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South Carolina's Catastrophic Floods

South Carolina’s Catastrophic Floods Caused By One of the Most Prolific Rainfall Events in Modern U.S. History

By Nick Wiltgen
Published Oct 6 2015 12:57 PM EDT
weather.com

The catastrophic floods striking South Carolina will go down in the history books, not only because of the lives they’ve taken or the destruction they’ve wrought, but also because of the sheer amount of rainfall. By the time the last raindrop is counted, the October 2015 storm will go down as one of the most prolific rainfall events in the modern history of the United States.
Rainfall totals from the early October storm have shattered or jeopardized countless records in South Carolina.

  • The official statewide 24-hour rainfall record was exceeded in several locations; an official determination may take months to complete.
  • The unofficial state record for 5-day total rainfall, which had stood for 107 years, has been surpassed at more than a dozen reporting sites.
  • The rainfall exceeded that of any tropical cyclone in South Carolina history
  • Six sites with more than 50 years of data have had their wettest Octobers on record.
  • The October monthly precipitation record for any location in the state has likely been broken.

Data are still being assimilated into the National Oceanic and Atmospheric Administration’s database, but here is how the Carolinas flood disaster stacks up in various categories.

Greatest 24-Hour Rainfall Total

The National Oceanic and Atmospheric Administration (NOAA) has a process in place for vetting measurements of “potentially record-setting extreme meteorological elements” for each state. The process focuses on a small number of high-profile weather variables – all-time high and low temperatures, all-time greatest snow depth and 24-hour snowfall, and all-time heaviest 24-hour precipitation total.
Any time a standing record is called into question – either because of a new record-breaking event or new doubts about the scientific validity of the official record value – a State Climate Extremes Committee (SCEC) can be convened to investigate the matter.
For South Carolina, the standing 24-hour precipitation record was set during Hurricane Floyd when 14.80 inches of rain fell in Myrtle Beach on Sept. 16, 1999.

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Storm Total Rainfall

Storm total rainfall in South Carolina from Oct. 1 through early evening Oct. 5, 2015.

South Carolina’s committee will surely be called upon to investigate the astronomical rainfall totals recorded this week, as some appear to surpass the standing record.
One possibility is the Weather Underground personal weather station on White Birch Circle in Columbia, which reported 18.71 inches of rain between 3:55 p.m. EDT on Oct. 3 and the same time Oct. 4.
The site piled up 17.72 inches on Oct. 4 itself, of which 15.12 inches – enough to break the 24-hour record – fell in less than 10 hours during the morning. However, a neighboring site about a mile away received only 7 inches of rain in 48 hours Oct. 3-4. Such a sharp difference may be evidence that one or both gauges is not properly calibrated.
In any case, several other sites may also have a case for claiming the new state record:

Regardless of the state record, several individual cities and towns saw unprecedented rainfall.
In fact, of the 59 South Carolina sites in NOAA’s climate database with at least 50 years of weather observations, nine set all-time one-day records for the month of October. More impressively, six of those nine sites also broke their one-day rainfall records for any month.

Historic One-Day Rainfall Totals

Columns mark cities that broke October one-day rainfall records; green columns are all-time records for any time of year. From left to right: Antreville, Little Mountain, Batesburg, Columbia, Darlington, Manning, Andrews, Summerville and North Charleston.

Charleston International Airport, which is actually in North Charleston, led that group of long-time stations. Its Oct. 3 total of 11.50 inches bested all other days in the site’s 77-year period of weather data.
The U.S. 24-hour rainfall record of 42 inches set in Alvin, Texas, will stand untouched. It was set during Tropical Storm Claudette in July 1979.

Greatest Five-Day Rainfall Total

Part of the reason this week’s floods have become so catastrophic is that the tropical downpours have been lashing the same areas for several days.

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This graphic represents rainfall totals for one city in each county of South Carolina from Oct. 1-5, 2015. The height and color of the columns correspond to the magnitude of those rainfall totals; the tallest columns, in light magenta, received more than 18 inches of rain.

While NOAA does not maintain official state rainfall records for intervals other than 24 hours, its climate database allows us to make those comparisons on other time scales.
This week’s storm deluged the Carolinas for the better part of five days, so we took a look at the wettest five-day intervals on record in South Carolina history. At the start of this month, the top five were:

  • 17.44 inches in downtown Greenville (Aug. 22-26, 1908)
  • 17.37 inches in Kingstree (July 12-16, 1916)
  • 17.08 inches at Charleston International Airport (July 8-12, 1973)
  • 16.93 inches in Gillisonville (Aug. 27-31, 1898)
  • 16.80 inches in Myrtle Beach (Sept. 15-19, 1999)

The 1916 event in Kingstree is the highest valid rainfall total from a tropical cyclone in South Carolina history. It remains to be seen whether NOAA will consider rainfall from the 2015 flood to be associated with Hurricane Joaquin, since its role in the deluge was indirect.
Even though quite a bit of information hasn’t yet made it into NOAA’s database, this storm has already re-written the record books for South Carolina. As of early Monday evening, these were the wettest five-day intervals on record in South Carolina history – all of them from the past week:

Wettest Five-Day Periods in South Carolina

These locations all exceeded the previous all-time record five-day rainfall total for the entire state of South Carolina. These figures will change as more data are validated by NOAA.

As more reports are checked for validity and added to the NOAA database, the list continues to evolve. As of late Monday evening, the five highest rainfall totals on record in South Carolina were these:

  • 24.50 inches in the Park West neighborhood of Mount Pleasant (Oct. 1-5, 2015)
  • 23.50 inches at Georgetown County Airport in Georgetown (Oct. 1-5, 2015)
  • 20.52 inches on James Island in Charleston (Oct. 1-5, 2015)
  • 19.85 inches in Andrews (Oct. 1-5, 2015)
  • 19.61 inches near Wappoo Creek in Charleston (Sept. 30-Oct. 4, 2015)

At least eight other sites have also broken the 1908 record from Greenville, including Charleston International Airport with 17.70 inches of rain from Sept. 30 through Oct. 4.
These five-day totals stack up impressively against the all-time five-day rainfall records for other states in the region:

  • Virginia: 20.96 inches in Colonial Beach set Sept. 6-10, 2011; NOAA officially recognizes this as the top rainfall total from Tropical Storm Lee that year.
  • North Carolina: 24.06 inches near Southport set Sept. 15-19, 1999; this is the official top rainfall total in any state from Hurricane Floyd.
  • Georgia: 27.85 inches in Americus set July 4-8, 1994; this is the official top rainfall total from slow-moving Tropical Storm Alberto that year.
  • Florida: 27.84 inches in New Smyrna Beach set May 19-23, 2009. The setup was similar to this week’s disaster: strong high pressure over the Northeast, low pressure in the upper atmosphere over Florida, and a long fetch of onshore winds from the western Atlantic.

Wettest October on Record in Six Cities

As if all those short-term rainfall records weren’t enough, six longer-term reporting stations in South Carolina have already logged their wettest October on record – with 26 or more days to spare!
Perhaps the most devastating example is the co-operative observation site 4 miles west of Summerville, just north of Charleston. After 117 years of recordkeeping, that site has broken its previous record for the entire month of October (10.76 inches in 1968) less than a week into the new month. The month-to-date total (17.95 inches as of Oct. 6) surpasses the old record by a margin of more than 7 inches.
This month is already the second-wettest of any on record at the Summerville co-op site, behind only the 19.42 inches in June 1973. That said, several historical months notorious for record rainfall elsewhere have missing data in Summerville; it’s possible the area was too dangerous to stay in during floods in the early 20th century.
It’s no coincidence that one of the first areas to deal with swift-water rescues in South Carolina was that area west of Summerville in Dorchester County on Saturday night, Oct. 3.
Other places that have sealed the deal for October include the nearby Charleston International Airport in North Charleston (17.32 inches) and Andrews, in northeast South Carolina (19.85 inches).

New State October Rainfall Record?

There are two ways to look at the question of the “wettest October” in South Carolina history. One is to look at the statewide average, something NOAA will do in early November in its monthly State of the Climate report.
The other is to look at individual cities and observation sites. As of this writing, none has surpassed the standing October rainfall record for the state, which is 25.81 inches measured in Hilton Head in October 1994.
Several places are closing in, however. The top three rainfall totals noted in the five-day section above now rank second, third and fourth, respectively, on the statewide list of wettest Octobers. Closing out the top five – for now – is the 20.49-inch total from Marine Corps Air Station Beaufort in 1994.
Hilton Head is likely to lose its record status. The Mount Pleasant site reported another 2.65 inches of rain in its Monday morning 24-hour rainfall report, according to the Community Collaborative Rain, Hail & Snow Network (CoCoRaHS) website. If NOAA accepts that report into its database, that site’s October total will reach a new state record of 27.15 inches.

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