Colorado’s exceedingly rare flood in 3 maps

New visuals from the National Oceanic and Atmospheric Administration reveal the exceptional nature of rainfall  that flooded parts of 17 counties in Colorado last week.

The map below – as an example – shows the likelihood of the maximum 24-hour rainfall totals (in any given year) that occurred along the Colorado Front Range between September 9 and 16.

Annual exceedance probabilities for the worst case 24-hour rainfall. (NOAA)

(The technical term for this metric is annual exceedance probability or AEP. NOAA provides similar regional AEP maps for the maximum 48-hour, and 7-day rainfall totals)

Notice the areas colored in deep purple.  These areas, including Boulder, experienced maximum 24-hour rainfall totals with a likelihood of less than 1 in 1,000 in any given year.  So some would call this event a 1,000-year flood.

“This doesn’t mean that such a rainfall would literally be expected once every thousand years, like clockwork,” writes Bob Henson of the National Center for Atmospheric Research. “Rather, it’s a statement of probability: a 1000-year rainfall has a 0.1% chance of occurring in any given year.”

But Roger Pielke, Jr., a professor of environmental studies at the University of Colorado, says the notion of a 1,000-year flood is misleading.

“[T]he entire notion of the N-year flood is predicated upon a view of stationarity in the statistics of climate that has come into question in the flood research community,” Pielke writes in a blog post titled “Against the 100-Year Flood.”

No matter how you choose to describe this event, the amount of rain that fell along the Colorado Front Range was extraordinary.

Doppler-estimated 14-day rainfall. (National Weather Service)

Notice the expansive area with double digit rainfall totals in the map above.

See how the flood transformed the land surface in this before (Sept. 7)-after (Sept. 13) satellite image sequence from NASA:

Colorado land surface on Sept. 7 vs. Sept. 13 (NASA)

The light blue shade apparent in the swollen rivers and tributaries downstream of the Front Range, according to NASA, reveals “sediment-laden water and muddy ground”. (The bright blue shades over the city/towns west of Greeley are clouds.)

Presently, flood waters continue to recede and a massive helicopter mission has led to the rescue of hundreds of stranded people.

The Colorado Office of Emergency Management says the number of unaccounted people – at one point over 1,200, is down to around 200.

Property losses from this massive flood are expected to exceed $2 billion dollars, the Associated Press reports.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Brian McNoldy · September 19, 2013

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