The same winter storm can deliver a nuisance to one town and a major disruption to another an hour away. The difference is the position of a boundary that depends on temperatures throughout a deep layer of atmosphere.
Why the surface temperature is not decisive
Snow forms high in the cloud and must survive the entire descent to reach the ground intact, which means every layer it passes through matters.
A shallow warm layer aloft can melt falling snow into rain even when the surface is below freezing, and a cold surface layer can leave sleet or freezing rain instead.
Forecasting precipitation type therefore requires predicting the vertical temperature profile correctly, not just the reading at ground level.
What makes the boundary so sharp
Melting snow absorbs heat from the air around it, which cools that air toward freezing and helps the snow that follows survive the same descent.
The process reinforces itself, so once the transition begins in one place it tends to hold there and the changeover zone stays narrow rather than spreading gradually.
The result is a boundary that can be a matter of miles wide, with heavy snow on one side and plain rain on the other.
How terrain anchors it in place
Temperature falls with elevation, so higher ground frequently sits in the snow while nearby valleys sit in the rain during the same hour.
Cold air is dense and drains into valleys, and it can remain trapped there beneath warmer air arriving above, which reverses the expected pattern.
Water bodies add their own influence, since air crossing a lake that has not frozen arrives warmer than the air over the surrounding land.
Why forecasts change so close to the event
Models predict the position of the boundary with an inherent uncertainty of tens of miles, which is small in meteorological terms and decisive for any single town.
A change of a degree or two in a layer a few thousand feet up shifts the line enough to reverse the outcome for a large population.
This is why accumulation forecasts are revised close to the storm and why the range quoted for a city is often wide until the last hours.
Why the worst impacts cluster at the line
Precipitation near the boundary frequently falls as a mixture, and freezing rain within that zone produces the most damaging outcome of any winter precipitation type.
Heavy wet snow also concentrates just on the cold side, where snow near the melting point carries far more water and weighs far more per inch.
Power failures and tree damage therefore concentrate along a narrow band rather than where the deepest snow totals occur.