The heavy, persistent rainfall that caused historic flooding across South Carolina in October 2015 was the result of a perfect confluence of a few atmospheric ingredients. There is indication that the event exhibited some of the classic synoptic features of a predecessor rainfall event (PRE). PREs are concentrated, clearly distinguishable areas of heavy rainfall that usually form to the north and west of tropical cyclones and the right rear quadrant of an upper-level jet streak (a localized area of very enhanced wind speeds usually located within the jet stream). They are distinct from the main precipitation shield of a tropical cyclone and are located nearly 600 miles away on average. Recent research from the National Weather Service has identified 47 PREs from 1998 to 2006 resulting from 21 tropical cyclones.

In the South Carolina flood event, tropical moisture transported from Hurricane Joaquin, located east of the event, enhanced its rainfall. However, other features of the event, such as its location relative to the upper-level jet streak, were not as indicative of a PRE. The final verdict will have to wait until more detailed data analysis is available.