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What Caused the Historic Flooding in South Carolina?

What Caused the Historic Flooding in South Carolina?

The images on the news last weekend were shocking and heartbreaking: South Carolinians wading through waist-deep water as they escaped from submerged cars or trying to salvage belongings from their water-ravaged homes. In the span of three to four days, rainfall totals topped a foot in some areas, and more than two feet in others. Never in recorded history has South Carolina ever seen so much rain.

Forecasters expected that Hurricane Joaquin would hit the Carolinas and the rest of the southern East Coast, but fortunately, after the hurricane pummeled the Bahamas, it moved out into the Atlantic Ocean. However, even without effects of a direct hit from Joaquin, some 40,000 people throughout South Carolina lost power. Tragically, 15 people are reported to have lost their lives in the flooding – 17 total, counting two flood-related deaths in North Carolina.

By all accounts, this weather event was incredibly unusual – especially since there wasn’t a direct hit from a tropical storm or hurricane. So what caused this heavy rain? We’ll take a look at all the unique factors that combined to produce this “storm of the millennium.”

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SEAN RAYFORD / GETTY IMAGES

An Unusual Recipe for Disaster
The flooding throughout South Carolina was the result of three different weather systems that happened to line up perfectly to create untold amounts of rain. It started with moisture from a non-tropical storm from the south, combined with a strong area of high pressure coming from Canada. The third component was Hurricane Joaquin. As the hurricane, non-tropical storm and high-pressure area converged, airflow patterns essentially formed a funnel that directed loads of moisture to the Carolinas and other parts of the Eastern Seaboard.

Most storms move in and out in just a day or two, but the Carolina rainfall lasted for five days because as the Canadian high-pressure zone drifted eastward, the non-tropical storm and Hurricane Joaquin stalled. To make matters worse, a front reversed its course during the early stages of the event, causing yet more rain. Fortunately for everyone involved, South Carolina avoided a direct impact from Hurricane Joaquin. The hurricane channeled more moisture to South Carolina, but the storm itself (and its damaging winds) remained over the Atlantic Ocean, hundreds of miles to the southeast.

Five Days of Record Breaking Rains
The massive amounts of rain in South Carolina have been called a “1-in-1,000 year” event. Most weather records around the United States date back to the late 1800s or early 1900s, so for a storm to be classified as a once-per-millennium event, there must be a 0.1% or less chance that the event could happen in a particular location over the course of a year. In other words, rainfall and flooding on this scale can only reasonably be expected to happen in South Carolina one time in any 1,000-year period.

To give you an idea of the magnitude of this event, coastal cities like Georgetown receive approximately 56 inches of rain per year, while midland cities such as Columbia see an average of 46 inches of rain per year. During this event, nearly 24 inches of rain fell on Georgetown – almost half of their yearly average over the course of five days. Columbia received about a quarter of their yearly rainfall over the five-day stretch, with 12.45 inches of rain inundating the city during this historic event. Charleston broke a 24-hour rainfall record (10.52 inches on September 21, 1998) on Saturday, October 3 with 11.5 inches of rain.

According to Weather.com, prior to this event, the wettest 5-day span on record in South Carolina happened in Greenville, between August 22 and August 26, 1908. During that storm, the city received 17.44 inches of rain. However, between October 1 and October 5, 2015, at least five South Carolina locations broke that record:

• Mount Pleasant recorded 24.5 inches
• Georgetown recorded 23.5 inches
• Charleston (at James Island) recorded 20.52 inches
• Andrews recorded 19.85 inches
• Charleston (at Wappoo Creek) recorded 19.61 inches

There are likely even more areas that broke the 5-day record – Mount Pleasant, for instance, recorded 27.15 inches of rain – but as yet, forecasters are still checking gauges and recording equipment for accuracy.

Here at the Farmers’ Almanac, our thoughts are with those who continue to deal with the aftermath of this historic storm.

 

Freelance writer, Amber Kanuckel, contributed to this story.

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