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Thursday, October 10, 2013

Colorado extreme rainfall and flood event of 2013

Like many people around the world I watched and followed the astonishing rainfall and flood events in Colorado in September this year that resulted in at least 10 deaths, damaged some 18,000 homes, caused the evacuation of more than 10,000 people, washing away roads and bridges and isolating communities for a time. The rain and flood event is estimated to have caused $2 billion worth of damage.

The extent of the rainfall was unprecedented in meteorological records that stretch back a little more than 100 years. The extreme rainfall event has been described as a 1 in 1,000 year event. An Extremely rare combination of weather factors combining to produce the event.

Climate change however stacks the dice in several ways in increasing the probability of this event occurring in the future. A primary contribution is the increase in atmospheric capacity to carry water vapour which scientists have worked out to be 6 per cent to 7.5 per cent for each degree Celsius in average global warming. By the end of this century with average global surface temperatures projected to increase by 4 to 5 degrees with current business as usual scenarios, the atmospheric carrying capacity may be an extra 30 per cent, making extreme precipitation events like the Colorado event much more likely to occur.

The video above from I-news at Rocky Mountains PBS features Matt Kelsch, a hydrometeorologist with UCAR Comet Program. He says in the video:

"What we saw in this particular flood event is the rainfall amounts were truly unprecedented and have been estimated to be a 1000 year return interval. As we get more moisture in a warmer atmosphere, warmer air can hold more moisture, we can have longer periods of time when it doesn't rain because the atmosphere is holding all this moisture. That increases the drought or dry spells, but what climate change appears to be doing is increasing the odds or the chances of this kind of flood happening.

The world warms and the oceans warm, there is more moisture in the atmosphere, so when the natural factors come together - the natural variability - more moisture for more rain.

One of the problems people have with wrapping their heads around this is what we are talking about with something like Hurricane Sandy or the 2103 Colorado Floods is going from a very rare event to just a rare event. But then when the conditions come together, that natural variability , when we have the conditions that do make rain there is more moisture available. And that is the trend we are seeing around the world and what is expected in the future."

Matt Kelsch detailed some of the statistical records set in this Climate Central Article:

Five of the past 7 days set daily rainfall records.
The 9.08 inches that fell on Sept. 12 was an all-time single-day record, nearly doubling the previous record of 4.80 inches set on July 31, 1919.
As of 7 a.m. on Tuesday, Boulder's monthly rainfall during September stood at 17.18 inches, all but 0.02 inches of which fell during the past week. The previous all-time monthly record was 9.59 inches in May of 1995.

While the flooding was extensive and caused much damage to roads and houses, in Boulder itself, flood adaptation and mitigation reduced the potential flooding impact in Boulder itself, while it was felt much worse in many other towns and locations.

I sent out a few tweets and re-tweets about the event

The floods also submerged and damaged thousands of fracked wells and fracking infrastructure located on flood plains.

"Once those chemicals hit flood water, they get across a large swath of the landscape," Gary Wockner of Clean Water Action told "Our big concern is oil and gas, and fracking chemicals, in the water. We have seen photos of oil slicks on top of the floodwaters and we are continuing to monitor all of the flooding and cleanup efforts. Oil, gas and fracking chemicals are poisonous to people and animals, and could pollute farms and drinking water supplies."

The CIMSS Satellite Blog on the Flooding rains in the Front Range of Colorado details the record-setting rainfall across much of the Front Range of Colorado in a blog post on 12 September. They provide a Colorado flooding update blog on September 17 which examines the progress of the flood down the South Platte River and tributaries. It notes in particular "that the peak river crest of 18.79 feet at Kersey eclipsed the previous record of 11.7 feet set in 1973."

We can expect more of these record precipitation and flood events as we leave our historical climates that we have known and developed our civilization and cultures in.

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