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malaise

(268,682 posts) Fri Jul 29, 2016, 06:45 PM Jul 2016

The Big Thompson Disaster in Colorado: Reverberations of a Flash Flood, 40 Years Later-MUST READ

https://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3369
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What began as a celebratory Saturday in the mountains ended in tragedy 40 years ago this weekend, when a catastrophic flash flood ripped through the narrow Big Thompson Canyon of Colorado’s Front Range. A total of 144 people were killed on that Saturday evening, July 31, 1976--the eve of the 100th anniversary of Colorado’s statehood. On just about any summer weekend, the canyons northwest of Denver are packed with vacationers and day-trippers. With the state’s centennial falling on this particular weekend, the mood was especially festive, and the weather seemed no more threatening than on many other summer days. Forecasts through the day called for a 40% to 50% chance of showers and thunderstorms, but there was no particular concern about flood risk. Only a few hours later, critical gaps in weather data, communication, and public awareness had teamed up with a slow-moving deluge to create a true disaster--one that’s had a noteworthy influence on how we deal with flash floods today.

The cold truth about warm rain
The gaping observational holes of 1976 were accompanied by a limited understanding of the meteorology that drives flash floods in the mountainous West. We now know that the heaviest rainfall comes from “warm rain” processes, when the atmosphere is so unusually warm and moist that much of a storm lies below the freezing level (a hard thing to achieve in this high-altitude region). The warm-rain process can yield radar returns that are misleadingly low for the amount of rain actually being produced. The official NOAA report on the event notes that NWS staff were puzzled by a seeming contradiction: the storms extended upward to an impressive 62,000 feet, but the strongest radar returns were surprisingly weak (only about 30 dbZ). We don’t know exactly how much warm-rain processes may have boosted the rain totals, but the available data suggests there was at least some impact. If a higher-resolution radar had been available and located closer to the canyon, and if the warm-rain process had been recognized at the time, forecasters might have picked up on the gravity of the threat. As it happened, the storms were addressed with a fairly routine severe thunderstorm warning and a cursory reference to potential flooding.

Unbeknownst to virtually everyone outside the canyon, torrential rain was falling at the time, with amounts topping 12” in less than five hours toward the western (higher) end of the canyon. Before long, an enormous pulse of high water cascaded down the canyon, pushing 10-foot-wide boulders ahead of it. The flood wave demolished more than 570 structures and hundreds of vehicles--as well as much of U.S. Highway 34, the primary route into and out of the canyon for some 1800 full- and part-time residents and hundreds of visitors that night. Many tried in vain to escape in their vehicles, and a highway patrolman who drove into the canyon to investigate was among those killed. Some of the worst damage occurred near the downstream end of the canyon, where relatively little rain fell. Based on the 139 bodies recovered from the flood (several others were never found), the vast majority of victims were killed by traumatic injury, not by drowning. With communication tools so limited by today’s standards, it took many hours for the full scope of the tragedy to become evident. Not until the next day did most Coloradans find out anything about the Big Thompson disaster.