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OUT OF THIS WORLD | Science Behind the Weather

How the Great Lakes contribute to dangerous wintry weather


Scott Sutherland
Meteorologist/Science Writer

Tuesday, November 7, 2017, 2:59 PM - Get ready, southwestern Ontario! A wintry mix of weather, mid-week, is going to set up the perfect ingredients for bands of lake effect snow to develop. Here's the recipe for whipping up this weather phenomenon.

With a low pressure centre moving in to southern Ontario by Wednesday, bringing a mix of rain, freezing rain and even snow across parts of the southwest, Weather Network meteorologists are not only watching for the timing of this system, but also its aftermath.

In the wake of this wintry mix, conditions are expected that will be perfect for the development of lake effect snow bands. But what exactly are these ingredients, and why do they matter?

Water

The very first ingredient for lake effect snow is the lake, or specifically the water in the lake. Even more specifically, though, is that we need warm lake water - at least warm enough that the surface remains liquid, but the warmer the 'better'.

Right now, lake water temperatures are well above average for this time of year. They're not record high - that was 2016 - but they're still warm for early November.


Lake Huron average surface water temperatures for 2017, compared to the average for 1992-2016. Credit: NOAA GLERL

According to NOAA's Great Lakes Environmental Research Laboratory (GLERL), the average water temperature across Lake Huron on November 6 was 11.5oC, or roughly 2 degrees Celsius warmer than the 1992-2016 average. This will mean a lot more as we get farther into the season, and into winter, as it will determine how long it takes the lake waters to freeze over. For the moment, it's sufficiently warm to set up the scenario for lake effect snow.

So, why is warm lake water so important when we're talking about cold snow forming? Partly this is because the water acts as a source of water vapour evaporating into the air, so if the surface freezes over, that source is cut off. For a complete answer to this question, however, we need to take a look at the next ingredient.

Wind

Paired with warm lake water, we need wind blowing over that water, specifically a cold wind, with an air temperature below freezing, and at least 13 degrees colder than the water temperature.

This temperature difference provides a strong impulse for the warm, moist air just above the water's surface to rapidly rise, and produce moisture-laden clouds that the winds can then carry over land.

Just having cold winds over warm waters isn't quite enough, though. The winds must satisfy two more requirements. The first is that there must be low directional shear.

Low Shear

In meteorology, directional shear is a term that describes how the wind direction changes between the surface and a point higher up in the atmosphere.

The more the winds are lined up with one another with height (low directional shear), the more 'focused' the weather conditions are, delivering energy and moisture from the water directly into the clouds, the stronger the potential for snow squalls.

Now, if we have warm lake water, cold air at least 13 degrees cooler than the water blowing over the lake, and those winds are well-aligned with height, we need only one more component: Sufficient fetch.

Fetch

Fetch is simply the straight-line distance of lake water that the wind blows over, and it basically defines exactly how much water the winds can pick up to turn into clouds and snow.

If there's only a short fetch, such as when winds are blowing from west to east across the southern part of Lake Huron (between White Rock, MI and Goderich, ON, for example), snow squalls are unlikely. Move just 40 km or so further to the north, however, to Point Clark, where the westerly winds can also tap into Saginaw Bay as a source, and the chance increases.

Give the winds an even longer fetch, and they tap into more water vapour. This is especially true when they're blowing from the northwest or north-northwest across Lake Huron and Georgian Bay, or from the west-southwest up the long axis of Lake Erie or Lake Ontario. As long as the other components are present, you are virtually guaranteed snow squalls, and the longer the fetch, the most persistent the squalls will be, and the more snow they will dump on the ground.


Some typical wind directions across the eastern Great Lakes that produce lake effect snow squalls. Credit: Google Earth/Scott Sutherland

Watch Below: Weather expert Chris St Clair gives us another look at how lake effect squalls develop.




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