Seasons & Weather
Lake Effect Snow Needs Open Water to Work
Great Lakes shore towns get enormous snow totals from a mechanism that requires unfrozen water and a long wind path, and it shuts off once the lake ices over.
Some Great Lakes shore communities receive snow totals that have no relation to the weather a short distance inland. The mechanism is entirely local and it depends on the lake being liquid.
Cold air over warm water becomes unstable
Continental air arriving in late autumn is far colder than the lake beneath it, because water gives up its summer heat slowly.
The lowest layer of that air is warmed and moistened from below, becomes buoyant and rises, forming convective clouds along the wind path.
The larger the temperature contrast between the water surface and the air aloft, the more vigorous the overturning and the heavier the resulting snow.
The path across water sets the intensity
A longer trajectory over open water gives the air more time to collect heat and moisture, so the wind direction determines which shore receives the band.
Downwind communities can sit under a narrow, nearly stationary band producing extreme accumulation rates while a neighboring town a few miles away stays clear.
A slight shift in wind direction moves the band, which is why totals in these events vary enormously over very short distances.
Terrain finishes the job
Rising ground inland from the shore forces the air upward further, squeezing out additional moisture and concentrating the heaviest totals on the higher terrain.
That is why the largest seasonal snowfall figures in the region come from communities set back from the water on elevated ground rather than from the shoreline itself.
The same effect explains why a highway can be clear at the lake and impassable a few hundred feet higher a short drive away.
Ice cover switches it off
Once a lake develops substantial ice cover, the heat and moisture supply is cut off and the mechanism stops regardless of how cold the air is.
Seasons with limited ice cover therefore extend the snow season into late winter, while heavy ice years end it early.
This is why the snowiest period is generally the early part of winter rather than the coldest part of it.
What it means for travel on those shores
Conditions can change from clear to zero visibility within a mile of driving, and the transition is abrupt rather than gradual.
Local highway agencies issue travel advisories and close roads during the strongest bands, and those closures are the practical constraint on any winter itinerary there.
Forecasts for these events are issued as warnings for specific counties rather than broad regions, because the affected area really is that narrow.
Questions readers ask
Why is the coast windier than the forecast says?
Land forecasts are produced for typical inland exposure, and models smooth out coastal terrain. An open shoreline, a headland or a cliff top routinely experiences considerably more wind than the grid square value.
Which forecast should I use for a beach day?
The inshore waters forecast for the relevant stretch, together with a wave model and a local tide table. The general land forecast tells you about temperature and rain, and very little about what the sea will be doing.
