What is lake-effect snow?

While the term 'lake-effect snow' may have little meaning for people who live well away from the Great Lakes, people living near the Great Lakes are used to its drastic effects on travel and daily routines.

Lake effect occurs when the air is significantly colder than the surface of the water, according to AccuWeather Senior Meteorologist Brian Wimer.

"The difference in the surface water temperature to the air about 5,000 feet above the ground generally needs to be at least 13 degrees Celsius or 26 degrees Fahrenheit to have lake-effect clouds, rain and snow to develop," Wimer said.

As the moist air from the Great Lakes is forced over the land and rises in elevation, the rain or snow is released.

Early in the autumn, the air may not be cold enough, so rain falls instead of snow.

Later in the autumn and into the winter, as air and water temperatures trend downward, all or mostly snow typically falls.

The same sort of phenomenon can occur with any large body of water, such as another lake, inland sea or the ocean. Examples of this are Great Salt Lake effect, Black Sea effect, Chesapeake Bay effect and sea effect.

However, in the United States, unless otherwise stated, that lake-effect snow refers to the snow produced over and downwind of the Great Lakes.

Typically, lake-effect snow will develop in narrow bands, sometimes called streamers. The streamers tend to align with the wind direction.

Beneath the streamer, it may snow heavily and the landscape may resemble a winter wonderland. Outside of the streamer, the sun may be out with no snow on the ground.

This is why it may be sunny and dry at your location, but a friend or family member only a few miles away may be experiencing whiteout conditions at the same time.

These streamers can extend for more than 100 miles downwind of the Great Lakes. In very rare cases, streamers may extend from Lake Superior to the mid-Atlantic and New England coasts.

However, most often, the lake-effect snow tapers off east of the Appalachians. This is because the air typically descends and dries out when winds blow from the northwest and west.

Lake-effect snow is a given during the late autumn and winter months for those around the Great Lakes. However, for newcomers to the region or those traveling through, it can be quite a shock.

During a lake-effect event, the intensity of the snow usually depends on how much colder the air is above the ground, when compared to the surface temperature of the lake.

Sometimes, a streamer may gather moisture over more than one lake. In these cases, the snowfall rate within the streamer can be quite heavy.

It is not uncommon during an outbreak of arctic air in December or January for snow to accumulate at the rate of 3-6 inches per hour where streamers linger over a local area. A typical lake-effect snow event can bring 1-2 feet of snow to localized areas.

Quite often, the streamers will shift around during a lake-effect snow event. This is because the streamers generally align with the flow of air.

A typical lake-effect snow event will begin with a west to southwest flow of air and streamers aligning accordingly. In this case, Buffalo, New York, may be hit hard during the first part of a lake-effect snow event, since the city is located on the northeastern end of Lake Erie.

As a typical lake-effect event progresses, winds tend to become more northwesterly and the streamers tend to shift southward. So even though it may get rapidly colder at some locations without any snow to start, heavy snow may yet arrive as cold air continues to flow across the Great Lakes.

There are some cases where winds fail to swing around and bands of snow may persist not only for hours, but days. It is in these situations where feet of snow may pile up, such as the case in Buffalo during the middle of November 2014. Up to 88 inches of snow fell from Nov. 15-21. Cars were buried and people were stranded for days after the record lake-effect event.

In some cases, on average of one to three events per winter, cold air may rush in so violently to trigger snow squalls, or the winter equivalent of summer thunderstorms.

In the case of lake-effect snow squalls, there can be thunder and lightning as well as blizzard conditions. The visibility can transition from unlimited to near zero, and roads can become snowcovered in a matter of seconds, hence the concern for unsuspecting motorists traveling the highways.

Interestingly, lake-effect snow can fall when the lake surface is completely covered in ice. This is because a frozen lake is relatively smooth, when compared to the hilly terrain of the land. In this case, it is the effect of friction that causes some lake-effect snow to fall over the land. However, lake-effect snow in this case is often light and spotty, when compared to when the lakes are completely free of ice.

Lake effect is much more likely to occur during the autumn, instead of the spring, since the air generally becomes warmer than the surface of the lake.

Download the free AccuWeather app to see if lake-effect snow is forecast for your area.

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