ROCHESTER, NY (WROC) – Lake-effect precipitation in the Great Lakes amounts for about half of seasonal snowfall in Rochester (~50″) and we will break down how lake-effect snow works here. NOTE: While most lake-effect precipitation falls as snow, there is lake-effect rain in certain circumstances when a good portion of the surface is above freezing. There are also situations in which lake enhancement boosts precipitation within a storm system. This description will focus on exclusively Lake-effect snow for simplicity.
- Cold air: This may seem obvious, but it really is the main driver of the instability necessary to drive lake-effect snow. When a cold air mass moves over a warmer body of water, a large temperature gradient (difference) develops between the water temperature and temperatures aloft. The cold air acts as a dense sponge that soaks up moisture from the lake (ideal gas law). This leads to clouds and eventually snow. Analysis is done at around 850 hPa (mb), about 1.5 km above the surface. When the difference in temperature is above 10°C, and especially above 13°C, the threshold for lake effect snow is reached.
2. Wind direction: Wind flow within the column of air dictates where the snow will form. A west wind (remember, wind flows FROM the direction) will result in snow for the Tug Hill Plateau. A northwest wind will send lake-effect snow bands across the southern shores of Lake Ontario and further inland. Wind direction must be generally uniform through a deep column of air. A good rule of thumb is <30° wind “shear” through 700 hPa (mb) will result in more organized bands of snowfall.
Fetch: Defined as the distance traveled by wind or waves across open water, this helps determine strength of bands. A longer fetch translates into a longer time the air has to travel over the warm waters and collect moisture. The maximum fetch for Rochester is a west-northwest wind over the entirety of Lake Ontario. Remember that once the bands move over land, the moisture source ends and the snow cannot continue to strengthen.
4. Time: It is important that the cold air and wind are persistent over several hours in order to get the lake-effect bands going. However they set up, a good rule of thumb is that they need to remain in that setup for at least six hours to produce snow.
5. Wind Speed: Too strong of a wind speed and the bands can get sheared apart. Too weak of a wind speed and no lake bands will develop. Usually a wind between about 10 mph and 30 mph will allow for lake effect. Too strong can sometimes send snow bands well inland, even hundreds of miles at times.
5. Topography: Based on how the land is shaped once the bands move over land, snow can be enhanced. The classic example is along the Tug Hill Plateau, where a west wind not only brings snow bands into the plateau, but the bands get a boost from lift as they slide up the plateau.
6. Moisture: There are situations in which winds upstream can pick up moisture before Lake Ontario. Georgian Bay, Lake Superior, and Lake Huron can all contribute moisture to lake effect. That would mean more snow. Some cold blasts can originate from central Canada, and that can result in a lower moisture value and lower snowfall chances and totals.
Remember that we can also see lake-effect rain showers when temperatures are above freezing. During these rain events there can often be some rotation within the clouds and a funnel can form. That may produce a waterspout, like seen on August 18th, 2020.