ANSWERS PAGE


Temperature & Synoptic Conditions

Water has a higher capacity to hold heat than air does.  Because of this, in the early winter months lakes tend to be warmer than the air around them.  This is especially true for the Great Lakes region as cold air moves over the area.  When the cold polar or arctic air moves over a warmer water surface, the cold air is heated and moistened.  This process makes the air very unstable and helps to generate convective snow bands downwind of a body of water.  In looking at the image with the pressure contours, you should be able to observe a high pressure system (cold air mass) situated to the northwest of the Great Lakes and a low pressure system is situated to the northeast.  These conditions are favorable for lake effect snow.  The flow vectors show that the main zonal flow over the western Great Lakes region is NW and W over the eastern Great Lakes Region.  This flow creates cold air advection over a warmer lake, increasing the potential for the formation of lake effect snow bands.
Temperature Page



Instability

A main contributor to lake effect snow is the unstable air generated by the warmer temperatures over the lakes compared to the colder air over land during the winter season.  This instability in the boundary layer (the bottom layer of the troposphere that is in contact with the surface of the earth) helps to enhance mesoscale snow production and convection.  Only large bodies of water, such as the Great Lakes, can generate enough instability to produce lake effect snow.  Normally, the temperature difference between the water surface and 850mb needs to be about 13°C to support convective lake effect snow development in the Great Lakes.  In heavy snowfall events, the boundary layer can be very deep, up to 700mb where a strong cap is present.  A cap is a region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent.  This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC.  Most of the ingredients needed to create lake effect snow happen below 500mb.
Instability Page


Convection

Convective snow bands develop and are enhanced by frictional convergence and orographic lifting over and inland of the leeward shore. The vertical extent of the convection is limited by the stable capping inversion (mentioned above) that is typically 1-4 km above the surface, making lake effect convection shallower than that normally associated with thunderstorms.  The formation of convective snow bands, with stronger vertical motions and enhanced precipitation production, results in limited areas of intense snowfall.
Convection Page



Fetch

Fetch (the red lines in the previous image) is the distance that cold polar air travels over the relatively warm water, and is very important in the formation of lake effect snow bands. The minimum threshold for flurries is about 80 km, but a minimum fetch of 160 km is generally needed to produce significant lake effect snowfall.  The red distances are the fetch for Lake Huron and Lake Ontario, since the winds are out of the west. 
Other Factors Page


Wind Speed

Wind plays a key role in generating snow bands.  The optimal wind speed near the surface (up to 850mb) for lake effect snow is 15-20 knots.  Land breeze circulations will dominate when wind speeds are less than this, preventing intense snow bands from forming.  Also, snow bands are unlikely to form with wind speeds more than 20 knots.  Wind shear is another important factor since the formation of intense snow bands is most likely when the low-level wind direction is less than 30 degrees within the mixed layer. 
Other Factors Page



Humidity/Cloud Microphysics
Humidity plays a very important role in the formation of lake effect snow as well.  In heavy lake effect snow events, the pre-event 850mb dewpoints are usually high compared to lighter snowfall events.  This is because high humidity in the boundary layer causes rimming (supersaturation) to occur in clouds.  Supersaturation is the condition existing in a given portion of the atmosphere when the relative humidity is greater than 100%, that is, when it contains more water vapor than is needed to produce saturation with respect to a plane surface of pure water or pure ice. Heavy snow events are more favorable in environments where dendrites, which are ice crystals that form near -15 Celsius, and rimming are present. 
Other Factors Page



Snow Bands

Lake effect snow bands develop parallel to the prevailing wind flow in the mixed layer.  For the most part, these snow bands occur as a large single band or an area with multiple small bands.  Single bands usually are associated with heavy snowfall over a small region and occur when the prevailing winds are parallel with an elliptically shaped lake, making for a long fetch.  In this case Oswego, NY (east of Lake Ontario) was hit with a single band which left some surrounding areas with over 70 inches of snow.  The other main type of lake effect snow, multiple bands, are associated with light snowfall over a large area and occur when the flow is perpendicular to an elliptically shaped lake, making for a short fetch.  Since Lake Huron is not very elliptical, and the fetch over the lake is shorter, many areas in Ontario received some lighter lake effect snow showers compared to what they received in Oswego, NY.
Snow Bands Page

                                                                  
References and Other Educational Sites
"Topics in Lake Effect Snow Forecasting." COMET/UCAR. 2005. http://meted.ucar.edu/norlat/snow/lake_effect

Bikos, Dan and Weaver, John.  "Lake Effect Snow."  Virtual Institute for Satellite Integration Training.  2005.  http://rammb.cira.colostate.edu/visit/les/title.asp

American Meteorological Society. Glossary of Meteorology Online.  Allen Press.  2005.  http://amsglossary.allenpress.com/glossary


    
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