Lake effect snow is highly localized snowfall, sometimes intense, that forms downwind of large bodies of water such as the Great Lakes. They usually take the shape of narrow bands and can produce significant snow accumulation within very short time periods. As shown on the map below, the Great Lakes have a tremendous influence on the amount of snowfall that falls downwind of their location. On the map below, 100 cm equals about 40 inches.
Lake effect snow actually requires a specific set of conditions involving the atmosphere, land, and water surface. There are 5 primary ingredients the play a role in the formation of lake effect snow.
1. Cold air over a relatively warmer body of water - this results in instability in the atmosphere, and creates a situation where heat and moisture is lifted from the water and transported by the wind downstream. The warm moist air rises and snow showers and squalls form. Typically a temperature difference of 13 degrees C or 24 degrees F between the lake water surface and the air at the 850 mb pressure level or roughly 4500 feet above the lake is needed to reach absolute instability where heat and moisture can be vigorously lifted from the water into the air. The relatively warmer air from the lake cools as it rises and condenses into clouds which produce snow.
2. A layer of cold air at the earth's surface that is sufficiently deep - For the Great Lakes Region, cold air masses originate in the high latitudes of North America and then "spill southward" with an upper level trough or buckle in the jet stream. The depth of the cold air is an important player in determining the intensity of the snowfall possible. Generally, an arctic air mass at least 3000 feet deep is needed to generate good lake effect snow development, and usually air masses greater than 7500 feet deep are associated with the strongest lake effect snowstorms.
The above satellite image shows the lake effect snow band streaming south across Lake Michigan into northwest Indiana on Tuesday, November 18, 2008 at 9:46 am CST.
3. Fetch - Fetch here relates to the distance the air travels over the water. Greater fetches can produce heavier and more intense lake effect snow because of the opportunity for greater "heating" and "moistening" by the relatively warmer waters. In the Great Lakes, wind direction plays a key role in determining the fetch. Below are a few examples of wind direction and fetch.
4. Little if any wind shear (change in wind direction with height) through the layer of cold air - If there is little wind shear, this favors the type of convection necessary to produce intense and well organized lake effect snow bands. If the wind changes significantly with height through the cold layer (for example by 60 degrees in direction or more), the convection process that leads to lake effect snow is disrupted and diminished. This will typically result in only flurries rather than strong well organized lake effect bands, all other things being equal.
The above Doppler radar image shows an intense lake effect snow band impacting northeast Illinois and northwest Indiana on March 7 of 1996.
5. Upstream Moisture - How moist an air mass is before it even comes in contact with the lake has been shown to play a key role in determining how heavy snowfall can be. If an air mass has lower moisture, it is more difficult to get condensation, clouds, and snow. On the other hand, if an air mass has higher moisture content (relative humidity of 70 percent or greater), heavier snowfalls are possible. Upstream lakes can actually add to the amount of pre-existing moisture. For example, if an arctic air mass moving across Lake Michigan has also moved across Lake Superior, there has already been additional moistening of that air mass before it even gets to Lake Michigan! This can result in heavier snowfalls for areas downwind of Lake Michigan.
Lake Effect Snow Formation - The diagram below summarizes the key ingredients for lake effect snow. Arctic air with a temperature difference of at least 13 deg C or 24 deg F between the water and the air at about 4500 feet flows over the lake surface. The depth of the cold air (marked by the dashed line labeled "capping inversion") must be enough to support convection to develop as heat and moisture is transported from the water into the air. Greater snows can result if the fetch (distance the air travels across the water) is longer, wind shear minimal, and initial moisture content of the airmass greater. Rising air moving over the water condenses to form snow showers and squalls, and snow bands develop and extend inland downwind of the shoreline.
After looking at the numerous parameters the influence the creation of lake effect snow and modulate its intensity, and seeing the complex atmospheric circulations that exist and interact, it is easy to see why predicting lake effect storms is so challenging!