Hurricane Joaquin and the South Carolina Flood of October 2015

In early October, 2015, severe flooding took place across the state of South Carolina, with damages exceeding $10B.  During this event, Hurricane Joaquin was located offshore to the south and east of the heavy precipitation.  Prior research, storm summary reports, satellite imagery, and media accounts all suggest that Joaquin contributed to the flooding event.  Here, we utilize numerical simulations to analyze moisture transport mechanisms and to elucidate Joaquin’s role in the flooding.  Specifically, we investigate: (i) whether the presence of Joaquin enhanced water vapor content or transport over the southeastern US, and (ii) if Joaquin altered the synoptic-scale and mesoscale forcing for ascent through changes in the upper-level flow.

Given that precipitation generally exceeds evaporation in the vicinity of tropical cyclones, and that the lower-tropospheric flow is strongly convergent in such systems, our working hypothesis was that Joaquin’s role, if any, was due to upper-level dynamical alterations. Given that a robust and slow-moving upper trough was located to the west of the flooding region, with moist tropical air in place, we further hypothesized that heavy precipitation would have taken place even in the absence of Joaquin.  In order to test these hypotheses, we analyze precipitable water (PWAT) anomalies and Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model back trajectories over South Carolina in the days preceding the event. Additionally, we compare control simulations using the Weather Research and Forecasting (WRF) model with experimental simulations in which Joaquin is removed from the model initial conditions. 

Anomalous moisture was transported into South Carolina in the week preceding the event from both the Gulf of Mexico and Atlantic Ocean. During the event, moisture transport into South Carolina was maximized in an intense, narrow, and persistent band (aka an “atmospheric river”).  This feature was characterized by a southeasterly low-level jet that was located immediately to the northeast of a diabatically generated lower-tropospheric potential vorticity anomaly, consistent with a previously identified positive feedback mechanism.  What happened when Joaquin was removed?  The spatial location of heavy rain shifted northward, with only slightly lower total accumulation.  Regional water vapor content was largely unaffected.  The most pronounced influence of Joaquin was in an altered upper-level outflow jet, which shifted the location of strongest forcing for ascent.