An extraordinary meteorological event; was one of its results a 1000-year flood?

By: Stu Ostro , 6:01PM,GMT on October 5,2015

Image credit: NASA Earth Science Office

A remarkable combination of a deep cutoff low aloft, an old front and a new one developing with a non-tropical surface low, a potent low-level jet, very strong ridging aloft and surface high pressure to the north, and extra moisture/fetch contributed by Hurricane Joaquin (whose steering was affected by a system that was associated with the remnant of Ida), resulted in an extraordinary convective band, extreme rainfall and disastrous flooding in the southeastern United States; wind damage from gusts and wet soil with loosened tree roots; and, from parts of the Northeast down across the Mid-Atlantic to the Southeast, coastal flooding and severe beach erosion from a persistent onshore wind gradient through many astronomical tide cycles.


Image credit: NOAA/ESRL Physical Sciences Division



Image credit: Levi Cowan, Tropical Tidbits


There have been many other cases of repetitive convection in various places, Pineapple Express type atmospheric rivers into the West Coast, and persistent lake-effect snow squall bands, but I've been forecasting the weather since the 1970s and I can't remember having seen anything in the southeast U.S. or anywhere else quite like the nature of this long solid discrete relentless band of torrential thunderstorms training (from ESE to WNW no less) hour after hour after hour after hour after hour after hour after hour (the loop is 7 hours, and the band was going long before and long after that), along with the combination of synoptic ingredients that existed. 


Image credit: College of DuPage NEXLAB


Certainly the event was exceedingly rare.  But exactly how unusual?  In particular, was this a "1000-year flood"? 

Such a specific description can be problematic in a number ways.

● An X-year rainfall is not necessarily an X-year flood, as a number of factors affect how a given rainfall total translates into impacts. 

● That type of term itself is misunderstood, as it's not meant to imply that such events occur every X years, it's that they occur with that frequency on average, and thus there's a particular probability in any given year of such an occurrence.  But the way it appears is usually not a headline or soundbite of "A 0.1% Probability of A Flood Like This in Any Given Year," it's "1000-year Flood!"  (And that's reported as if it's an absolute fact.) And in regard to precision and rounding off, do the statistical calculations equal precisely 1000 years? Or 950? 1098?

● What are the spatial characteristics of a particular "rainfall event"?  An extreme amount that happens to fall at a particular location for which recurrence intervals are calculated?  Or extreme amounts collectively, covering an area like this in the graphic below?  And given that X-year floods/storms can occur in multiple places in a given year, that further complicates the issue.  Furthermore, now that there is an easily-accessible calculator, to what extent is that affecting communication and perception?


Image credit: NOAA


● The frequency can be established for events that occur within a known period of record measurement.  For example, let's say that 10" of rain has fallen in 24 hours, at a given location where such observations have been recorded since 75 years ago, six times during that period of record.  We can say that since 1940, that amount of precipitation has occurred at an average frequency of eight times per century.  For longer return intervals, there are statistical methods that are used to extrapolate.  But what if during the period of record, no such event has occurred.  How accurate is an extrapolation?  And do we really know what the frequency has been over the course of centuries and millennia, without the data?  No.  How many times have 10-20+" across that size of an area fallen in South Carolina during the past 10,000 years?   Were there storms in October 1011 or August 827 B.C. that brought such a deluge?  Or could this be a 2000-year flood?  And I'm not being flippant, I'm asking serious questions.

[Addendum 10/6: The order of magnitude is important, given the 100-year flood regulatory standard for flood insurance.]

● And what if atmospheric water vapor and circulation patterns are changing, resulting in a recent trend in the frequency of extreme precipitation events such as that below?  Is an X-year rainfall still an X-year rainfall?   For that matter, how will this weekend's event change the average recurrence interval calculations in South Carolina?


Image credit: NOAA