Because most of my scientific research focuses on the understanding and prediction of storms that produce heavy rainfall, and because this event has hit so close to home (both literally and figuratively), I wanted to try to summarize some of what we know about this event so far.
This entry will focus on the rainfall -- where it happened and how it fits into historical context. The next one will focus on how well it was predicted. Future entries will cover other aspects of the meteorology, flooding, and impacts of this event.
The rainfall totals, especially from Wednesday night, September 11, to Friday afternoon, September 13, were remarkable. Boulder County appears to have seen the largest amounts, with numbers that are difficult to comprehend. The previous wettest September in Boulder had 5.5" of rain -- this storm produced 9.08"...in one day. The storm total at Boulder was 17.15" from Monday the 9th through Monday the 16th. And this was not an isolated event: nearly every automated rain gauge in the foothills and mountains of Boulder county reported more than 10" of rain, with many reporting similar totals to the official station in the city of Boulder. Some of these numbers come from the excellent summary by Bob Henson of UCAR, which can be read here: http://www2.ucar.edu/atmosnews/opinion/10250/inside-colorado-deluge.
As a preliminary way to put these rainfall totals into historical context, we compared the NOAA Stage IV precipitation analysis, which incorporates radar estimates of rainfall and rain gauge observations, to NOAA's "Atlas 14", which uses statistical methods to estimate the probability of a given amount of rain to occur in a given amount of time at different locations around the country. These values have recently been updated for Colorado and much of the rest of the country. For example, the Atlas 14 will tell you, for a point in Colorado, the amount of rain that is associated with having a 0.1% chance of happening in any given year. Some refer to this as a "1000-year rainstorm," but that terminology is misleading, since it implies that if an event occurs once, it won't happen again for 1000 years. Instead, what it really is is a very low-probability event, but next year will have that same low probability, and thus it could happen again if the atmosphere brings the ingredients for heavy rainfall together in a similar way. It's also important to separate the rarity of the rainfall from the rarity of the flooding that resulted. Based on the above analysis, we can say pretty confidently that this was a "1000-year rainstorm" (or, more correctly, a 0.1% annual probability rainstorm) over a large portion of northern Colorado. But it's much more difficult to place the flooding in this same perspective yet. This depends on which river or creek you're interested in, and hydrologists will be doing research to sort out exactly where this flood fits in historical context.
This first map is for the 12-hour period from 6 pm Wednesday (the 11th) to 6 am Thursday (the 12th). This is the time when the heaviest rain fell in the Boulder area -- some gauges reported over 6" of rain in this 12-hour period. When comparing to the expected recurrence intervals, we see that the Boulder area, as well as much of Boulder county, experienced 12-hour rainfall amounts that exceeded the "500-year" recurrence interval, with a few areas exceeding the "1000-year" value. (Remember that these really represent a 1/500 and 1/1000 chance of happening in a given year.)
Now, extreme rainfall is occurring almost all the time *somewhere* in the world, and we've found that, depending on the duration of rainfall you look at, if you consider the United States there are roughly 50 rain events each year that exceed the "100-year", or "1% annual chance" recurrence threshold. And there is also an important history of extreme rainfall in northern Colorado: the 1976 Big Thompson Canyon flood resulted from a localized, nearly stationary cluster of thunderstorms that developed in the worst possible place: near the top of a very steep canyon. Similarly, the July 1997 Fort Collins flash flood was caused by a localized, slow-moving group of thunderstorms.
But this event was different in a few important ways:
- it happened in September, which is not typically a rainy month in northern Colorado. There have been times with heavy rains in September -- State Climatologist Nolan Doesken recalled a case with over 3.5" of rain in one day in 1938 at Fort Collins -- but the totals in this event were several times that.
- it covered such a large area. As noted above, most of what we know about truly extreme rainfall amounts in Colorado come from relatively isolated thunderstorms in late July and early August. This is what happened in 1976, 1997, and other notable Colorado flash floods. This event, instead of covering a single canyon or watershed, had extraordinary rains over entire large counties.
- it lasted a long time. At most locations, the heaviest rains fell on Wednesday night and Thursday morning, but the entire week was rainy, which exacerbated the flooding problems.
Now, a couple comments about estimating return frequencies of extreme rain events, and the relationship between the rainfall and the flooding. You may ask, "how do we know what amount of rain represents a 1000-year event when we only have a little over 100 years of records?" And you'd be right that we can't know these with perfect certainty with such a short data record. However, estimating the probabilities of rare events is a very active and robust area of research in statistics. I don't claim to understand all of them, but there are several methods which can give both a "best estimate" and the error bars around that estimate...and those error bars can be fairly large for the low-probability events.
Finally, the results shown above are somewhat preliminary. The Stage IV precipitation analysis can be problematic in mountainous areas because it uses radar data, and radar beams are blocked in regions of high terrain. However, from the rain gauge reports that are available, it appears that if anything, it underestimated the actual amount of rain that fell, so the real story might be even worse. This is where my colleagues here at CSU are asking for your help! Even though there are now far more rain gauges around Colorado than in the past---thanks in large part to the CoCoRaHS project that started here at CSU after the 1997 Fort Collins flash flood---there are still many areas that don't have observations. If you measured rainfall during the event and it wasn't reported to CoCoRaHS, they still want that information! See here for more details: http://www.today.colostate.edu/story.aspx?id=9071