Atmospheric circulation and inland flooding in twentieth century North Carolina, USA: Implications for climate change impacts?

Widespread inland floods for 20th century North Carolina, USA were defined from stream flow records as events where flow was more than one standard deviation above the mean annual peak for at least two contiguous drainage basins simultaneously. Thirty-one events were identified. One snowmelt flood was detected. For the others, synoptic causes were identified from precipitation and circulation data. Eight events were directly related to hurricanes. Each required a precursor storm, often another hurricane, to provide sufficient precipitation to overcome the dry soils and low stream flows of the autumnal hurricane season. The decadal frequencies of these floods were poorly correlated with the total number of hurricanes, with no hurricane floods between 1955 and 1999 despite frequent hurricanes. Further, most events involved slow-moving decaying systems, not intense ones. An increase in hurricane intensity, often suggested as a consequence of climate change, may lead to fewer floods. The other floods were produced by either extra-tropical storms or squall lines, and precursor systems were also needed. These floods were common in the first and last three decades of the century, virtually absent in the middle four. This corresponded to a small dip in the total number of cyclones, and to periods of rising temperature statewide. This suggests a future increase in North Carolina floods as global temperatures increase. However, the synoptic causes of the relationship are not clear, and detailed quantitative analyses of recent events are required.