The Role of the Great Lakes in Northwest Flow Snowfall Events in the Southern Appalachian Mountains

Abstract

Northwest flow snowfall (NWFS) events are a regional forecasting challenge that affects much of the southern Appalachian Mountains. These events can be defined as snowfall accompanying upslope flow and low-level northwesterly winds in this region, and typically feature irregular snowfall distributions and highly variable total accumulations. Previous research done by Perry and Konrad (2004—2007) provides an excellent climatology of NWFS events, and shows that NWFS accounts for nearly 50% of mean annual snowfall along the higher elevations of the southern Appalachians. Additionally, through analysis of backward air parcel trajectories, their research shows that NWFS events that featured a Great Lakes connection exhibited increases in composite mean and maximum snowfall totals. This body of work clearly suggests that the Great Lakes can enhance snowfall in NWFS events by warming and moistening the low-level airmass upstream of the southern Appalachians.

The specific objective of this study is to quantify and evaluate the role of the Great Lakes in NWFS events for select cases via model experiments using the Weather Research and Forecast (WRF) model. The selected cases occurred 5–6 March 2001, 18–20 December 2003, and 10–11 February 2005, and were investigated using a case study approach. In order to determine the effect of the Great Lakes on NWFS precipitation in these cases, two experimental runs were designed to isolate the role of the lakes. First, surface fluxes of heat and moisture were set to zero across the entire model domain (NOFLX). Second, surface fluxes of heat and moisture were set to zero across only water points (LKNOFLX). The sensitivity of the selected NWFS events to planetary boundary layer (PBL) scheme was also tested (MYJPBL).

Overall, it was found that the Great Lakes play an important role in some NWFS events and can be responsible for 20–30% of the precipitation that occurs in these events. Of the selected cases, the March 2001 and February 2005 events showed large decreases in precipitation in the LKNOFLX model run compared to the control (CTRL) run. In these two events, the role of the Great lakes was to destabilize the upstream airmass and increase the Froude number. At a point roughly halfway between the Great Lakes and the southern Appalachians, the LKNOFLX model run in the February 2005 event had an average 950?850 hPa Froude number of 0.99, which was 0.40 less than the CTRL value of 1.39. Similarly in the March 2001 event, the LKNOFLX model run had an average 950–850 hPa Froude number of 1.28, which was 0.42 less than the CTRL value of 1.70. In both cases, the reduced average low-level Froude number in the LKNOFLX run compared to the CTRL shows that when the effect of warming and moistening of the low-level upstream airmass caused by the Great Lakes is removed, a more stable upstream airmass occurs which reduces the Froude number and reduces NWFS precipitation.