Lake-effect snow is a meteorological phenomenon that occurs when a mass of cold air (typically Arctic or polar continental) crosses a relatively warm water surface (large lake or sea), absorbing heat and moisture that trigger intense convective bands of snow, rain, or showers concentrated on the downwind shore.
Mechanism
When the temperature difference between the water surface and the air at 850 hPa exceeds 13 °C, the instability generated is sufficient to produce vigorous convection. The cold air warms and loads with moisture as it crosses the water, forming cumulonimbus organised in bands parallel to the wind. These bands deliver extremely intense and localised precipitation upon reaching the opposite shore.
The classic example is the Great Lakes of North America, where cities like Buffalo or Cleveland receive snow accumulations of 1-2 metres in 24-48 hours. In Europe, similar phenomena occur in Scandinavia (Baltic Sea), the Black Sea, and on a smaller scale in the Mediterranean: cold continental air crossing the Adriatic or Tyrrhenian Sea generates intense showers on the Italian coasts.
In Spain, an analogous mechanism is observed when cold air masses from the north cross the relatively warm Bay of Biscay and generate intense snow showers on the Cantabrian coast and western Pyrenees. See also: convective precipitation, powder snow.