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|
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.
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.