Mechanisms for Transient and Long-Duration Mesoscale Snowbands in Northeast U.S. Winter Storms

The distribution of snowfall accumulation attending winter storms is a product of both precipitation intensity and duration. Many cold-season extratropical cyclones (ECs) exhibit distinct mesoscale snowbands in radar reflectivity observations, the presence of which strongly modulates snowfall accumulation. Mesoscale snowbands are known to be favored within environments characterized by frontogenetical forcing in the presence of weak moist symmetric or gravitational stabilities. These environments, in turn, are often fostered by synoptic-scale processes accompanying cyclogenesis. Although the development of mesoscale snowbands can often be successfully anticipated at 24-36 h forecast ranges, the mechanisms that control band duration over a region are not well understood. Thus, better assessment of snowfall duration in mesoscale snowbands presents an opportunity to improve snowfall accumulation forecasts.

The current study investigates synoptic and mesoscale features responsible for transient and long-duration snowbands in ECs. Long-duration snowbands are defined as exhibiting (1) a length-width ratio >5 in radar reflectivity and (2) reflectivity values ≥25 dBZ for ≥3 h at any geographic location. Transient snowbands are defined as exhibiting criterion (1) alone, as these fail to persist over any single geographic location regardless of their intensity or longevity. Using archived WSR-88D data, Northeast U.S. winter storms spanning the years 2002-2012 are examined for the presence of transient or long-duration mesoscale snowbands. Gridded data from the 0.5° resolution NCEP Climate Forecast System Reanalysis and 13-km Rapid Update Cycle are used to identify synoptic and mesoscale features associated with both transient and long-duration snowbands.

Preliminary results indicate that long-duration mesoscale snowbands may be ascribed to either of two mechanisms, both of which are absent in transient snowbands: lengthwise translation and pivoting. Lengthwise translation occurs when the cross-axis component of band motion is approximately zero, thereby favoring heavy snow training along a narrow, down-axis corridor. This mechanism is most common in cases of weak cyclogenesis. Pivoting occurs when the cross-axis component of band motion changes sign, typically about a slow-moving pivot zone, yielding a quasi-stationary band in the pivot zone. Pivoting is most common in cases of rapid cyclogenesis. Characteristic distributions of potential vorticity and resultant deformation serve to distinguish between transient and long-duration snowband occurrences, and further delineate between lengthwise translation and pivoting for the latter occurrences. Composite analyses and case studies will be presented to document the antecedent and concurrent synoptic and mesoscale features associated with transient snowbands and long-duration snowbands exhibiting lengthwise translation and pivoting.