Impact of the Southern Appalachian Mountains on the SEMPE IOP-4 event

On 1 February 2007 a winter storm was forecasted to impact the Asheville, NC region with accumulations of 2 to 5 inches of snow, sleet, and/or freezing rain. What followed were large numbers of school and university closings, along with the cancellation of local events before any significant accumulation of precipitation had occurred. The actual weather produced by the passing storm system was a mixture of snow, sleet, and freezing rain in Northeast Georgia, Upstate South Carolina, and the Piedmont of North Carolina. In Asheville and the surrounding communities just east of the Blue Ridge, there was very little accumulation of precipitation.

Participants in the Sounding-based Experiment on Mixed Precipitation Events (SEMPE) launched five rawinsondes during the 1 February winter storm as it passed over the Asheville region. The SEMPE experiment, sponsored by the Cooperative Program for Operational Meteorology, Education and Training (COMET) and the National Environmental Modeling and Analysis Center (NEMAC), was funded to observe potential mixed-precipitation weather events in an attempt to better understand cold air intrusions into the mountains of western North Carolina during the 2006, 2007 winter season.

The emphasis of this study is to compare an analysis of the 1 February 2007 storm using operational and SEMPE observations to RUC, NAM, and GFS forecasts produced within 24 hours of the storm passing over Asheville. In particular, the comparison focuses on the role of the nearby mountains in modifying the storm such that the actual total accumulations were much less than anticipated. Radar imagery from this event indicated a distinct dry "wedge" downstream of the Appalachian Mountains that failed to erode completely as the storm moved through the region. This study will analyze the source of the drying and seek to explain why the operational model forecasts were unable to predict the observed dry wedge.