When a snowstorm deposits denser snow over less dense snow, creating a slab/weak layer combination.
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Upside-Down Storm:

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.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab