The Influence of Vertical Shear on Observed Lake–Effect Snow Bands

The magnitude of vertical shear is considered an important parameter for winter weather forecasting of lake-effect snow storms. Operational forecasting experience and several previous research studies have suggested that a significant change in wind direction with height can often hinder the development and maintenance of intense lake-effect shoreline snow bands. A recent investigation using fifteen idealized model simulations showed that the vertical shear between the surface and 3.0 km (~ 700 hPa) had a significant impact on the evolution, intensity, and position of lake-effect snow bands; yet for each simulated environment a low-level convergence zone and mesoscale snow band developed in the vicinity of the lake. These contradictory findings suggest an increased understanding of the influence of vertical shear on lake-effect snow bands is necessary.

The current study used 23 months of visible satellite imagery during the winters of 2000-2004 to identify 40 shoreline bands over Lakes Michigan, Erie, and Ontario. Subsequently, archived soundings collected upwind of each of the lakes (Green Bay, WI for Lake Michigan, Detroit, MI for Lake Erie, and Buffalo, NY for Lake Ontario) were used to investigate the influence of environmental wind conditions on band position and evolution. Shoreline bands were found to exist with surface-to-700-mb vertical shear values from 0° - 90° and near-surface flow oriented along each lakes major axis. This finding is contrary to previous observational reports that suggest snow band development will be inhibited with vertical shear values > 60°. Results showed that vertical shear was important for determining snow band position. The upwind extent of shoreline bands was found to decrease with increasing positive directional shear. Observations of Lakes Erie and Ontario snow bands showed the magnitude of the cross-lake component of vertical shear was linked closely to band location, positioning the snow band over the downshear side of the lake. Further details of these results, additional findings from this observational study, and their relation to previous work with regard to the influence of vertical shear on lake-effect shoreline snow bands will be presented.