Over many years of operational forecasting, Great Lakes National Weather Service forecast offices have noticed that a significant change in wind direction with height is often detrimental to the development and maintenance of intense lake-effect snow bands. An operational forecast decision tree used to predict lake-effect snow band coherence and possibility of development incorporates three categorical criteria of the directional wind shear between the surface and 700 hPa. A small wind direction change within this layer (i.e., 0 – 30 deg) leads to forecasts of a strong, well-organized band; weaker, less-coherent bands are predicted for a 30 – 60 deg change; and conditions with wind direction changes of > 60 deg produce forecasts that indicate snow band development will be inhibited.

To date neither observational studies nor numerical model simulations have been undertaken to (1) quantify the influence of vertical directional wind shear on the structural coherence of lake-effect snow bands or (2) investigate the dynamical / physical basis behind these operational criteria. An array of 15 idealized mesoscale model simulations was used to investigate the atmospheric response and mesoscale circulation resulting from lake-effect conditions with different values of vertical directional wind shear and wind speed. Simulated ambient conditions included wind direction changes of 0, ±45, and ±90 deg from the surface to 3.0 km, with surface winds parallel to the long axis of the elliptical lake, wind speeds of 5, 10, and 15 m s^-1, and a lake-air temperature difference of 15 °C. Results show that coherent bands develop under all shear conditions for wind speeds of 5 and 10 m s^-1, the most intense snow bands associated with wind speeds of 10 m s^-1. The band strength and shoreline regions impacted by the heaviest snowfall varied significantly. Simulations with wind speeds of 15 m s^-1 under all wind shear conditions resulted in a mixture of cases with weak widespread snowfall or no organized mesoscale structure. In addition, results show a distinct asymmetry in the strength of lake-effect snow bands associated with the sign (+ or -) of the vertical directional wind shear. Findings from this study suggest that the operational wind shear criteria used to aid forecasting of intense lake-effect snow bands may not have general applicability to NWS forecast offices and shoreline communities throughout the Great Lakes region. Additional details describing the varied mesoscale structure of snow bands and quantifying the influence of wind shear across the array of simulations will be presented.