Session 10B.5 A remarkable mesoscale snowband event over southern PA: A snow lover's dream and a forecaster's nightmare

Wednesday, 3 June 2009: 2:30 PM
Grand Ballroom West (DoubleTree Hotel & EMC - Downtown, Omaha)
John R. Scala, WGAL-TV, Lancaster, PA; and D. Beachler, M. Dangelo, and R. H. Grumm

Presentation PDF (1.0 MB)

A remarkable, meso&beta scale snowband delivered up to 12 inches of snow in five hours over central Lancaster County, Pennsylvania during the late evening of February 3, 2009 and early morning hours of February 4, 2009. Observed snow rates exceeded 3 inches per hour during the height of the event which was characterized by a quasi-linear arrangement of convective cells exhibiting a slow, south-southeast drift. A second, less defined and noticeably weaker band was present to the southwest over Cumberland, and Adams Counties.

The Lancaster County snowband developed in a region of weak surface convergence between a deepening Atlantic cyclone over the Canadian province of Nova Scotia and a surface anticyclone building south into the mid west. Upper-air analyses suggest the presence of a poorly-developed lower tropospheric low rotating cyclonically across the northeastern U.S., and around the departing cyclone. Further west, an anomalously cold upper low acted to not only amplify the eastern U.S. trough but greatly enhance the convective instability over southern Pennsylvania.

Operational guidance from the Nation Centers of Environmental Prediction (NCEP) NAM, GFS, and SREF fields indicated the potential for snow with little consensus on position or amount. Computed frontogenesis appeared to provide only a minor supporting role in the development of the snowband. The 12 km WRF-ARW forecasts of precipitation produced a band in eastern PA although erroneous in location and amount.

The extraordinary nature of the event is revealed when one considers the maximum observed five-hour amounts approached approximately 45% of the 30-year climatology for seasonal snowfall across central Lancaster County. The isolated and intense character of the snowband developed in the wake of a coastal cyclone that some prognosticators equated to the Superstorm of 1993 very early in the forecast cycle. The limited scale and duration of the event emphasized the limits and uncertainties associated with higher resolution forecasts. The event also proved to be a success for the recently implemented NWS chat at the State College forecast office.

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