Over the northeastern United States, northwesterly lower-tropospheric flow regimes are occasionally associated with production of heavy precipitation, especially over the considerable orography of northern New York and northern New England. Two cutoff 500 hPa cyclone events in the autumn of 1999 produced heavy snowfall over the mountains of northern New York State and northern Vermont. These two cases provided an operational impetus for studying the characteristics of upslope snowfall events. The goal of this research is to produce ingredients-based conceptual models and operational methodologies for the purpose of improved prediction of the precipitation patterns produced by these flow regimes in the complex terrain of the northeastern United States.

Cases selected for this study were limited to events occurring with prevailing deep-tropospheric northwesterly flow, which excluded any cases involving rapidly deepening coastal cyclones (i.e., Nor’easters). Northwest-flow scenarios generally produce a significant low-level flow component orthogonal to the Green Mountains and Adirondack Range, which is favorable for the generation or enhancement of heavy precipitation by orographic lift. Six scenarios have been examined in this study: three events which produced heavy snowfall in northern Vermont and northern New York State (including the two aforementioned events from 1999); two events which had been forecast to produce heavy snowfall, yet significant precipitation failed to occur; and one weak northwest-flow event.

NCEP/NCAR Reanalysis data were used in determining the synoptic-scale characteristics of each of the cases, supplemented with ETA model BUFR sounding analysis data in order to interrogate the mesoscale structure of each event. Additionally, 5-km grid spacing mesoscale ETA model simulations have been performed both to isolate mesoscale signatures, and for comparison with their coarser-resolution operational ETA model (40-km) counterparts. Diagnostic findings from this study suggest several meteorological factors significant to the development of heavy precipitation from this type of flow regime: (a) the low-level moisture profile; (b) the strength and orientation of the low-level wind with respect to the orography; (c) the low-level static stability profile. Low- and high-resolution model comparisons have yielded some measure of forecast success by the 5-km ETA. Diagnostic analyses and model simulations of two events will be discussed. Operational forecast procedures and techniques currently under development will be presented as well.