A satellite image shows clouds as frigid air pours over the Great Lakes region Wednesday. (CIMSS/SSEC/University of Wisconsin)

If you’re even a casual follower of weather news, the term “lake-effect snow” probably conjures images of snow measured in feet, not inches, covering cars and hiding homes in places such as Rochester, N.Y.; Cleveland; and Michigan’s Upper Peninsula.

The National Weather Service says lake-effect snow accounts for 30 to 50 percent of the annual snowfall on the eastern and southern shores of the Great Lakes, a region famous for the large amounts of snow that falls on it.

This snow falls from bands of clouds that average roughly 10 miles wide and maybe 300 or more miles long. Amounts of snow vary across a region, with the most snow usually falling on hills inland from the lakes.

Thanks to lake-effect snow, cities on or close to the Great Lakes such as Syracuse, Rochester and Buffalo in New York, plus Erie, Pa.; Cleveland; Grand Rapids, Mich.; and Duluth, Minn., are among the snowiest large U.S. cities.

This week, the Weather Service hoisted blizzard warnings for parts of western New York because of an onslaught of lake-effect snow and high winds. The lake-effect snow bands dispensed as much as 19 inches in Buffalo where winds gusted over 40 mph creating whiteout conditions. As temperatures plunged to zero and wind chills below minus-20, the Weather Service described a “particularly dangerous situation."

Lake-effect snow forms when frigid air flows over relatively warm water. Its formation requires that temperature 5,000 feet above the water must be at least 25 degrees colder than the lake’s water. This cold air needs to blow at least 60 miles over the warmer water to produce significant snow.

(AMS Weather Book)

Temperatures aloft that are much more than 25 degrees lower than the water can sometimes add enough energy to create thundersnow. Thundersnow can be especially intense, sometimes falling at a clip of two to three inches per hour or more.

Lake-effect snow often falls in waves with intermittent gaps in snowfall. The relatively clear air between lake-effect snow bands forms when air that rose to create the snow bands sinks, which warms the air and stifles cloud formation.

A good deal of the early winter lake-effect snow that falls on Buffalo and neighboring Niagara Falls occurs when westerly winds blow across Lake Erie. When the lake freezes over, it cuts off that source of lake-effect snow.

Some large Great Lakes cities, including Detroit, Toledo, Milwaukee and Chicago, receive little lake-effect snow because they are on the west side of their lake, and the prevailing winds are from the west.

The heaviest lake-effect snow in the United States falls on the Tug Hill Plateau, south of Watertown, N.Y., at the eastern end of Lake Ontario, making this region the snowiest place in the United States, except for some Western mountains.

Syracuse, which is south of Tug Hill, is one of the snowiest big cities in the United States.

Unlike the relatively shallow Lake Erie (201 feet at its deepest) the 802-foot-deep Lake Ontario stays warmer than Erie. In addition, water from all of the other Great Lakes flows past Buffalo, over Niagara Falls and into Lake Ontario all year. This continuous water movement also helps keep the lake from freezing.

The Weather Service says that about half of the annual average of 90 inches of snow that falls on places along or near the southern and eastern shores of Lake Ontario each year is lake-effect snow.

Forecasting lake-effect snow

As with most kinds of forecasting, advances in forecasting computer models, weather radar and satellites are helping forecasters make better lake-effect snow forecasts, said Michael Fries, a forecaster at the Buffalo office of the Weather Service.

For example, he said, dual-polarized weather radar that the Weather Service has installed nationwide shows forecasters where rain, snow, sleet and big or small snow crystals are falling.

He said improvements in weather satellites give faster updates of cloud bands and show where the clear air between cloud bands is located.

In addition, he said, the Buffalo office is now receiving Canadian weather radar images, which show what’s heading for the United States.

These improvements all helped the Buffalo forecasters to produce graphics showing forecast updates as a snow band with more than six inches of snow moved across the city during rush hour one day before Christmas.

“All in all, we’re making our forecasts more specific,” Fries said.

Lake-effect snow and the Mid-Atlantic region

Fortunately for Washington-area skiers, lake-effect snow from Lake Erie and sometimes Lake Michigan reaches the ski areas closest to Washington in West Virginia, Western Maryland and southern Pennsylvania but usually not Washington and its suburbs.

“Occasionally 'popcorn’ lake-effect snow clouds make it over the Blue Ridge west of Washington, but 98 percent of the snow falls on the west side of the Allegheny mountains,” said John Hopewell, earth science specialist for the Capital Weather Gang.

Hopewell, who is an avid skier, said lake-effect snow often makes the drive to the region’s ski areas worthwhile. Sometimes lake-effect snow occurs over the course of several days, and the snow totals go well into the double digits.

Ski resorts west of the Appalachian Mountains tend to get much more snow than resorts on the eastern slopes because lake-effect snow bands are intercepted before crossing over the mountains. Shown are average snowfall amounts at Mid-Atlantic ski areas, which illustrate the clear disparity in amounts west vs. east. (Jordan Tessler)

Lake-effect moisture reaching the ski areas arrives in bands of snow clouds and invisible water vapor injected into the air by the lake-effect mechanism, Hopewell said.

“I am a big fan of lake-effect snow,” said Shawn Cassell, public relations manager at the Snowshoe Mountain ski resort in West Virginia. “People tend to picture very heavy wet snow when they think about lake effect, but due to our high elevation (4,848 feet above sea level), here along the western escarpment of the Alleghenies, we tend to see a much lighter, more powdery snow, which is great for skiing."