by Terri Cook Thursday, January 2, 2014
In early September last year, the weather along Colorado’s Front Range, the urbanized corridor paralleling the eastern slope of the Rocky Mountains, swung from one extreme to another. The first week of the month was exceptionally hot and dry, with high temperatures averaging 7 to 9 degrees Celsius above normal. For three days in a row, the city of Denver matched or exceeded its record high temperatures, according to National Weather Service (NWS) data.
During the second week, rain — initially a welcome relief from the heat and drought conditions plaguing much of the state — began to fall. With heavy precipitation on both Sept. 9 and 10, the ground quickly became saturated, and still the rain continued.
By the morning of Sept. 12, the NWS reported that there was a “major flooding/flash flooding event underway” in the Denver-Boulder area, with “biblical rainfall amounts” reported in many areas in or near the foothills. The unwelcome prediction on the Denver-Boulder area forecast page was for “lots more rain” that day.
September is typically a dry month along the Front Range, with average precipitation of 40.9 millimeters in Boulder, a university town of 100,000 located 40 kilometers northwest of Denver. But on Sept. 12, Boulder’s Cooperative Observer Program weather station, which has records extending back to 1893, measured the one-day total precipitation at 230.6 millimeters, nearly double the previous September month-long record high of 139.7 millimeters. The same station also measured new two-day (292.6 millimeters) and seven-day (429.3 millimeters) records. Boulder’s average annual precipitation (rain plus melted snow) is 525.3 millimeters per year, meaning the city received 82 percent of its average annual precipitation in just that one week.
Boulder — and the other communities along the Front Range — are accustomed to flash floods. One of the worst in recent history occurred in July 1976 in the Big Thompson Canyon about 50 kilometers north of Boulder. On the weekend of the state’s centennial celebrations, a stalled weather system dumped more than 300 millimeters of rain over a four-hour period near Estes Park, creating a 6-meter wall of water that crashed down the Big Thompson Canyon, killing 143 people, destroying 418 homes, and ripping out the highway that ran along the river.
In anticipating future storms, emergency planners and officials had thus focused their preparations on a scenario similar to that which played out during the Big Thompson storm — a very intense but short and localized summer event occurring due to a convective storm system. But the September storm had none of these characteristics, so many of its impacts, which lasted longer and were much more widespread, came as a surprise to officials.
The unprecedented rainfall came courtesy of an unusually persistent, stationary weather pattern that funneled extremely moist air from the Pacific and the Gulf of Mexico toward the Front Range, according to a preliminary report led by researchers at the CIRES Western Water Assessment at the University of Colorado at Boulder (CU). Circulation around an upper-level, low-pressure system over the Great Basin drew a strong plume of monsoonal moisture from the Pacific Ocean toward Colorado. Continued circulation around the low, which was blocked from moving north or east by a large dome of high pressure, drew in additional moisture from the Gulf of Mexico. Local southeasterly winds created “upslope” conditions that, along with a stalled cold front, trapped the combined moisture against the foothills, causing the humid air to rise, cool, condense and produce heavy and extensive rainfall.
The Front Range lies at the interface between the lofty Rocky Mountains and the vast Great Plains. Along this north-south corridor, a series of east-flowing drainages tumble from steep, rocky canyons, channeling precipitation from alpine areas to the relatively flat plains, where the streams spread out and meander toward Nebraska and Kansas.
Because of the arid environment, many communities are situated alongside these drainages. With the record and near-record precipitation measured along much of the Front Range, the channels swelled dangerously, overtopping dams, undercutting banks, wiping out roads, bridges and buildings, and mobilizing huge amounts of rocks, sand and silt.
The flooding, which tragically killed eight people, was the worst the area has experienced in decades. Flood conditions stretched for more than 240 kilometers, from Colorado Springs to Ft. Collins, washing out an estimated 320 kilometers of roads and 50 bridges, destroying 1,882 homes, and damaging another 17,500. Property damages alone may exceed $1.3 billion, and total losses could reach $2 billion, according to initial estimates by the catastrophe modeling firm EQECAT, Inc.
Located at the mouth of a large river canyon and in the center of the precipitation bull’s-eye, Boulder was one of the most affected communities. In addition to extensive damage to human infrastructure, 150 kilometers, or 64 percent, of the city’s hiking trails suffered significant to severe damage, and much of Boulder’s 182 square kilometers of open land — which span a range of ecosystems, including plains, foothills, montane and riparian environments — were dramatically altered.
Smaller communities tucked into the foothill canyons to the northwest, including Jamestown, Lyons and Estes Park, the gateway to Rocky Mountain National Park, suffered serious, and in some cases devastating, damage. And many isolated residents — as well as several school groups attending a mountain retreat — had to be evacuated by helicopter because access roads were impassable.
On the plains farther east, where the swollen tributaries merged, the resulting high flows severely damaged homes, wastewater treatment plants and agricultural land, much of it unharvested, located on or near floodplains, particularly along the South Platte River. Oil producers in the Denver-Julesburg Basin were temporarily forced to shut down 1,900 wells, and at least two dozen aboveground oil tanks were toppled by the raging floodwaters, spilling at least 43,000 gallons of oil.
Unusual meteorological conditions, combined with the Front Range’s distinctive topography, together created a “perfect storm,” the effects of which are still being measured.
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