The 1,000-year Tennessee flood of 2010 — what are the odds?

It started raining on Saturday, May 1, 2010. By the time the rain stopped 36 hours later, large areas of middle and western Tennessee were under water. Fiftytwo of the state’s nintyfive counties would later qualify for disaster assistance. The amount of water that bucketed down that weekend was epic. The meteorologists called it ‘a thousand-year flood.’ What’s remarkable about the weather system that caused so much damage is that it showed up unannounced. No named storm was involved.

Map of Tennessee showing rainfall distribution May 1 & 2, 2010

The rains that inundated Houston, Texas, in 2017, were carried in from the Gulf by hurricane Harvey. The rains that dumped on the Carolinas in 2018, were transported from the Atlantic by hurricane Florence. People knew those tropical storms were coming, days in advance. We could watch the approaching cyclones on our TV screens. The deluge that swamped Tennessee in 2010 arrived without any warning at all. Here’s what the Memphis Office of the National Weather Service had to say:

“A significant weather system brought very heavy rain and severe thunderstorms from Saturday, May 1 through Sunday morning, May 2. A stalled frontal boundary coupled with very moist air streaming northward from the Gulf set the stage for repeated rounds of heavy rainfall. Many locations along the I-40 corridor across western and middle Tennessee reported in excess of 10 to 15 inches, with some locations receiving up to 20 inches according to Doppler radar estimates.”

It was an ordinary weather system — except for the “very moist air.” Apparently that’s what made the difference between a typical Tennessee rain storm and a thousand-year flood.  What is a thousand-year flood, anyway? The National Oceanic and Atmospheric Administration website (climate.gov) contains an engaging article titled, “How can we call something a thousand-year storm if we don’t have a thousand years of climate observation?”  Here’s my interpretation of the statistics it covers:

Records gathered over the past 100+ years showing the correlation between rainfall amount and flooding are available for most parts of the country. Flood predictions are derived from the statistical analysis of these records. The term ‘thousand-year flood’ means that the chance for a flood of a certain magnitude to occur at a particular place, in any given year, is one in a thousand or 0.1%. For Tennessee, it means that the chance for a 2010-sized flood to re-occure this year (2019) or in any following year, is one in a thousand.

But wait a minute. If the meteorologists are doing their job, they are constantly updating the available records with the most recent data. And if (as news reports from around the world suggest) the existing records are being broken with increasing frequency, statistical predictions will eventually reflect that trend. Floods that were once labeled 1,000-year floods, may now more properly by labeled 500-year or 100-year floods. For Tennessee, it means that the chance for a 2010-sized flood to re-occure this year, could be one in a hundred rather than one in a thousand.

How should politicians, concerned about the safety of the people they represent, respond to an increasingly dangerous climate? Since the problem is global, the response must be global. Hence The Paris Agreement. The Paris Agreement aims to strengthen the international effort to halt the rise in temperature of the world’s atmosphere and thereby limit its destabilizing effect on climate.

The U.S. Climate Alliance is a coalition of 16 (and counting) U.S. States committed to upholding the objectives of the Paris Agreement. What are the chances that the State of Tennessee will join the Climate Alliance? Considering Tennessee’s current political leadership, about one in a million. The following YouTube video, published December 2009, records the position of GOP House Rep. Marsha Blackburn, on the question of Climate Change — she says: it’s cyclical; the science is not settled; humans are not responsible. Blackburn is now a U.S. Senator representing Tennessee.