When a snowstorm deposits denser snow over less dense snow, rapidly creating a slab/weak layer combination.
Lucky for us, most storms deposit new snow with denser snow on the bottom and lighter snow on top—just the way we like it. This is because most snow comes from cold fonts, which usually start out warm and windy but end up cold and calm. But sometimes snowstorms deposit denser, stiffer snow on top of softer, fluffier snow. We call this “upside down” snow. We often call it “slabby” or “punchy” meaning that you punch through the surface slab into the softer snow below, making for difficult riding and trail breaking conditions. It also means that we need to carefully monitor avalanche conditions within the new snow because—by definition—a denser slab has been recently deposited on top of a weaker layer, which should make anyone’s avalanche antennae stand at attention. Most instabilities within upside-down snow stabilize within a day or two.
The kind of weather conditions that often produce upside-down snow include warm fronts, storms in which the wind blows harder at the end of the storm than the beginning, or storms that end with an unstable airmass, which can precipitate a lot of graupel within instability showers.