Synoptic climatology of northwest flow snowfall in the Southern Appalachians.

摘要

Snowfall in association with low-level winds out of the northwest is a common occurrence at higher elevations and along windward slopes in the Southern Appalachian Mountains. These northwest flow snow (NWFS) events typically have low temperatures and considerable blowing and drifting snow. Due to the high degree of spatial variability of snowfall and limited ability of numerical models to predict these events, forecasting NWFS remains a challenge. This dissertation analyzes the synoptic climatology of NWFS events in the Southern Appalachians for the period 1950 to 2000. Hourly observations from first-order stations, daily snowfall data from cooperative observer stations, and National Center for Environmental Prediction (NCEP) reanalysis data are utilized to identify NWFS events, defined here as snow events with 850 hPa northwest flow (270 to 360 degrees) at the hour of greatest snow extent. Atmospheric fields of temperature, wind, moisture, and associated variables are analyzed for heavy and light snowfalls separately by calculating composite field values and constructing composite plots of the synoptic patterns. The NOAA Hysplit Trajectory Tool is used to calculate 72-hour antecedent upstream air trajectories, and composite trajectories are mapped in a geographic information system (GIS). The sample of events in the trajectory analysis is limited to those with synoptic-scale subsidence, a frequent occurrence with NWFS. Analyses of vertical soundings are coupled with NCEP data to determine the synoptic characteristics associated with different air trajectories. Results indicate that NWFS accounts for as much as 56 percent of mean annual snowfall along the higher elevation windward slopes. Heavy NWFS events are tied to higher values of synoptic-scale ascent and relative humidity in the lower troposphere, as well as lower 500 hPa heights and longer event durations. Additionally, upstream air trajectories with a Great Lakes connection have higher composite mean areal and maximum point snowfall totals along the, higher elevation windward slopes than other northwest trajectories. Little Great Lakes influence is noted at lower elevations and on leeward slopes.