A Climatology of Snow Squalls in Southern New England 1994–2018

Abstract

Snow squalls are sudden snow events, lasting less than one hour, characterized by low visibility and gusty winds, which can result in notable societal impacts. This analysis develops a climatology of non-lake effect snow squall events in Southern New England for 1994–2018 and investigates the synoptic environment and mesoscale factors conducive to their formation. National Weather Service surface observations were used to identify events; sea-level pressure maps, composite radar charts, and a cell tracking algorithm were used to determine their organization and movement; and ERA5 hourly reanalysis data were used to analyze the associated synoptic and infer mesoscale features, as well as convective and symmetric instability. One hundred events were identified and categorized into four distinct types based on the direction of movement of the associated radar echoes, which is closely linked to characteristic synoptic structures and mesoscale factors. The four types are Classic (squall movement from the northwest, 72 events), Atlantic (from the southwest, 15 events), Northern (from the north, 9 events), and Special (varying, 4 events). All types have a 500-hPa trough over the Northeast but differ in the structure of the trough and its relation to lower-level flow, which accounts for the differences in movement of the squalls. The snow events occur in shallow, convective squall lines and the ingredients for convection were present in all cases. Both upright and symmetric instability are typically present, all cases had at least one lower tropospheric layer with cyclonic differential vorticity advection, and many cases were also associated with frontogenesis.