Forecasting wintry weather is always interesting. The atmosphere is in a constant state of chaos while the steep temperature gradient exists over the sun-deprived middle latitudes. Powerful extratropical cyclones dump massive amounts of snow, sleet, ice, and freezing rain across much of the country. There is another threat, however, that threatens those in close proximity to any significantly-sized body of water, and that is lake effect snow. For those who live around the Great Lakes, it becomes a part of daily weather discussions in the early months of winter.
The interactions between water and land are often taken for granted, but we are reminded of its power every winter when a foot of snow falls from a lake effect band of snow. The large-scale features generally involved in lake effect snow, include an exiting extratropical cyclone (weather system) with a strong high pressure system building behind it, ushering in an air mass of very cold air for Canada. The key for lake effect snow development, is the temperature difference between the air mass and the water. The greater the difference, the more favorable the conditions are for big snow totals.
Once the cold air moves across the warm water, heat is transferred rapidly from the water to the air. The air above the lake warms up fast and has moisture evaporating into it as well. Since cold air has a smaller capacity for moisture than warmer air, the moisture condenses and becomes clouds. As the air continues to heat, the low-level air becomes unstable (instability occurs when cooler air resides above a layer of warm air) resulting in deepening convection and the falling of precipitation in the form of snow. Once the air moves over land on the downwind shoreline, friction with the land causes the air to decelerate, resulting in convergence. Consequently, the air is forced upward even more. The heaviest of these bands generally falls just downwind of the shoreline.
So, we know lake effect snow requires cold air and warmer water, but there are also other factors to consider when forecasting the significance of a lake effect event. some of the others include wind direction, wind speed, amount of ice cover over the lake, and topography of the land downwind. The length of water (also known as "fetch") the air must travel is also a big factor. The direction of the wind over a section of the lake can dictate the extent of snowfall over a specific region. This is why places like Porter, Indiana receive so much snow every year. Northerly winds over Lake Michigan present a significant fetch that result in an abundance of convection and thus, higher snow totals.
Topography plays a significant role by creating a forcing mechanism more potent than just the land by itself. Not only does the higher terrain enhance surface friction, contributing to increased convergence, but it also forces the air to rise, creating a higher level of upward motion, enhancing convection.
All-in-all, there are many factors to consider when forecasting lake effect snow. It is a very tricky event to forecast for as the slightest shift in wind direction, speed, or surface temperature can alter the distribution of snowfall, not only quantity-wise, but on a locationally as well. It is certainly a huge part of winter weather forecasting, but just one among several types of weather phenomenon that many people have learned to live with.