The rainfall amounts are staggering and shattering records, with an extreme weather pattern involving a ribbon of tropical moisture from Hurricane Joaquin interacting with an intense, slow-moving weather system over the Southeast.
One to two feet of rain have fallen in central and eastern South Carolina, amounts that should occur — on average — once every 1,000 years. At the coast, flooding is occurring not only because of the deluge from the sky but also exceptionally high tides pushing water ashore.
Late Sunday afternoon, the National Weather Service warned that “flash flooding remains likely, which could be locally catastrophic” through the evening. An additional three to six inches of rain was forecast with rates up to three inches in one hour possible in a few areas.
But rainfall totals are already beyond incredible in the Charleston area. More than two feet of rain has fallen in central Charleston County.
On Saturday, Charleston International Airport received 11.5 inches of rain, which broke the all-time record for wettest day — 10.52 inches set on Sept. 21, 1998, during Tropical Storm Hermine.
With a total of 14.78 inches between Thursday and Saturday, Charleston International also broke the all-time record for the month of October, as well as the two-day and three-day rainfall record for any time of year according to the National Weather Service.
To make matters worse, high tide in Charleston surpassed “major flooding” on Saturday afternoon, peaking at 8.29 feet — the highest water level in Charleston Harbor since Hurricane Hugo in 1989, according to the National Weather Service.
The Columbia, S.C., fire department is reporting multiple dam breaches and warning people to stay in safe and dry places. Images from social media show a dangerous river of water flowing through parts of the state capital.
(above image from Columbia, SC)
The National Guard was called in to assist with rescue efforts.
With heavy and persistent rain continuing to fall, a large portion of central as well as coastal South Carolina was covered in many feet of water, and there were flash flood warnings extended into the evening.
Even after the rain ends some time Monday, flooding will remain possible for days in South Carolina as rivers and streams take some time to crest.
According to statistics compiled by the National Oceanic and Atmospheric Administration, South Carolina’s torrential weekend rain has well surpassed a 1,000-year rainfall event. “Hydrologists would refer to a 1-in-1000-year rain as having a typical ‘recurrence interval’ of 1000 years,” says Weather Underground’s Bob Henson. “The idea is that such events are not always separated by 1,000 years; the same amount of rain could conceivably occur the very next year, or might not occur until thousands of years later.”
A three-day, 1,000-year rainfall event for Charleston County would have been 17.1 inches. A four-day, 1,000 year event would have been 17.5 inches. Boones Farm Plantation, just north of Mt. Pleasant, S.C., in Charleston County, reported more than 24 inches of rain through Sunday morning, which essentially blows NOAA’s 1,000-year events scale out of the water.
Even though Joaquin tracked well off-shore over the weekend, a moisture-laden tentacle swept over the Carolinas. Some meteorologists have been calling this peculiar, spindly rainfall feature a predecessor rain event, or “PRE,” which sometimes occurs ahead of tropical storms that interact with separate areas of low pressure and lingering surface fronts — exactly what this hurricane is doing.
The atmospheric river pumped extremely high moisture toward the Southeast like a fire hose, with rainfall rates that exceeded three inches per hour. Downpours were exacerbated in the mountainous regions, where winds from the west will push very moist air uphill, wringing it out like a wet rag.
The previous week of rainfall compounded the flooding situation for the Southeast. A deep trough of low pressure pushed through the United States last week, funneling tropical moisture north across the Eastern Seaboard. Rainfall from this lingering system was been widespread up and down the East Coast. Many rivers and streams across South Carolina already peaked well beyond flood stage earlier last week.
Rain will begin to taper off in the Carolinas on Monday, though flooded roads and power outages will linger well beyond the duration of this historic rainfall event.