Orographic effects during a severe wintertime rainstorm in the Appalachian Mountains

The evolution of precipitation features during a severe wintertime rainfall and flooding event associated with a cold front that crossed the central Appalachians on 19 January 1996 is illustrated through the analysis of radiosonde, rainfall, and streamflow gauge data, and WSR-88D images. Striking evidence of the linkage between heavy precipitation cells and orography was obtained by tracking the movement of the center of mass of storm precipitation, which closely followed the contours of regional orographic features. Higher intensity precipitation cells were consistently located windward of the orographic crest, and the trajectory described by the center of mass of precipitation was also consistent with the spatial arrangement of the river basins where hazardous flooding occurred. Persistent, low-intensity ( less than or equal to 5 mm h super(-1)) rainfall was registered in these basins during the 12-h period that preceded the arrival of frontal storm activity. It is argued that this prefrontal precipitation had a critical impact on watershed rainfall-runoff response and snowpack conditioning during and after the passage of the front. The intent here is to investigate the links between the observed space-time variability of rainfall and the influence of terrain features on mesoscale circulations in the lee side of the Appalachians. In particular, the viability of orographic mechanisms such as forced ascent, lee-wave interference, and precipitation scavenging of shallow orographic clouds was assessed using simple models and the available meteorological and hydrological data.