Climatology of Floods and their Associated Weather Systems in North Carolina

In the United States, 90% of natural disasters involve flooding. Therefore, it is important to understand flood frequency on a global, national, and regional scale to be better prepared for flood events. Since state level studies are not as common, this analysis focused on creating a climatology of flood events along with their associated weather systems in North Carolina from 2008 – 2022.

Six North Carolina river basins were selected for analysis. A United States Geological Survey (USGS) stream gauge station within each watershed of the river basins were used in the climatology. A flood event was defined as when the stream gauge height was above its 99^th percentile gauge height for at least an hour. Archived radar and satellite imagery was used to determine if the meteorological source of the flood events was a tropical cyclone, extratropical cyclone, warm season convection, or cold season convection.

The most common source of flooding in each river basin was extratropical cyclones or cold season convection. In most cases, these two regimes had a similar number of total cases. Across all six river basins, flood event frequency decreased from average values in the spring to a minimum during the late summer. This may be explained by a soil moisture minimum and evapotranspiration maximum typically observed in the summer. There was no correlation in the interannual variability in flood frequency between the six river basins except for the Neuse and Catawba, which are very similar in land composition.

There were 18 flood events that impacted the three representative stations of the Catawba River basin that were analyzed more in depth to understand the timing of issued flood watches and warnings for the events. Of the 38 flash flood and flood watches issued, there was a 50/50 split of if they were issued before or after the 99^th percentile gauge height was reached. Most of the 76 warnings were issued after the 99^th percentile gauge height was reached. These results were not surprisingly due to the definition of a watch versus a warning.