The Colorado Event: Struggling to Find a Precedent
September 17, 2013
On average, Boulder gets about 1.7 inches of rain during September, based on the 1981-2010 average. So far this month, Boulder has received 12.3 inches of rain. This smashes the record for the wettest month ever in Boulder, which was set in May 1995 when 9.59 inches of precipitation fell — and September isn’t even half over! Not only that, but the average yearly rainfall in Boulder is 20.68 inches. This means that Boulder picked up well over half its annual precipitation in just a couple of days.
Colorado has a long and tragic history of flash flood events, most notably the Big Thomson Canyon Flood in 1976, which resulted in 139 deaths after a slow-moving thunderstorm dumped a foot of rain in just four and a half hours, causing a massive wall of water to blast through the canyon. According to the NWS forecast office in Denver/Boulder, the river stage at the North Fork of the Big Thompson river so far has exceeded the Big Thompson Flood of 1976 by more than 1 foot.
Extreme rainfall events have become more frequent across the U.S. during the past several decades in part due to manmade global warming. Increasing air and ocean temperatures mean that the air is generally carrying more water vapor than it used to, and this moisture can be tapped by storm systems to yield rain or snow extremes. Trends in extreme precipitation events vary by region, though, and in general the biggest increases have taken place in the Midwest and Northeast. However, most parts of the U.S. have seen an increase in extreme precipitation events, according to the draft National Climate Assessment report that was released this past January. The report goes on to note that in the future, “increases in the frequency and intensity of extreme precipitation events are projected for most U.S. areas.”
Amusingly, in the comments following Freedman’s post, crank-blogger “Stephen Goddard” wrote:
By Steve Goddard (Fort Collins CO 80525)
on September 13th, 2013
The 1976 Big Thompson River flood brought 12 inches of rain in only 4 hours. It was much more intense than what we have had this week. Same for the 1972 Rapid City, SD flood.
I don’t buy the statistics. They are not even remotely credible
For a photo comparison of the two events, see above.
“Goddard’s” post was followed by these responses from Freedman and Glacier expert Mauri Pelto :
on September 13th, 2013
The 1976 flood was the result of a thunderstorm that sat over a small area. This event occurred over a far broader region, and from totally different atmospheric dynamics. Also the Rapid City flood was in South Dakota, which the last time I checked, is not in Colorado.
By Mauri Pelto (West Boylston)
on September 14th, 2013
The widespread nature of this flood combined with the magnitude makes this a larger flood event than Big Thompson in 1976, which I was in the state to witness. This is obviously a quite rare event. the Calgary region of Alberta experienced a similar event in June. It was not the same type of storm, but equally unusual. From Chris Burt Heavy rainfall began falling across the Bow River Basin on Wednesday night with up to 190 mm (7.51”) falling in some areas over just a 24-hour period. However, it was the widespread nature of the heavy rainfall, with an average of 50 mm (2”) blanketing the entire river basin that has led to the massive flooding.” The Calgary event also featured a rare rain on snow event higher in the mountains for the second consecutive year, combined with high intensity. I could go on with the unusual rain events in the North Cascades of Washington from this summer.
“Some areas of Larimer County experienced a hundred-year flood, some a thousand-year flood,” said Jennifer Hillman, a spokeswoman for the Larimer County Sheriff’s Department. “When you’re talking about rivers cresting 10 feet over their banks, no one can prepare for that. Blizzards and wildfires — those are things we do prepare for. A flood is a whole different beast.”
Even though the weather is improving, the scope of the ongoing rescue efforts remained large, and the toll of flood devastation takes in about 4,500 square miles — about the size of Connecticut. Helicopters remained the best way to reach the thousands still isolated in mountain areas, cut off with no power or food and surrounded by undrinkable water.
The air rescue operations “are believed to be the largest airlift rescue since Hurricane Katrina” said 1st Lt. Skye Robinson, spokesman for the Colorado National Guard. “You just never thought it would happen here.”