Catastrophic rainfall from an upslope thunderstorm in the central Appalachians: The Rapidan Storm of June 27, 1995

Authors

  • James A. Smith,

  • Mary Lynn Baeck,

  • Matthias Steiner,

  • Andrew J. Miller


Abstract

A storm system near the Blue Ridge Mountains of Virginia produced peak rainfall accumulations exceeding 600 mm in a 6-hour period during the morning and early afternoon of June 27, 1995. The peak flood discharge of 3,000 m3 s−1 on the Rapidan River at a drainage area of 295 km2 places this event on the envelope curve of flood discharge for the United States east of the Mississippi River. Observations of radar reflectivity factor and Doppler velocity made by the Sterling, Virginia, WSR-88D (Weather Surveillance Radar–1988 Doppler) are used for analyses of the storm. The temporal and spatial variability of rainfall are examined on a 1-km grid scale and 6-min timescale. Like many heavy rainfall events, storm motion played a key role in the production of heavy rainfall for the Rapidan storm. Storm motion and storm evolution for the Rapidan storm were closely linked to topographic features at the scale of the ridges which extend southward from the Blue Ridge and delineate the Rapidan basin. Key elements of the storm environment included strong boundary layer winds directed upslope toward the Blue Ridge, weak upper level winds, high precipitable water values, and a near-saturated atmospheric column up to 6 km. An important element of storm structure was the low-reflectivity centroid of the storm. This feature of the storm was related both to the exceptional rainfall rates of the storm and to the underestimation of storm total rainfall by the operational WS-88D precipitation products. Components of the atmospheric and land surface water budgets are derived. The cumulative discharge from the Rapidan River was 0.87×108 m3 (296 mm over the 295-km2 catchment). The storm total precipitation for the Rapidan basin was 1.01×108 m3 (344 mm over the catchment). The precipitation efficiency of the storm, that is, the ratio of storm total rainfall to atmospheric water vapor inflow, was approximately 90%.

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