What is Lake Effect Snow and How is it Forecasted?

Lake Effect Snow @ WFO IWX
Justin Arnott, Senior Forecaster - WFO IWX
1.   What is Lake Effect Snow? 
In our region, Lake Effect Snow (LES) develops when arctic air moves over the relatively warm waters of Lake Michigan. Air in contact with the lake gradually becomes warmer and more moist. This warm, moist air near the surface, combined with much colder air aloft is a very unstable scenario which will result in the warm and moist air rising, creating clouds and eventually snowfall over the lake. These clouds and snow may then move inland with the wind, bringing hazardous conditions into parts of our region. This sequence of events is shown schematically in the conceptual diagram below. 
Image 1: Lake Effect Conceptual model from NWS WFO Gaylord, Michigan
The visible satellite picture below shows an example of the development of cloudiness responsible for LES.  
Image 2: Visible satellite picture of lake-induced clouds during an LES event.
LES can take different forms, depending on the length of water traversed by the arctic air, something called fetch. A shorter fetch results in numerous small bands of snow called multi-bands. A longer fetch results in fewer, stronger bands of snow, and in some cases, an intense single band.
Image 3: Multi-band LES radar image.
Image 4: Single-band LES radar image.
2.   When does Lake Effect Snow Occur? 
As described above, cold air moving over a warm body of water is required for LES. Research has found that LES develops when the air about 5,000 feet above the ground is approximately 25 degrees (F) or more colder than the surface of the lake. This difference in temperatures is typically only found in the late fall and winter months.
3.   Who is impacted by Lake Effect Snow?

Areas impacted by LES typically depend on the movement of air across the lake. In our region, the direction of the wind is the single biggest factor in determining where LES will impact. Here are the areas at the highest risk for significant snow accumulations when the wind is blowing from the:

4.   How is Lake Effect Snow Forecasted? 
The two characteristics of LES that NWS forecasters concentrate on in their forecasts are LES intensity (how much snow is going to fall) and location (where is the snow going to fall). 
The intensity of LES depends on numerous parameters, including:
·         The air-lake temperature difference (bigger difference = stronger LES).
·         Cloud microphysics (dense snow vs. fluffy snow).
·         Amount of time air resides over water (longer fetch = stronger LES).
·         Lower atmospheric wind direction (strongest LES when winds are parallel to the long axis of the lake). 
As discussed in section 3, the location of LES depends on:
·         Lower atmospheric wind speed.
·         Lower atmospheric wind direction.
NWS IWX forecasters use guidance from a variety of numerical models, in addition to personal experience and pattern recognition skills to anticipate the location of expected LES as well as expected snowfall accumulations. 
5.   What is a Lake Effect Snow Watch/Warning/Advisory? 
Lake Effect Snow Advisory
Imminent or ongoing lake effect snow expected to bring 3-5 inches of snow accumulation.

 Lake Effect Snow Watch

Possibility (in the next 12 to 48 hours) of lake effect snow accumulations of >6 inches in 12 hours or >8 inches in 24 hours. 
Lake Effect Snow Warning
Imminent or ongoing lake effect snow expected to bring >6 inches of snow accumulation in 12 hours or >8 inches of snow in 24 hours.

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