Lake effect snow produces some of the sharpest gradients in weather, with heavy accumulation in one community and dry pavement a short drive away. The narrowness is a consequence of how the snow is generated.
What the lake supplies
A lake that has not frozen holds heat well into the cold season, because water releases heat slowly compared with land.
Cold air moving across that surface is warmed from below and takes up moisture, which makes it buoyant and unstable relative to the air above it.
The rising moist air forms clouds and precipitation, so the lake acts as a continuous source rather than the system being a passing storm.
Why the temperature difference matters most
The strength of the transfer depends on the contrast between the water surface and the air passing over it, and a larger contrast drives more vigorous convection.
This is why the heaviest events occur when very cold air arrives over relatively warm water, typically early in the cold season before the lake has cooled.
Ice cover shuts the process down by separating the air from the water, which is why activity declines sharply once a lake freezes over.
How fetch organizes the snow into bands
The distance air travels over open water, called fetch, determines how much heat and moisture it collects before reaching the far shore.
Wind aligned along the long axis of a lake gives the longest fetch and organizes the convection into a single intense band rather than scattered showers.
That band can be only a few miles wide while extending far inland, which is the source of the extreme differences in accumulation over short distances.
Why small wind shifts move everything
The band's position is set by the wind direction over the lake, and a shift of a few degrees pivots the whole band across the shoreline.
A community under the band for several hours can accumulate rapidly, then see the snow stop entirely as the band moves to a neighboring county.
Forecasting the band's placement is consequently harder than forecasting its intensity, and warnings often cover a wider area than will ultimately be affected.
What terrain adds after landfall
Air moving inland encounters rising ground, which forces additional lifting and enhances snowfall on the upslope side.
Elevated terrain downwind of a lake therefore records the highest totals, well beyond what the shoreline itself receives.
Further inland the air loses its moisture supply and stabilizes, so bands weaken with distance even where the terrain continues to rise.