Mesoscale Band Formation in Three Major Northeastern United States Snowstorms

The National Centers for Environmental Prediction's 29-km version Meso Eta Model and Weather Surveillance Radar-1988 Doppler base reflectivity data were used to diagnose intense mesoscale snowbands in three northeastern United States snowstorms. Snowfall rates within these snowbands were extreme and, in one case, were close to 15 cm (6 in.) per hour. The heaviest total snowfall with each snowstorm was largely associated with the positioning of these mesoscale snowbands. Each snowstorm exhibited strong midlevel frontogenesis in conjunction with a deep layer of negative equivalent potential vorticity (EPV). The frontogenesis and negative EPV were found in the deformation zone, north of the developing midlevel cyclone. Cross-sectional analyses (oriented perpendicular to the isotherms) indicated that the mesoscale snowbands formed in close correlation to the intense midlevel frontogenesis and deep layer of negative EPV. It was found that the EPV was significantly reduced on the warm side of the midlevel frontogenetic region as midlevel dry air, associated with a midlevel dry tongue jet, overlaid a low-level moisture-laden easterly jet, north of each low-level cyclone. The continual reduction of EPV on the warm side of the frontogenetic region is postulated to have created the deep layer of negative EPV in the warm advection zone of each cyclone. The negative EPV was mainly associated with conditional symmetric instability (CSI). Each case xhibited a much smaller region of conditional instability (CI) on the warm side of the frontogenesis maximum for a short period of time. The CSI and, to a lesser extent, CI are postulated to have been released as air parcels ascended the moist isentropes, north of the warm front, upon reaching saturation. This likely was a major factor in the mesoscale band formation and heavy snowfall with each snowstorm. The results indicate that model frontogenesis and EPV fields can be used to predict the potential development of mesoscale snowbands. When a deep layer of negative EPV and strong midlevel frontogenesis are forecast by the models, forecasters can anticipate the regions where mesoscale snowbands may form. Inspection of saturation equivalent potential temperature in conjunction with EPV is suggested to determine whether CI is present in a negative EPV region. If CI is present in addition to CSI, then upright convection may dominate over slantwise convection leading to heavier snowfall rates. The region where the frontogenesis and negative EPV are forecast to persist the longest (usually left of the 700-hPa low track) is where the heaviest storm total snowfall will occur. Once mesoscale bands are detected on radar, accurate short-term forecasts of areas that will receive heavy snowfall can be made.