A process-oriented methodology toward understanding the organization of an extensive mesoscale snowband: A diagnostic case study of 4-5 December 1999

A case study of a long, narrow band of heavy snowfall is presented that illustrates those processes that force and focus the precipitation in a unique linear fashion. System-relative flow on isentropic surfaces shows how the trough of warm air aloft (trowal) formed to the north-northwest of a weak synoptic-scale surface cyclone. To the north of the trowal, midtropospheric frontogenesis formed as the warm, moist, high-theta(c) air in the trowal canyon became confluent with cold, dry air to the northwest of a closed midlevel circulation. Within the trowal airstream, isentropic uplsope is shown to contribute to vertical motion, while transverse to this flow, mesoscale lift is enhanced on the warm side of a frontogenetical zone in the presence of weak symmetric stability and conditional symmetric instability. Further, it is shown that a sloping zone of small positive to negative equivalent potential vorticity forms to the southeast of the midtropospheric system-relative closed circulation as low-theta(c) air associated with the dry conveyor belt, seen in water vapor imagery, overruns warm, moist high-theta(c) air associated with the warm conveyor belt. In this way cold season instability forms due to differential moisture advection on the warm side of the frontogenesis axis. Finally, a conceptual model is shown that encapsulates the key processes that contributed to the extensive, narrow band of heavy snow in the presence of a weak synoptic-scale surface cyclone.