut off from ambulances in case of emergencies. They had enough food, thanks to their experience in preparing for snowstorms, to have organized other residents for a potluck. But they’re getting restless.
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