Colorado Deluge: "Could Be Classified as a 1,000-Year Event"

An animated gif consisting of water vapor images from the GOES-15 and GOES-13 weather satellites. (Animation: Cooperative Institute for Meteorological Satellite Studies) | UPDATE 8:30 p.m. MDT 9/12/13: I've just confirmed with NOAA data that for the areas with the highest rainfall tallies so far, this was indeed very likely a once in a thousand year precipitation event. See below for details. | The Satellite Blog of the Cooperative Institute for Meteorological Satellite Studies has just posted some dramatic remote sensing imagery of the continuing deluge here in Colorado. And in the explanation of what's been happening, the author of the post, Scott Lindstrom, concluded that the extraordinary amount of rainfall we've experienced here in the Boulder area "could be classified as a 1,000-year event." Make no mistake about it: The rainfall has simply been astonishing — upwards of 12 inches in less than 24 hours in some locations near Boulder. And it has caused quite a bit of devastating flooding, as well as two deaths in this area, and another one further south. But could it really be something that occurs only once in a thousand years? Earlier in the day, the National Weather Service did say that "biblical rainfall amounts" have been reported. So it's clear that the deluge here has been impressing the experts. Even so, the thousand-year statement is an extraordinary claim, one that deserves some justification. I tried getting in touch with Lindstrom to see why he thinks it's justified, but he has gone home for the day. I've left a phone message and emailed him, and I'll report what I learn in an update once I get hold of him. | UPDATE 9:05 p.m. MDT 9/12/13: Lindstrom emailed me back and directed me to NOAA's Precipitation Frequency Data Server, where you can search for data on recurrence intervals for rainfall events occurring at a particular place, lasting for varying durations, and comprising various levels of precipitation. I looked at the relevant data table for the Boulder area, where more than a foot of rain has fallen in less than two days. And unless I'm reading it incorrectly (I don't think so), it shows that rainfall events like the one we've been experiencing tend to recur only once every thousand years. Check it out for yourself here. (Update ends here.) | Scott Bachmeier a research meteorologist at CIMSS, a leading scientific institute devoted to remote sensing applications in meteorology, shares the writing duties at the satellite blog with Lindstrom. He told me that he finds the 1,000-year number to be "believable." Particularly in light of "how unusual it is to get that large amount of rain in such a small amount of time." What has accounted for the unusual confluence of weather conditions that has produced an event so astonishingly rare? The animated gif above showing water vapor in the atmosphere can help explain the answer to that question. First, look for the counter-clockwise circulation pattern over the western United States. This, according to Lindstrom, has been drawing moisture north from the coast of Mexico — some of it possibly remaining from the dissipated tropical cyclone Lorena. A monsoonal flow like this is not unusual in Colorado — during the heart of the summer. But it usually dissipates by late Labor Day. This year it has not. Instead, it has been on steroids. Add to that an anticyclonic circulation (clockwise) in the Midwest. This has been pushing water vapor into the Front Range region as well. In other words, two circulation patterns have come together in just the right way and just the right time to draw large amounts of water vapor into Colorado. You can see this happening in the animation. Look for just to the right of the yellowish plume. All of that water vapor has pushed up against the Front Range (the easternmost extent of the Colorado Rockies) and has had trouble getting over it, according to Lindstrom. In addition, a stationary cold front that had moved in from the north also has tended to keep the moisture pinned over us. These factors have helped total precipitable water (see an explanation here) to build up to more than 200 percent of normal. And it's not over yet.