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Saturday, May 29, 2010

Stunning NOAA map of Tennessee’s 1000-year deluge: 15 sites had rainfall exceeding maximum associated with Hurricane Katrina landfall

Stunning NOAA map of Tennessee’s 1000-year deluge

15 sites had rainfall exceeding maximum associated with Hurricane Katrina landfall

by Joseph Romm, Climate Progress, May 26, 2010
What is a 100 year flood? A 100 year flood is an event that statistically has a 1% chance of occurring in any given year. A 500 year flood has a 0.2% chance of occurring and a 1000 year flood has a 0.1% chance of occurring. The map below relates [the] amount of rainfall that fell to the chances of that amount of rain actually occurring.
Nashville1 5-10

Climate Progress has been documenting the woefully underreported Tennessee deluge of 2010 aka Nashville’s ‘Katrina’. It was an off-the-charts extreme weather event that human-caused global warming set the table for and almost certainly made more intense, as a leading climate scientist explained to me (interview to be posted next week).

But I didn’t understand just how unprecedented this superstorm was until I saw the above map from the Office of Hydrological Development at NOAA/NWS.  I have never seen a map like this before, but then that may be because there simply aren’t many events to rival this one.  Look at the red streak, which is the area hit by a greater than 1000-year deluge.  And look at how much of western Tennessee was slammed with a greater than 500 year downpour.  This is the “high water” of Hell and High Water.

The NWS has more maps that put the deluge in perspective, including how it compared to Hurricane Katrina’s rainfall:

 Here are some amazing factoids:
  • Fifteen (15) observation sites had rainfall measurements exceeding the maximum observed rainfall associated with Hurricane Katrina landfall.
  • The two day rainfall of 13.57 inches at Nashville International Airport shattered the monthly rainfall record for May which was 11.04 inches.
  • The rainiest month in Nashville is 13.92 inches in January 1950.
  • Nashville International Airport experienced its 1st and 3rd rainiest days on back to back days.
  • The heaviest rainfall occurred in a swath across Davidson, Williamson, Dickson, Hickman, Benton, Perry, and Humphreys Counties.  An average of 14 to 15 inches of rain fell equivalent to 420 billion gallons of water in just two days.
And here is what Katrina did:
Hurricane Katrina Rainfall Totals

So yes, this superstorm deserve to be called Nashville’s Katrina.  It is all the more stunning for having generated so much rain without actually being associated with a hurricane, similar to the Georgia superstorm from September (see Weather Channel expert Stu Ostro’s discussion of Georgia’s record-smashing global-warming-type deluge).

I suppose people can stick their head in the sand water if they want, but CP readers understand that this is the shape of things to come for many of the world’s great cities if we stay anywhere near our current greenhouse gas emissions path.  More on the way.

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Peter said...

Can you give some more context for the "1000 years" statistic? The core of the storm covered less than 1/1000 of the surface area of the US: doesn't that mean you'd expect a similarly extreme event to be happening somewhere in the US every day?

Clearly that's not the case, but without a lot more context it's not at all clear how to interpret that statistic.

Tenney Naumer said...

Dear Peter,

Thanks for your comment.

These statistics are meant for regions, not the entire country.

For example, back in the 1990s, the Mississippi River flooded with like a 1 in 500 years chance of it happening.

That Tennessee flood was so extreme (dropping more rain in that area than was dropped during Katrina), that it would not be expected to occur in that area of Tennessee except for one year out of every 1000 years.

Climate change science tells us that with the atmosphere warming, there will be more water vapor up there over our heads. And the science tells us that we are going to get more extreme precipitation events.

And, indeed, all over the world, storms are occurring that are dumping phenomenal amounts of rain or snow.

Peter said...

... it would not be expected to occur in that area of Tennessee except for one year out of every 1000 years.

Right. So, given that "that area of Tennessee" is a tiny fraction of the overall area of the continental U.S., shouldn't we be expecting a similar event to occur somewhere in the U.S. every couple of years?

Tenney Naumer said...

Just because it is expected to happen only once every 1,000 years does not mean that it cannot happen again. It is just a statistical calculation.

As for the rest of the U.S., there is always a chance that a record will be broken somewhere on any given day.

The story is in how the records are broken.

For example, we know that high temperature records are being broken during the day, more and more often.

We also know that the temperatures during the night are increasing in most places around the U.S., but not all.

It is the trends that tell us the story.

Single events can't tell us a lot.