In lieu of direct impact from Hurricane Joaquin, what led to historic rainfall in the Carolinas this past weekend?
According to AccuWeather Chief Meteorologist Elliot Abrams, “I have never seen rainfall this intense, in this large of an area and during this short of a period in absence of direct impact from a tropical storm or hurricane.”
From 1 to 2 feet of rain has fallen over a broad area in the Carolinas, over a span of three to four days.
A very unusual combination of weather factors came together in just the right way to produce rainfall rates rarely experienced and total rainfall never precisely recorded before in part of the Southern United States.
Weather Setup for Historic Carolina Flooding
The setup not only disrupted travel and drenched ballgames in the East but also resulted in major flooding in some locations.
The key players included a strengthening non-tropical storm in the South, a strong area of high pressure in Canada and converging tropical moisture from Hurricane Joaquin and near the equator.
Another player during the early stage of the event was a front that reversed direction along the Atlantic Seaboard.
Impact from Joaquin was indirect as the system remained hundreds of miles away to the southeast.
While the setup may seem relatively simple, it is the manner in which all of the systems interacted to produce the exceptional rain and major flooding event.
Both the non-tropical storm and Hurricane Joaquin stalled, while the high from Canada drifted eastward during the weekend.
According to AccuWeather Meteorologist Anthony Sagliani, the flow of air around the systems helped to funnel a narrow zone of intense rainfall.
“You had cool air to the north and dry air to the south that began to squeeze moisture into a narrow band of intense rainfall, which extended inland from the Atlantic Ocean,” Sagliani said.
The strong flow of air around the three weather systems worked together to produce the gusty winds, rough seas, beach erosion and significant coastal flooding from the Carolinas to New Jersey.
Every similar and large-scale torrential rainfall event in the Eastern United States most likely involved a tropical storm or hurricane.
Hurricane Camille (August 1969), Hurricane Irene (August 2011), Hurricane Isabel (September 2003) and Hurricane Hugo (September 1989), to name a few, all made landfall in the U.S.
The interaction between Tropical Storm Lee, which made landfall, and offshore Hurricane Katia in 2011 was perhaps the most similar event to the October 2015 flooding. Rainfall during the Lee event reached 21 inches in Virginia.
However, the axis of torrential rain was much farther north, and Lee was originally a tropical storm that transitioned to a non-tropical system.
Another similar event was Agnes (June 1972). In this case, a non-tropical storm captured the tropical system and caused it to make landfall. However, total rainfall paled in comparison to the early October 2015 deluge in the South. The maximum rainfall during Agnes was 19 inches in Pennsylvania.
Other events, such as the Johnstown Flood(s) and the January 1996 Flood, were smaller in scale but were the result of melting snow combined with rain or repeating thunderstorms.
No two storms and atmospheric setups are exactly alike.
The output of the weather pattern produced historic rainfall this weekend.
How Rare Was This Rainfall Event in the Carolinas?
The vast majority of locations in South Carolina experienced a once-in-50-years to once-in-200-years event over a three-day period.
Locations near the coast in the Carolinas may have had rainfall approaching a once-in-1,000-years event, depending upon the accuracy of gauge and recording equipment.
This is known as “rainfall return period” or “rainfall recurrence period” to describe the rarity of a particular event.
The rainfall return period is based on historical records and is an average of how often an event will occur at a particular location. The vast majority of rainfall records date back to the early 1900s and late 1800s.
According to AccuWeather Certified Consulting Meteorologist Stephen Wistar, “Due to the variability of the atmosphere, the actual time between such rainfall events can vary widely.”
It is possible to have a once-in-100-years or once-in-1,000-years rainfall event more often than every 100 or 1,000 years respectively.
A once-in-200-years rainfall may not result in a once-in-200-years flood, due to changes and/or improvements of a stream’s watershed.
While the bulk of the flash and small stream flooding have already been realized, river flooding will continue for days in some areas.