Understanding Lake Effect Snow

Written by: Scott Hetsko
Image Courtesy: USA Today

Lake effect snow is so much more than "cold air over relatively warmer lake waters". It's hard to get into the nuts and bolts of these mini blizzards on television. Below is the science: The 7 most important parameters to really get the snow guns firing!

Heating: The water of the Great Lakes lags behind the atmosphere in cooling through the fall and early winter. Cold air traveling over these warmer lakes creates low level instability. Rising air quickly reaches saturation, and the result is shallow clouds, usually in bands parallel to the low-level wind direction.

Moisture: The humidity of the boundary layer air is vital to the production of lake effect snow. Dry air (Relative humidity < 50%) means the airmass works too hard to reach saturation. Significant lake snow is difficult under this scenario.

Wind Fetch: The length of trajectory of the wind across the lakes has a great bearing on the development of lake-effect snow. Longer distance means heavier snowfall downwind of the lake. The best example locally is a WSW wind over lake Erie usually pummels Buffalo with snowfall measured by the foot!

Wind direction: Which way the wind blows not only "steers" the snow bands but also determines the intensity of the snowfall. Winds that blow perpendicular to the lake usually result in multiple bands of lake snow. Snow is more spread out and less intense. Winds the blow parallel to the lake fire a single band of heavy lake snows, sometime with thunder snow!

Frictional Difference. As winds carry the air from lakes to land, it encounter friction and thus slows down. This results in convergence of air and lifting. The frictional convergence increases snowfall amounts due to the added lift.

Upslope lift: The Tug Hill region East of Lake Ontario is the perfect example of enhanced snow due to upslope lift! Rising air adds about 50-75" of snowfall to this region each winter.

Large-scale forcing: General lifting from a large scale storm system often enhances lake effect. We saw a classic example of that earlier this week when the region received 10-20" of snow.