An overshooting top is a dome or plume of cloud that rises above the flat anvil of a thunderstorm, marking a place where a strong updraft has enough momentum to briefly carry cloud material past its equilibrium level. It sits directly above the storm's most intense updraft core and is visible from the ground, from aircraft, and on satellite imagery.
The physics of overshooting
An updraft rises as long as the parcel inside it is warmer than the surrounding air. When it reaches the tropopause, that surrounding air becomes very stable and the parcel loses its buoyancy advantage. In a weaker storm the updraft simply stops and spreads sideways into the anvil.
In a strong storm the rising air still has upward momentum when it reaches the equilibrium level. That momentum carries the parcel a short distance further, into air that is denser than it is. Once above equilibrium the parcel is negatively buoyant and rapidly decelerates, which is why overshoots tend to look like short-lived bubbles that surge and collapse rather than steady towers.
How forecasters and spotters use it
An overshooting top that persists for ten minutes or more, or one that rebuilds continuously in the same spot, points to a durable, strong updraft. That kind of updraft is the engine behind large hail, damaging winds, and, in the right shear environment, tornado potential.
On satellite, overshooting tops appear as small, bright, cold bumps on the cirrus anvil, often surrounded by warmer air where the anvil has thinned. Modern GOES imagery and infrared brightness temperature products flag them automatically, and forecasters use their location and behavior to identify the strongest cell in a cluster.
Important limits
Overshoot behavior varies with storm environment. A storm with a very high tropopause and modest CAPE can produce a modest, ragged top even while producing severe hail, and a compact storm with strong CAPE and a lower tropopause can produce dramatic overshoots without producing tornadoes. The visual is a clue about updraft strength, not a direct measurement of any ground hazard.
A collapsing overshoot can precede a downburst or a rapid intensity change at the surface. If the storm's top suddenly disappears and the anvil smooths out, that is worth noting rather than dismissing.