21st Conf. on Severe Local Storms and 19th Conf. on Weather Analysis and Forecasting/15th Conf. on Numerical Weather Prediction

Monday, 12 August 2002: 10:45 AM
Common characteristics of heavy banded snowfall events in the central United States
James T. Moore, Saint Louis University, St. Louis, MO; and S. Ng and C. E. Graves
A conceptual model for heavy banded snowfall is described from a process-oriented approach involving frontogenesis, potential symmetric instability (PSI), and the warm and dry conveyor belts. The trough of warm air aloft (trowal) is associated with that portion of the warm conveyor belt that "wraps around" the cyclone due to the system-relative flow. This warm, moist, high qe air stream is confluent with a northern dry, cold air stream, producing a zone of increased baroclinicity (i.e., a zone of frontogenesis), creating a lower tropospheric direct thermal circulation. The dry air stream, typically entering from the southwest of the cyclone, is instrumental in altering the qe topography in mid- tropospheric levels to create a convectively neutral to unstable layer. The latter region is located southeast of the area of heavy snow and is a zone of PSI. Results from several case studies reveal a consistent pattern of these parameters with mid-level frontogenesis to the northwest, potential symmetric instability to the southeast, and the trowal axis along and south of the region of heavy banded snowfall.

Several case studies will be shown to illustrate the applicability of the above conceptual model. In each case diagnosis of the trowal, frontogenesis, and the region of decreasing equivalent potential vorticity (to diagnose the region of PSI) will be shown to document how these parameters can be used to isolate the meso-beta scale region likely to experience heavy snowfall. Emphasis will be placed on describing how these features can be diagnosed and evaluated using both observed and gridded model data.

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