Coupling atmospheric and hydrological modeling toward flash flood forecast improvement: An HMT-Southeast case study from the 2013 Flash Flood and Intense Rainfall Forecast Experiment

Successful flash flood prediction requires forecast skill in both the atmospheric driving fields (i.e., precipitation) as well as the hydrologic processes responsible for transporting surface water. Historically, these two parts of the flash flood forecast process have been largely treated as separate entities. In July 2013, the NOAA Hydrometeorology Testbed (HMT) and the NWS Weather Prediction Center (WPC) hosted a Flash Flood and Intense Rainfall (FFaIR) forecast experiment to test and evaluate experimental forecast products designed to better interface atmospheric and hydrologic forecast guidance.

This work examines one flash flood event that occurred in western North Carolina on 27 July 2013. Nearly a foot of rainfall falling in less than 24 hours drove flash flooding that was responsible for over 50 road closures, at least 4 destroyed homes, and over 700 damaged properties in western North Carolina. The flash flooding was also blamed for two deaths when swimmers were swept away in a fast-moving rural creek. The event occurred at the end of an already-record-setting month for rainfall in the region: most of western North Carolina received monthly precipitation amounts exceeding 500% of normal in many locations. Western North Carolina is the focus region of HMT-Southeast, a project focused on conducting research to better understand extreme rainfall and its impacts in the Southeast US. Thus, an examination of the high-impact event, its meteorological causes, and the associated forecast successes and challenges is warranted.

The 27 July 2013 flash flood event is examined to compare (i) traditional forecast guidance (i.e., based only on precipitation) using both operational and experimental forecast models, (ii) complementary atmospheric-hydrologic approaches in which a human forecaster or forecast algorithm compares standard flash flood guidance information with atmospheric model output, and (iii) directly-coupled atmosphere-hydrology model output.

Operational models (e.g., NCEP's NAM, SREF) and human-issued operational forecasts from both WPC and National Weather Service forecast offices will be used to assess forecast skill for this event. NSSL's Flooded Locations and Simulated Hydrographs Project (FLASH) will be used in combination with experimental forecast guidance [e.g., NOAA's High-Resolution Rapid Refresh (HRRR) and NOAA's Experimental Regional Ensemble Forecast System (ExREF)] in order to examine a pseudo-combined forecast approach. The novel aspect of this work will be the use of the new Weather Research and Forecasting (WRF)-Hydro model to drive directly-coupled atmosphere-hydrology simulations. WRF-Hydro will be driven by several different input forecasts, including HRRR and ExREF. Results will demonstrate the strengths and weaknesses of the various approaches to the flash flood forecast problem, and illuminate future research directions offering potential future benefit to operational forecasters.