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Londoners know it when they see it, lake-effect snow closing in and blanketing the region. Bands of intense snow coming off Lake Huron cancel school buses and dash travel plans each winter, often with little warning. Our Jennifer Bieman reports on the science behind the streamers.
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What is lake-effect snow?
The Great Lakes warm up in the summer and take a while to cool down and freeze in the fall and winter. The lakes are not balmy in any sense of the word – hovering around 4 C on average in Lake Huron in December – but they’re unfrozen and warmer than the air temperature.
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Enter chilly winds blasting in from the Arctic. The cold air masses sweep across the lake, picking up moisture from the comparatively warm water as they hurdle into the region.
The saturated air then gives way, dropping precipitation that turns into snow or freezing rain in the cold Southwestern Ontario air. Snow squalls happen when the winds line up just right, dumping a significant amount of snow in an area.
Why is Lake Huron the main culprit in the London region?
Lake Huron is a large body of water and in the direct path of Arctic air flowing into Southwestern Ontario, Environment Canada warning preparedness meteorologist Geoff Coulson said.
In Toronto, east winds coming across Lake Ontario are the culprit for their lake-effect snow, Coulson said. Lake-effect snow does happen on Lake Erie too, Coulson said, but Lake Huron is usually the cause of the big lake-effect snow events in the London region.
“We don’t tend to get east winds as often as we get west or northwest winds. These winds tend to happen more frequently over the course of the fall and winter,” he said. “Our traditional snowbelt areas tend to be to the lee of Lake Huron and Georgian Bay.”
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Why is lake-effect snow unpredictable?
There are a lot of variables at play in lake-effect snow, from water temperature to air temperature and wind direction, Coulson said.
If you’ve got really warm water, very chilly air and winds that line up in just the right way, you’ve got a snow squall. Any slight deviations in that formula will net different results.
“The winds at the low levels have to line up. If there’s any wind shear, or changes in wind direction, that tends to spread out the lake-effect snow and we get a general snowfall, not intense bands,” Coulson said.
Those intense bands can throw people for a loop, he said.
“It could be brilliantly sunny where you are, but you could be driving en route through one of these snow bands,” Coulson said, recommending people consult weather radar to see where the bands are setting up.
What lessens lake-effect snow?
A frozen lake can do the trick, but this doesn’t usually happen until later in the winter, if at all, Coulson said.
“The water temperature gets colder, eventually ice starts to form, and that inhibits, somewhat, the development of lake-effect snow,” he said.
“Usually our prime-time for the biggest lake-effect snow events tends to be anywhere from the second half of November, through December and at least into the first part of January. But in milder winters, we can get lake-effect snow the whole winter.”
Low-level winds typically weaken as the winter draws on, Coulson said, also inhibiting the development of snow squalls.
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