Thundersnow in Conjunction with Heavy Snowfall Events over the Northeast United States

Convective snow, commonly referred to as thundersnow, can present major forecasting challenges. Thundersnow, which often occurs in conjunction with mesoscale snowbands, may be associated with regions of locally heavy snowfall (6–12 in) and intense snowfall rates (2–3 in h^-1). Although winter thunderstorms are less common than their warm-season counterparts, they are likely to have been underreported in the past by human observers because falling snow can obscure lightning and dampen the sound of thunder. In recent years, the availability of advanced lightning detectors and sensitive weather radars has allowed meteorologists to identify small areas of enhanced snowfall produced by thundersnow within winter cyclones. The purpose of this presentation is to take advantage of these and other contemporary observing systems in order to construct composite and case study analyses of the atmospheric environment preceding and during the occurrence of thundersnow events over the Northeast U.S.

Thundersnow over the Northeast U.S. can occur in conjunction with coastal cyclones, Alberta Clippers, lake-effect storms, and elevation storms, and is hypothesized to result from the contributions of dynamical and thermodynamical processes to thundersnow development. The particular processes important to the development of thundersnow appear to be strong dynamic and/or orographic lifting and elevated convection with surface temperatures near 0°C. These processes motivate the construction of a thundersnow phase space to assess the relative importance of dynamical versus thermodynamical forcing in generating thundersnow in various synoptic-scale flow regimes. Thundersnow cases spanning 1994–2013 will be identified from archived METAR surface observations and NLDN data. WSR-88D and dual-polarization radar data will be used to identify possible convective signatures associated with the thundersnow cases. The NCEP CFSR(v2) gridded datasets will be used to populate the aforementioned phase space and construct the various composite and case study analyses.

The occurrence of thundersnow relative to low-level and upper-level jets and jet-related vertical circulations also will be analyzed. Sounding and cross-section analyses will be produced using RUC hourly 13-km datasets. These datasets will be used to determine the CAPE profiles attending thundersnow. In cases where there are slantwise updrafts in the presence of high concentrations of ice crystals, minimal CAPE thresholds required to enhance updrafts and induce charge separation and lightning will be quantified. The frequency of thundersnow events in the absence of CAPE also will be assessed, and minimal ascent and ice crystal thresholds required for thundersnow to occur in nonconvective events will be quantified.