Investigating the Influence of Orography on Extreme Precipitation during the 2015 South Carolina Flood

Observed precipitation accumulations and reanalysis datasets will be examined to characterize the role of terrain in the intensity, placement, and evolution of rainfall during the 2015 October 1-5 South Carolina flooding event associated with non-landfalling Hurricane Joaquin. Orographic precipitation enhancement is influenced by terrain height and flow properties such as wind speed and direction, moisture, and stability. This enhancement can heighten the risk for flooding depending on the intensity, duration, and placement of that precipitation in a watershed. The presence of a nearby tropical cyclone could potentially influence some or all these properties, although questions remain as to the nature and extent of their influence. An understanding of non-landfalling hurricanes in their influence on variables conducive to orographic enhancement of precipitation can thus serve as a crucial aid in weather forecasting and the mitigation of damages caused in connection with stress on aging water management infrastructure.

The impacts are especially felt in regions such as the eastern seaboard of the United States that are predisposed to both tropical systems and orographic enhancement due to the being near the Atlantic Ocean and Appalachian Mountains. This case had rainfall totals exceeding 500 mm in some areas, making it important to examine the interactions between synoptic and mesoscale flow characteristics and how they are modified both by the Appalachian Mountains and the offshore hurricane. Preliminary results indicate an evolving Appalachian cold-air damming (CAD) event. A high pressure system located over northeastern Ontario, Canada induced north to northeasterly wind flow along the eastern seaboard, with cold air advecting southward parallel to and east of the mountain range. Examination of ASOS data determined that the CAD event, identified by the strongest temperature gradient, peaked on 3 October 2015 where the temperature difference between the coast and eastern side of the mountains was or exceeded 15ºC. The CAD coincides with a coastal front with a 5ºC temperature gradient near the coast as well as a distinct shift in wind direction as winds east of the front were easterly.

Areas of surface wind convergence align with regions of precipitation among all reanalysis and model datasets, though there are discrepancies in the spatial placement, intensity, and timing. Differences in the datasets further extend into temperature gradients, wind flow, and propagation of systems. Analysis will examine whether variations in precipitation outcomes among the different datasets may be attributed to these discrepancies. Future work will consider topographic modification experiments using WRF-ARW to further examine whether the Appalachian Mountains played a role in the location and duration of precipitation.