Mesoscale band formation in three major northeastern United States snowstorms

The National Centers for Environmental Prediction's 29-km version Meso Era Model and Weather Surveillance Radar-1988 Doppler base reflectivity data we re used to diagnose intense mesoscale snowbands in three northeastern Unite d 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 sno wfall with each snowstorm was largely associated with the positioning of th ese mesoscale snowbands. Each snowstorm exhibited strong midlevel frontogen esis in conjunction with a deep layer of negative equivalent potential vort icity (EPV). The frontogenesis and negative EPV were found in the deformati on zone, north of the developing midlevel cyclone. Cross-sectional analyses (oriented perpendicular to the isotherms) indicated that the mesoscale sno wbands formed in close correlation to the intense midlevel frontogenesis an d 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 midle vel 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 o f negative EPV in the warm advection zone of each cyclone. The negative EPV was mainly associated with conditional symmetric instability (CSI). Each c ase exhibited 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 par cels 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 laye r of negative EPV and strong midlevel frontogenesis are forecast by the mod els, forecasters can anticipate the regions where mesoscale snowbands may f orm. Inspection of saturation equivalent potential temperature in conjuncti on with EPV is suggested to determine whether CI is present in a negative E PV 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 stor m total snowfall will occur. Once mesoscale bands are detected on radar, ac curate short-term forecasts of areas that will receive heavy snowfall can b e made.