Lake effect snow is a phenomenon that is most common to areas around the Great Lakes and Upstate New York. It typically occurs during the late fall and early winter when cold air from Canada passes over the still warm lake waters, namely Lake Erie and Lake Ontario for us in the Northeast. Chilly air over the warm lake water allows the moist air near the lake surface to quickly rise, leading to clouds and eventually snow. One of the most important ingredients needed for lake effect snow is a large temperature difference between the lake waters and the air temperatures. Research shows that at least a 13°C difference between the warm lakes and the colder air temperature is most favorable. Of course, larger temperature differences will produce more intense lake effect snow bands and in some case even produce thundersnow. In addition to the temperature difference, winds oriented parallel to the lake have the most "fetch" or the most time for the cool air to travel over the warm lake waters. The more time chilly air has over the warm lake waters, the more moisture it gathers, and thus the more snow it produces. That's why lake effect snow bands are infamous for producing incredble snowfall rates and near zero visibility.
In most lake effect snow events, the winds shift at some point causing snow bands to travel from one locaton to another. However, if the winds are persistent enough in one direction, massive snow totals of 50 - 70”+ can occur due to the high snowfall rates of 2 - 4"+ per hour. Lake effect snow is especially fascinating to meteorologists due to the tight snowfall gradients that develop. One location could be dealing with white-out conditions and feet of snow whereas a location a few miles away could experience clear skies without a single flake. An example of this occurred on November 20, 2016 in Upstate New York. Moisture rich and narrow snow bands coming off Lake Erie and Lake Ontario pushed into Upstate NY resulting in very tight snowfall gradients. Places near Syracuse received well over 2 feet of snow while amounts quickly dropped off heading east on I - 90 with places like Utica, NY measuring less than 10 inches. In fact, in a matter of just 17 miles, totals over 30 inches in Oneida County decreased down to around 1 foot.
These snow bands are not only restricted to Upstate New York with strong west to northwesterly winds often carrying bands downstream into the Hudson Valley, northern New Jersey and northeast Pennsylvania. While most bands lose some punch by the time they reach these spots, rogue snow squall can break off the bands. These squalls can quickly drop visibilities to 1/4 mile or less with bursts of heavy snow and gusty winds. They are also notorious for causing accidents on highways due to the rapidly deteriorating conditions. (read more about snow squalls here)
By January and Feberuary, Lake Erie and Lake Ontario start of freeze, reducing the lake effect snow events during the second half of the winter. However, if the winter is mild, parts of the lakes may never freeze and lake effect snow can continue into March. For more education on winter weather in the Northeast, follow WeatherWorks on Facebook and Twitter.
