Extreme rainfall from orographic thunderstorms in the central Appalachians

The central Appalachian region has experienced some of the largest rainfall accumulations in the world at time intervals less than 6 h, including the 18 July 1942 Smethport, Pennsylvania, storm that produced 780 mm in less than 5 h. The envelope curve of central Appalachian flood peaks at "small" drainage areas (less than 1000 km 2) is dominated by orographic thunderstorm systems, like the Smethport storm. Orographic thunderstorm systems that have produced catastrophic flooding in the central Appalachians are examined through observational and numerical model analyses using the Weather Research and Forecasting (WRF) model. Analyses suggest that the frequency of catastrophic flooding varies over the central Appalachian region and that small-scale variation may be linked to the role of complex terrain in altering thunderstorm dynamics. Catastrophic flooding along the western margin of the central Appalachians is dominated by four storms that occurred on 18-19 July (1889, 1942, 1977, and 1996), which were associated with warm season extratropical systems and were characterized by rapidly moving storm elements in complex terrain. Along the eastern margin and interior of the central Appalachians, catastrophic flooding is linked to four "terrain-locked" orographic convective systems that occurred 17-18 June 1949, 18-19 August 1969, 27 June 1995, and 11 August 2003. WRF model simulations of the Little River storm (June 1949), Rapidan storm (June 1995), Smethport storm (18 July 1942) and Redbank storm (18-19 July 1996) are used to characterize “ingredients” of catastrophic rainstorms in the central Appalachians.