I’ve been waiting for this. Today, CIRES at the University of Colorado, NOAA, and the CSU Colorado Climate Center have released their preliminary assessment of the September 2013 Colorado Front Range flood event.
● An unusually persistent and moist weather pattern led to rainfall totals from September 9th – 15th that have been observed in only a handful of events on the Front Range in the past century.
● In the context of the entire Front Range this was a rare precipitation event, especially for September, and in some respects unprecedented.
● The very heavy rains caused severe to extreme flooding across the northern Front Range and downstream areas in northeastern Colorado; the peak flows at many gages and the overall extent of flooding were probably unmatched in at least 35 years.
● Research is underway at CIRES and NOAA to determine how human-caused climate change may have influenced this event and whether the risk of similar events occurring in the future will increase. The most plausible influence of climate change: Slightly more water vapor being made available for precipitation.
● The natural hazard of flooding for the Front Range includes not just smaller-scale convective events with very high rainfall intensity (e.g., Big Thompson, July 1976), but also rain-on-deep-snowpack events (May 1894), and broader-scale, long-duration rain events with mainly lower intensities (September 2013).
● Total societal exposure to flooding on the Front Range has increased in the past several decades due to population growth and development; recurrence of a previously experienced natural hazard will tend to cause comparatively more damage.
Martin Hoerling of NOAA in Boulder says that conditions for both Hurricane Sandy and this Colorado Flood had a blocking pattern that kept the weather from moving west to east as normal. How quickly a storm leaves an area is a big factor in the severity of any weather event.
UPDATE: Also recommended: The flood next time, by Roger Pielke Jr. who says, “Unfortunately, Time magazine set in motion an urban legend when it called our disaster a 1000-year flood, suggesting that it was an incredibly rare event, on with only 0.1 percent chance of occurring in any year. The claim subsequently has been repeated often across social and mainstream media. We cannot afford to get caught up in such hyperbole. What we know so far is that the flood event experienced by the city of Boulder, despite the record extent and magnitude of rainfall, is actually probably more accurately described under standard methods of flood frequency analysis as a 25-year flood, or one with a 4 percent chance of occurring in any year.”
Tremendous future growth is predicted for the Front Range of Colorado. Each weather event here, and elsewhere, is complicated by the expensive fact that more homes, roads, bridges, and supply chain infrastructure are damaged as growth trends upward.