Just how extreme was Colorado's record-setting rainfall?

Boulder County went from drought to its wettest year ever in less than a week


Lindsay Abrams
September 17, 2013 5:24PM (UTC)

The torrential rainfall that inundated Colorado for the better part of a week has finally cleared, leaving eight confirmed dead and 1,600 destroyed homes in its wake (see images from the initial flooding here).

Yesterday, meteorologist Robert Henson told Climate Central, the city of Boulder officially set the record for its rainiest year in recorded history. Five days in the past week set individual rainfall records, while the 9.08 inches that fell on Sept. 12 shattered the previous single-day record, set in 1919, of 4.08 inches.

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For this much rainfall to hit the county was only about .1 percent.

What's even more surprising, though, is that up until the rainfall -- described in the press as "biblical" -- began on Sept. 9, Boulder was in the midst of a long-term drought, and on track to have a record-setting dry year. This graph from Climate Central shows just how sudden, and drastic, the rainfall was:

 

The flooding, too, set records, as Big Thompson River flooded a foot beyond levels seen in 1976, when a flash flood killed 143 people:

It's too early to pin any blame for the disaster on climate change, but some are suggesting that an unstable climate could have helped make the rainfall and flooding a bit more likely. Climate Central's Andrew Freedman explains exactly what went down:

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The meteorological contributors to the flood included a strong area of high pressure over southwest Canada and an upper level low pressure area over the northwestern Rockies. The flow of air around the two weather systems tapped into an extraordinary moist feed of air off the Pacific Ocean and Gulf of Mexico, transporting it northward into Colorado. Once the warm, moist air reached Colorado, it encountered a stalled frontal boundary and the Rocky Mountains, both of which caused the air to rise, cool, and condense into clouds that produced heavy rainfall.

By adding moisture to the atmosphere, long-term climate change may have made flooding more likely to occur or worse than it otherwise would have been, although it will be several months until scientists can complete research that will allow them to state that with confidence. Research completed on other extreme rainfall events have shown that global warming is already tilting the odds in favor of more frequent and extreme events.

Calling the past week a 1-in-1,000-year event is a rough estimate, said Henson, given the scarcity of historical records and population shifts that have made such floods, when they occur, more likely to cause damage. We can no longer say such weather is unheard of, but as the state's recovery efforts begin, it will hopefully remain unusual.


Lindsay Abrams

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