Colorado Disaster: What Is a 100-Year Flood? Page 2


Destroyed river gauges

The Satellite Blog of the Cooperative Institute for Meteorological Satellite Studies has said the downpour could be classified as a 1,000-year event, or one that has a 0.1 percent chance of happening. But Doesken noted that this kind of terminology can be problematic, because one needs a lot of data to be able to talk about how likely something is to happen over a long period.

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"It's argued that to have a reasonable estimate of a 100-year storm, you'd like to have a couple hundred years of data," he said. "And of course that's a rare thing in the United States." By extension, that means that it would be best to have at least 1,000 years of data before accurately calling something a "1,000-year storm." (Colorado Flood Photos: 100-Year Storm)

Perhaps ironically, the flooding has been so bad that it destroyed three to five river gauges designed to measure floodwaters, Kimbrough said. "One gauge was totally swept away when the river bank collapsed," he said.

Flooding has also prevented researchers at the University Corporation for Atmospheric Research (UCAR) from getting to work — the offices of this large climate research center are closed.

Whence the Rain?

Jeff Weber, a scientist who works at UCAR and lives in Boulder (and whose basement flooded), said all this rain has come as the result of an unusually stationary low pressure system centered over Utah, paired with a high pressure system to the east. This confluence of patterns has sucked in a huge mass of warm, tropical moisture from the Gulf of Mexico and pushed it toward the west. But the Rocky Mountains happened to get in the way — as the air is pushed up, moisture has condensed and fallen out as rain.

"We're dealing with a tropical air mass in a mountainous environment, and that's just a disaster," Weber said. "As the air is pushed up by the mountains, it squeezes out all the rain."

The heavy rain looks likely to subside by this afternoon (Sept. 13), and while some isolated storms are expected this weekend, next week should be dry, he said.

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