Antecedent upstream air trajectories associated with northwest flow snowfall in the southern appalachians

Northwest flow snow (NWFS) events are common occurrences at higher elevations and on windward slopes in the southern Appalachians. Low temperatures and considerable blowing and drifting of snow, coupled with significant spatial variability of snowfall, substantially increase societal impacts. This paper develops a synoptic classification of NWFS events in the southern Appalachians using 72-h antecedent upstream (backward) air-trajectory analyses. Hourly observations from first-order stations and daily snowfall data from cooperative observer stations are used to define snowfall events. NCEP–NCAR reanalysis data are utilized to identify NWFS events on the basis of 850-hPa northwest flow (270°–360°) at the event maturation hour. The NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory tool is used to calculate 72-h backward air trajectories at the event maturation hour and composite trajectories are mapped in a geographic information systems format. Analyses of vertical soundings are coupled with NCEP–NCAR reanalysis data to determine the synoptic characteristics associated with each trajectory class. Significant variability of trajectories and synoptic patterns is evident from the analyses, resulting in four distinct backward air-trajectory classes. Trajectories with a Great Lakes connection result in higher composite mean and maximum snowfall totals along portions of the higher-elevation windward slopes when compared with other northwest trajectories, but little effect of the Great Lakes is noted at lower elevations and on leeward slopes.