Compiled by Copy Editor Susan Johnson
Lake-effect snow is produced during cooler atmospheric conditions when cold winds move across long expanses of warmer lake water, providing energy and picking up water vapor, which freezes and is deposited on leeward shores. The same effect occurs over bodies of salt water, producing ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic (relating to mountains) influence of higher elevations on the downwind shores. This uplifting can produce narrow, but intense, bands of precipitation, depositing many inches of snow each hour. This often results in large snowfall totals.
The areas affected by lake-effect snow are called snowbelts. This effect is found in many locations around the world, but is best known in populated areas of the Great Lakes of North America, and especially central and western New York, northwestern Pennsylvania, northeastern Ohio, southwestern and central Ontario, northeastern Illinois (along the shoreline of Lake Michigan), northwestern and north-central Indiana.
Under certain conditions, strong winds can accompany lake-effect snows, creating blizzard-like conditions. But the duration of this event is often a little less than that required for a blizzard warning in both the U.S. and Canada. If the air temperature is not low enough to keep the precipitation frozen, it falls as lake-effect rain. For lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface).
Lake-effect snow is produced as cold winds blow clouds over warm waters. Several key elements are required to form lake-effect precipitation. These determine its characteristics: instability, fetch (the distance an airmass travels over a body of water), wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover.
As a lake gradually freezes over, its ability to produce lake-effect precipitation decreases for two reasons. First, the open ice-free liquid surface area of the lake shrinks, which reduces fetch distances. Second, the water temperature nears freezing, reducing overall latent heat energy available to produce squalls. A complete freeze is often not necessary to end production of lake-effect precipitation.
Even when precipitation is not produced, cold air passing over warmer water may produce cloud cover. Fast-moving mid-latitude cyclones (Alberta clippers) often cross the Great Lakes. After a cold front passes, winds tend to switch to the northwest, and often a long-lasting low-pressure area will form over the Canadian Maritimes, which may pull cold northwestern air across the Great Lakes for a week or more.
The southern and eastern shores of the Great Lakes typically receive heavy snowfall each winter, especially from late November to early January. This lake-effect snow may lead to large regional differences. Researcher B. Geerts noted, “For instance, 50 cm of snow may accumulate over the course of a few days near the shore, and 50 km from the lake shore the ground may be bare. Lake-effect snow occurs elsewhere as well, e.g., near Lake Baikal in Russia, but nowhere is it so pronounced and has it such an effect on ground and air transportation.
“The local maxima in snowfall are not due to the proximity of mountains or an ocean. The difference is not because the southern and eastern shores are cooler than the surroundings; in fact, they are slightly warmer than the other shores. Snowfall typically occurs in this area after the passage of a cold front, when synoptic factors are not conducive to precipitation.”
Meteorologist Brian Edwards wrote for AccuWeather.com on Jan. 8, 2014, that with the arctic air still hanging over the Great Lakes, snow would continue to pile up in the snowbelts downwind of the Great Lakes. According to NOAA, ice coverage throughout the Great Lakes is limited to mostly coastal locations, and the lack of ice would lead to blinding snow developing downwind of the lakes. Some locations to the east and southeast of the Great Lakes could receive more than 3 feet of snow.
From the Feb. 19-25, 2014, issue