The return of a cold air mass to Ontario is bringing the occurrence of lake effect snow. This phenomenon that is very common across the Great Lakes is responsible for extremely vast variabilities in snowfall over short distances, and often ‘busts’ the forecasts from apps on your smartphone. The development of lake effect snow usually declines through the latter stages of winter as ice builds up on the lakes, although it can be still quite common in February. Let’s break down the ingredients that lead to the formation of lake effect snow to get a better idea on why we are seeing these events later in the season, and why forecasting for them can be quite complex.
The Need for Instability
When cold air passes over the relatively warmer waters of a large lake, the lower levels acquire moisture and heat. As a result, this air becomes less dense (i.e., lighter) than the air above it which enables it to rise. This is a process known as ‘convective instability’. It is the degree of instability that will ultimately determine the depth of the cloud layer that will develop. In order to get the development of lake effect snow, research has established that a temperature difference of at least 13 degrees Celsius is required between the temperature of the water surface and the air temperature at the 850 millibar pressure level (approximately 1.5 kilometers up into the atmosphere). As the temperature difference becomes larger, more heat and moisture is able to be transported vertically resulting in taller clouds and higher snowfall rates. In instances when extreme instability is present, lake effect snow has been known to generate ‘thunder snow’. Lake effect snow events usually set up in the wake of a larger scale weather system, or in the presence of a trough of low pressure within a cold, Arctic air mass.
Snowfall rates can also be enhanced when existing precipitation from a weather system moves over a lake in a phenomenon called ‘lake enhanced snow’. In this case, temperature differences slightly less than 13 degrees Celsius are required between the temperature of the water surface and the air temperature at the 850 millibar pressure level (approximately 1.5 kilometers up into the atmosphere).
With very cold air aloft becoming established over the Great Lakes early this February enough instability is present for the formation of lake effect snow. Since the Great Lakes are large bodies of water they are able to retain heat well into the winter months. As disturbances cross the Great Lakes basin, conditions will likely become favourable for lake effect snow on a number of occasions within the upcoming weeks.Fetch Across a Body of Water
The distance that air travels over a body of water is termed the ‘fetch’. The term fetch does not allude to air travelling over sections of the body of water that have become frozen. Generally, a fetch of at least 100 kilometers is required in the formation of lake effect precipitation. The longer the fetch is, the more residence time air has to acquire heat and moisture from the water. Of course, the fetch is not only determined by the size of the body of water, but also the prevailing wind direction.
As a result of the recent January thaw, many of the Great Lakes lack in ice coverage meaning that there is still ample fetch to be taken advantage of.Wind Shear
In order for lake effect snow bands to develop, a lack of directional wind shear is necessary. Directional wind shear refers to the change in wind direction with height. If the wind direction varies from the surface to the mid-levels aloft (about 3 kilometers up into the atmosphere) by more than 60 degrees, precipitation will be limited to flurries at best. If the wind direction varies within the aforementioned region by 30 to 60 degrees then relatively weak lake effect bands are possible. When the wind direction varies by less than 30 degrees from the surface to the mid-levels aloft (about 3 kilometers up into the atmosphere) then strong, well-organized bands of lake effect snow are able to develop.
Wind shear can also be defined as a change in wind speed with height. This speed shear is less important to the development of lake effect snow, however, if wind speeds at the mid-levels aloft (about 3 kilometers up into the atmosphere) are too much stronger than those at the surface then organization of the lake effect snow bands will be limited (as the tops become ‘sheared’ off).
Moisture Must Be Present
If little moisture content is available upstream then the overall process allowing for condensation to form clouds and precipitation will be more difficult to achieve. When higher amounts of moisture are present upstream, clouds and precipitation are able to form more efficiently. Large bodies of water upstream can influence lake effect precipitation by supplying additional moisture. Under the right conditions, a band of lake effect snow may use the fetch of multiple bodies of water and span hundreds of kilometers in length. This type of occurrence is most common during the latter stages of autumn and into early winter.
Topography is Important
As lake effect precipitation is released downwind of the body of water the role of friction comes into play. As wind travels over open water, friction is limited and pretty much negligible. When the wind reaches land, however, frictional forces slow down the speed of the wind creating convergence. Think of a car suddenly hitting the brakes and having faster moving vehicles coming from behind running into its rear. This creates lift near the surface, and serves to enhance the condensation to form clouds and precipitation. Another example of this occurs when elevated terrain exists downwind of a body of water which serves to promote even more lift. The shape of the coast line also has the ability to create convergence features that enhance snowfall rates. The inland penetration of lake effect snow — varying from roughly 50 kilometers to over 400 kilometers — depends on the strength of the wind in the low-to-mid levels, the availability of moisture, and the orientation of the bands.Complexities in the Forecasting of Lake Effect Snow
Several aspects of lake effect snow can make its forecasting quite difficult. After determining the amount of instability present by evaluating surface water temperatures and temperatures aloft, a forecaster must then assess to what level in the atmosphere mixing will occur. By determining the mixing level, the forecaster is able to get an idea of the strength of wind gusts that may be brought down to the surface. The forecaster will then use the knowledge of the mixing level to assist in determining the prevailing wind direction that will essentially steer the lake effect snow bands. The height at which these ‘steering winds’ exist for lake effect snow bands can often vary during a single event, and thus must be evaluated carefully. The steering wind is not necessarily from the same direction as the wind at the surface! With an average width of about 16 kilometers, lake effect snow bands can be quite narrow. Whiteout conditions with near zero visibility in one location may give way to clear skies just minutes up the road. This is one of the most difficult challenges in forecasting lake effect snow. If the atmospheric profile is not evaluated correctly, even a slight variation of a few degrees in the wind direction that is used to predict the orientation of lake effect snow may completely botch a forecast. Raw model data has been known to mishandle the orientation of lake effect snow bands. Forecasts that rely on raw model data often lead to people caught off guard by lake effect snow.
When forecasting snowfall amounts in general the challenge not only becomes how much precipitation will fall, but how much accumulation will occur. This varies significantly as a result of the structure of the snowflake and the Snow-to-Liquid Ratio (i.e. the ratio of water contained in a given volume of snow if it were completely melted). Lake effect snow events are known to have particularly high Snow-to-Liquid Ratios because of the cold and moist conditions in which they exist — perfect for dendritic growth (dendrites are snowflakes with a branching, tree-like structure). The structure of dendrites enable them to grow efficiently by aggregation as they descend from a cloud. When dendrites are able to grow, snowflakes also accumulate much more efficiently as air pockets within the branches allow them to pile up quicker. A typical weather model will assume a 10:1 Snow-to-Liquid Ratio (i.e. 1mm of liquid water = 1cm of snow), although lake effect snow events can have ratios that are 25:1 (i.e. 1mm of liquid water in the model = 2.5cm of snow). The Snow-to-Liquid Ratio can be evaluated subjectively by the forecaster through carefully analyzing the atmospheric profile during a given event. This will often allow for a better interpretation of the expected snowfall accumulation than by simply relying on raw model data. To make things even more complex, Snow-to-Liquid Ratios may change during the course of any given snow event. When strong winds accompany lake effect snow it can serve to break apart the dendritic structure of the snowflake, and thus limit accumulation.
A forecaster must carefully evaluate the presence of moisture when the risk of lake effect snow exists. This is crucial as an assumption of moist conditions may not always be necessarily correct. A forecaster must also look to the higher levels of the atmosphere to determine disturbances that may not be reflected on a surface map and that may assist in the formation or enhancement of lake effect snow.
In all cases above it must be noted that the forecasting of lake effect snow, like all other weather forecasting, is not two-dimensional. Rather it is four-dimensional. Not only does it consider all three dimensions, but it incorporates variances in time as well.