Lake Effect Again

January 24th, 2013 at 11:09 am by under Weather

Lake Effect Snow (How Does It Form?)

Many of us woke up this morning to a ‘surprising’ coat of powdery snowfall. How and why did this happen. This is what we refer to as ‘Lake Effect Snow’ or when we discuss it many times we call it LES.

Lake Effect Snow needs three factors to form:

*Instability

*Fetch (The distance the air mass travels over a body of water)

*low wind shear.

Instability

Instability is created when there are two air masses have extremely different qualities. For Lake Effect Snow to form we need as a general ‘rule of thumb’ to have a 13C or 55F difference between the temperature of the water and the air temperature. So as a general rule in a normal winter we would usually see Lake Effect Snow develop early in the season when the waters of Lake Michigan are very warm and the air temperature is very cold. The air mass we just experienced was -10C and the water temperature was 4.5C.

This difference in temperature creates instability between the water and air and the water begins to evaporate rapidly. The evaporation creates clouds and moisture and another process called convection. (Yep that’s a similar process that occurs when we talk about thunderstorms).

This is a fetch or movement of winds from Lake Michigan across NE Indiana

Fetch

This refers to how far this cold air mass advects or moves over the body of water. It further describes the wind that carries it. For example, we first look at our forecast models to see if it is possible for Lake Effect Snow to develop. Then we look at the fetch or the trajectories of wind flow to see how far the wind is blowing and more importantly for our are which direction the wind is blowing. For example a strong west wind might take the LES off the lake and create a ‘fetch’ all the way to Fort Wayne, or a southerly wind might create a ‘fetch’ to Indianapolis.

Low Wind Shear

Wind shear is defined as how quickly the winds change direction as you move up in the atmosphere. If the winds are changing quickly it creates almost a cutting moisture which would essentially tear apart any clouds that formed over Lake Michigan.

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