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

By Tom Yulsman | September 12, 2013 4:27 pm

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.

  • disqus_atlq8Zmtsd

    Generally speaking you wouldn’t expect the 1000 year event to be too much higher than the 100 year event in most areas. Afterall, the 100 year event is already approaching “perfect” conditions. I would probably expect even less difference between the 1000 year storm and the ten thousand year storm.

    It doesn’t strike me as too extraordinary of a claim if it noticably exceeds the events we have on record.

  • Eric

    Be careful with the probability language. A 100 year flood does not mean that if it is happening today it will by 100 years before it happens again. This is simply an event that has a 1% probability of occurring in any given year. This is why it is possible, not likely, but possible to have a 100 year flood two years in a row. I expect the 1,000 year event is also a probabilistic statement indicating that Lindstrom is saying that this is a very low probability event, 0.1% in any given year, but we don’t need a 1,000 year record to estimate this probability. Also, as unlikely as it is, it could happen again next year. Thanks for the updates and stay
    safe.

    • disqus_atlq8Zmtsd

      Very true. You would actually expect these rare events to happen in clusters, when the weather patterns make them more favorable. Take for example the clustering of extreme weather events in certain areas during El Nino or La Nina.

      Perhaps two thousand year events in a decade followed by a few thousand years where it doesn’t occur again. Of course, these probability estimates are calculated using the short term climate (I believe), so at some point the time frame descriptors become meaningless because the climatic conditions that the probability is utilizing won’t be stable for that period of time. Calling an event with .01% probability the “Ten thousand year storm” is meaningless. Ten thousand years ago (give or take) we were in a glaciation period with vastly different weather patterns.

  • andrew oh-willeke

    To be clear, the description “1000 year flood” or “100 year flood” is not primarily based upon a survey of historical floods (historical records don’t go back far enough to create a statistically significant sample, although estimates can be made from tree rings, debris patterns, etc.) or an engineering analysis of what a perfect storm of climate events and surface conditions would imply.

    Instead, these descriptions rely on the fact that event intensity logarithmically related to event frequency (earthquakes and floods, incidentally show the same pattern in this regard). You use high frequency, low intensity events to set parameters in an equation that is then used to determine the expected frequency of high intensity events using the mathematical relationship.

    In the case of flood prediction, flood intensity is usually operationally defined as peak 24 hour rainfall with water height at a given level of peak 24 hour rainfall in particular locations calibrated from low intensity historical events.

    Another “stupid” way to make an estimate is that flood insurance programs require places in a 100 year flood plain to be mapped by engineers on a comprehensive basis. If places well beyond the 100 year flood plain are inundated, there is a good chance that it is a 1000 year flood. If places only barely beyond (or within) the 100 year flood plain are inundated, it probably isn’t a 1000 year flood.

    • Tom Yulsman

      Thanks for this! Just to clarify: Lindstrom was speaking not of the flooding, which may be in the 100-year category, but of the astonishingly large amount of rainfall in such a short period of time. Going on 15 inches here in 24 hours or a bit more — almost a year’s worth of precip for this region in a day.

      • JonFrum

        The flooding is in the 100 year category? The television report last night cited a 1976 flood that killed many people.

        • susandanielspi

          The Thompson dam broke then.

        • andrew oh-willeke

          Deaths are as much a product of land use policy as event severity. The widespread current requirement that flood insurance be obtained for homes in 100 year flood plains has greatly discouraged development in those areas because flood insurance is wicked expensive.

    • JonFrum

      So in other words, a ’1000 year flood’ is not meant to imply a thousand year flood. It is a statistical creation – hey, presto!

  • David Graham

    On the night of July 19/20, 1977 Johnstown, PA received 12 inches of rain in eight hours. NOAA called that a 100 year storm, and it was a freak. geography changes everything.

    • Tom Yulsman

      Thank you David. Some clarification: The return interval for that level of storm will be much shorter in Pennsylvania, which obviously gets a lot more precipitation every year than we do. We average ~18 inches — and we’ve gotten almost all of that in a little over 24 hours.

  • Tom Yulsman

    I just want to commend everyone for the comments so far. You guys are awesome. I am learning a lot.

  • disqus_atlq8Zmtsd

    What kind of effect will an event have like this on water availability in the area? I assume a short duration event will have marginal impact on ground water beyond whatever is normal for an event that saturates the soil. However, I have no idea what the reservoir locations and capacities are for that part of the world.

    I’ve read plenty about diminished capacities year to year in places like Texas, but I’m not sure what the situation is in Colorado.

    • feralcamero

      We are in a drought in Colorado, if you can believe it. As far as water availability, some cities are better off than others (water rights are a big deal out West). Some water purification plants have been inundated which will create potable water shortages for those areas. As far as trying to capture this runoff, the water down low below the mountains is polluted now and as such low lying reservoirs could be at risk.

      • http://www.cyclelicio.us/ Cyclelicious

        A few small reservoirs along the Front Range have apparently failed due to this storm. To what extent this affects storage capacity is anybody’s guess right now.

        • Tom Yulsman

          I’ll be posting a picture of one of these small reservoirs that was breached. It’s less than a mile from where I live.

          • http://www.cyclelicio.us/ Cyclelicious

            I’d like to see it. Can you say which reservoir? I used to hike around the string of reservoirs just underneath Indian Peaks – Isabelle, Brainard, Long Lake, etc.

          • Tom Yulsman

            It was a small, private agricultural reservoir near Niwot. No name. There are hundreds and probably thousands of these things. An unknown number of them burst.

      • disqus_atlq8Zmtsd

        Well that is just hugely unfortunate that copious amounts of fresh water could actually end up hurting water availability in an area that could really some.

  • Shawn Huckaby

    Given that a reported 12-14 inches of rain fell above the Thompson Canyon area (25 miles north of Boulder) in just 4 hours a mere 37 years ago, I question their findings.
    If we’re talking about the the widespread nature, or that it’s happening in September, then maybe.

    • disqus_atlq8Zmtsd

      You could have two 1000 year events separated by two weeks. Granted, anecdotal evidence hurts, but by itself one or two events don’t disprove the theory.

  • tfosorcim

    If I were the experts, I wouldn’t be too quick to label this.
    Based on the acceleration of climate change, this could be a 5-year flood.
    All we have to do is nothing. Except wait.

  • Anthony_A

    How accurate is the model predicting probability of extreme events? We have a hundred years or so of data to predict, but are we fitting the data to the appropriate curves?

  • Psalmon

    So the headline is 1,000 year flood. Three larger floods have occurred in the last 92 years (1921, 1997 and 1976 where 14 inches fell in 4 hours). BUT that’s not what a 1000 year flood means, because of statistics, tree rings, logarithms, “mathematical relationships.”

    So we should ignore OBSERVATION and trust models again – this is a 1000 year event, even though it happens every 25 years.

    Ok I get it. Nice headline.

    You know Man jumped down out of the trees and started writing things down to learn from history and build knowledge, a civilization, above mysticism and witchcraft. How this lunatic stuff continues in the age of the internet is beyond comprehension.

  • Keith Harding

    “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…..You can see this happening in the animation. Look for the yellowish plume.”

    Just to clarify, yellow on the satellite image implies drier air. What we’re looking at in this image is the moisture in the upper atmosphere, which isn’t reflective of what’s at the surface. This yellow plume is dry air from the Southwest that is being transported above the moist surface air mass. You can see this pretty well if you look at a sounding as well, very dry air aloft and winds from the Southwest (above about 500 mb) but very moist air near the surface and winds from the SE (going uphill into the mountains).

    http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=GIF%3ASKEWT&YEAR=2013&MONTH=09&FROM=1212&TO=1212&STNM=72469

  • Human Capital

    Thanks for the discussion.

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ImaGeo

ImaGeo is a visual blog focusing on the intersection of imagery, imagination and Earth. It focuses on spectacular visuals related to the science of our planet, with an emphasis (although not an exclusive one) on the unfolding Anthropocene Epoch.

About Tom Yulsman

Tom Yulsman is Director of the Center for Environmental Journalism and a Professor of Journalism at the University of Colorado, Boulder. He also continues to work as a science and environmental journalist with more than 30 years of experience producing content for major publications. His work has appeared in the New York Times, Washington Post, Audubon, Climate Central, Columbia Journalism Review, Discover, Nieman Reports, and many other publications. He has held a variety of editorial positions over the years, including a stint as editor-in-chief of Earth magazine. Yulsman has written one book: Origins: the Quest for Our Cosmic Roots, published by the Institute of Physics in 2003.

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