Wednesday, September 16, 2015

Monday Was the Worst Weather Day in Utah History

In terms of loss of life, the Hildale Flash Flood was the worst weather-related disaster in recent decades, but was it the worst in Utah history?

It depends.

Tragically, 12 are confirmed dead in the Hildale Flash Flood, with one missing.  Records of weather disasters are better in recent decades but than in the distant past, but there is evidence of at least one weather-related disaster in Utah history that was worse.  On February 13, 1885, avalanches killed 15 in what was then the mining community of Alta, Utah.  This is documented in an article by Mark Kalitowski entitled The Avalanche History of Alta, which appeared in the December 1, 1988 issue of The Avalanche Review.

C. R. Savage photo of Alta, UT on July 3, 1885 following a winter in which avalanches nearly destroyed the town
However, late yesterday we learned of another tragic flash flood catastrophe that occurred on Monday in Zion National Park's Keyhole Canyon.  Four are confirmed dead, with three missing.  That increases the count of confirmed fatalities in the two events to 16, which likely makes Monday the worst weather day in terms of loss of life in Utah history.

Whether or not the Hildale and Keyhole Flash Floods are the same disaster depends a bit on one's perspective.  Keyhole canyon is only about 15 miles north of Hildale.  Without better time lines of when each incident occurred, it's difficult to ascertain if the same convective cell produced the floods that washed away the victims in both areas (I suspect not, but more careful analysis is needed).  On the other hand, even if the generating cells were distinct, they were both associated with the same large-scale system, so some people might combine them into a single event.

If you are wondering what the most expensive weather-related disaster is, it is probably the 1983 Thistle landslide, which dammed the Spanish Fork River, inundating Thistle and cutting off a major rail line and nearby state highways.

The USGS suggests that the total direct and indirect costs of the slide were $688 million in 2000 dollars.  At the time, it was the most expensive landslide in US history, but I wonder if has subsequently been topped by the 2014 Oso slide in Washington.

You'll notice that I use the term "weather-related".  Most Utah "weather" disasters aren't produced exclusively by meteorology, but a combination of meteorological, geological, hydrological, and/or human factors.  An unfortunate aspect of Monday's fatalities is that they occurred despite apparently good forecasts and warnings.  Similarly, the challenges in avalanche safety involve avoiding heuristic traps, ensuring your perception of risk matches the reality, and then making good decisions (e.g., McCammon 2002).  

Striving to produce more timely and more reliable forecasts is what we do as meteorologists, but ultimately, as we look back on these events, there may be more to learn about how to minimize human loss through better understanding of the human factors.  Following the 1997 Antelope Canyon flood disaster, which killed 11, significant improvements in flash flood forecasting and communication were made that I'm sure have saved lives.  At issue as we look back on Monday's events is whether or not further improvements can be made.  An optimist might say yes, a cynic no, but the only way to find out is to study and learn.  

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