Causes of Lake Effect Snow

Snow that is generated or enhanced by a lake is known as lake effect snow (LES). In the United States it is most common in the areas of the lee- wind sides of the Great Lakes. These events can be minor, or produce many feet of snow. Understanding what lake effect snow is and how to forecast an event is critical for those living in regions prone to these snow squalls.

There are several factors that contribute to the creation of lake effect snow.

1) Warmer air blowing over cooler water: It is commonly accepted that a temperature difference of 55ª F (13ªC) is sufficient for lake effect snow to occur. This temperature difference creates clouds as the warmer moist air from the lake rises through the cooler air and condenses. This process is known as convective lift. Buoyancy, or convective lift, is the primary driving force for lake effect snow making the forecasting of vertical temperature profiles important in the predicting of these events. This means that meteorologists will not only look at temperature differences from the water to air immediately above the surface, but also consider temperatures at higher levels.

2) The fetch of wind blowing over the lake: Fetch is the direction the wind is blowing over the lake. The more of the surface area of the lake that the wind is blowing over the more conducive the fetch will be for high amounts of lake effect snow. This is because it would provide more access to the warm moist air needed from the lake.

3) Speed of the wind over the lake: Wind speed will determine how long a parcel of air will stay over the lake. Slower winds will cause more convection to take place increasing storm intensity.

4) Convergence of air caused by differences in friction levels at the surface: Water will generally provide less friction to the air blowing over it than land. Once a parcel of air has blown over the water and hits land it will usually slow down, the faster winds keep blowing over the lake and cause a pile up of air on the land. This is wind convergence, which will force air to rise and once again increase storm intensity. The geography of the land will affect the amount of convergence. Land that causes more friction such as hilly terrain, or urban areas allow for more convergence than flat farm land.

5) Factors involving the overall weather environment: Traditional dynamic lifting mechanisms such as surface fronts, mid level troughs, and jet stream dynamics aid in lift needed for precipitation. These factors coming together with the convection caused by the lake can make for a particularly strong event.

Lake effect snows are an intriguing weather phenomena because of the specific variables needed to generate the snow. It is possible for lakes to generate other types of precipitation depending on the vertical temperature profiles in place. Snow from these storms is localized making, it imperative that local forecasters stay on top of a developing event.