The Science Behind Colorado’s Thousand-Year Flood [CORRECTION]

How severe is the rainfall that has drenched Boulder over the past week? The city has already broken the record for annual precipitation—with more than three months left to go in the year

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Photo by Marc Piscotty/Getty Images

Record-breaking flooding has drenched Boulder and the surrounding areas of Colorado

Correction appended 9/20/13

Just a few months ago, Boulder, Colorado was in the grip of yet another drought, and the state itself experienced its worst wildfire on record earlier this year. But after days of heavy rainfall that the National Weather Service called “biblical,” drought and fire is the last thing that Boulder and the rest of the northern Front Range of Colorado has to worry about.

On average Boulder receives about 1.7 inches of rain during the month of September. As of 7 AM on September 16, Boulder had received 17.17 inches of rain so far in the month, smashing the all-time record of 9.59 inches set in May of 1995. 9.08 inches fell on Sept. 12, nearly doubling the previous daily record of 4.80 inches set on July 31, 1919. In fact, Boulder has already broken its yearly record for precipitation—with more than three months left in the year, and the rain still falling.

(PHOTOS: Intense Rains Cause Deadly Flash Floods in Northern Colorado)

Parts of Boulder are experiencing a 1-in-1,000 year flood. That doesn’t literally mean that the kind of rainfall seen over the past week only occurs once in a millennium. Rather, it means that a flood of this magnitude only has a 0.1% chance of happening in a given year. This is historically bad luck, due in part to the combination of an active, drenching Southwest monsoon and a low pressure area that trapped over the region. A tropical air mass—unusual in the dry Rocky Mountains—is slowly being hauled across the Front Range by weak southwesterly winds. This is known as an orographic lift, which is converting the incredibly moist air into sheets and sheets of rainfall.

Here’s how the weather blogger Dr. Jeff Masters described the flash flooding on Sept. 12:

The floods were triggered by widespread torrential rains of 4 – 6″ that fell in less than twelve hours, thanks to a flow of extremely moist air from the southeast that pushed up against the mountains. These sort of upslope rain events are so-named because as the air flows uphill, it expands and cools, forcing the moisture in it to fall as rain. Balloon soundings from Denver last night and this morning recorded the highest levels of September moisture on record for the station.

But as Adam Andrew Freedman of Climate Central noted, it wasn’t just weather that was playing a role in the biblical Colorado floods:

An increase in the frequency and intensity of extreme precipitation events is expected to take place even though annual precipitation amounts are projected to decrease in the Southwest. Colorado sits right along the dividing line between the areas where average annual precipitation is expected to increase, and the region that is expected to become drier as a result of climate change. That may translate into more frequent, sharp swings between drought and flood, as has recently been the case. Last year, after all, was Colorado’s second-driest on record, with the warmest spring and warmest summer on record, leading to an intense drought that is only just easing.

Climate scientists will surely try to find the fingerprints of global warming on the Colorado floods, though not immediately—the National Center for Atmospheric Research, a major Boulder-based climate research institution, had to be closed because of flooding. We’re not likely to have answers for awhile—it’s usually taken years for researchers to attribute extreme events to climate change, and the don’t always find the evidence. The more immediate need to rescue the thousands of people left stranded by the floods obviously takes precedence. But make no mistake—weather doesn’t get more extreme than what has happened to Colorado this month. For a disaster of this magnitude, once in a thousand years is still too common.

[Correction: I made a mistake here—it’s not that the Boulder area experienced a thousand-year flood event. Rather, it experienced what could be characterized as a thosuand-year rainfall event during the particularly intense periods of precipitation earlier this month. Floodwaters in fact did not reach those much higher levels. So what we had was epic rainfall that might be characterized as a thousand-year event—though scientists differ on the utility of using that term, given the paucity of older data from rainfall events—but nowhere near a similarly high flood. University of Colorado professor Roger Pielke Jr. pointed out the error in a post that you can see here:

A big problem with Time’s article is that Boulder did not actually experience a “1,000-year flood.” In fact, according to an analysis presented by fellow CU faculty member John Pitlick yesterday, using standard hydrological methods, Boulder experienced between a 25- and 50-year flood. (I am focusing here on Boulder, I have not seen similar analyses for other Colorado streamflows, though they are sure to come.) Pitlick further noted that the flood waters did not reach the 50-year flood marker on the Gilbert White memorial (seen at the top of this post.)

The Colorado floods were devastating by any measure, but it does no good to call it worse than it really is. My apologies.]