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originally posted by: mrthumpy
Scud clouds otherwise known as pannus
Pannus clouds are formed as the warmer (and often more moist) updraft of a thunderstorm lifts the relatively warm air near the surface. These clouds condense as the warm, moist air saturates through ascent and is pushed outward from the storm. Scud clouds are very commonly found on the leading edge of a storm front. In this area of a storm, scud are commonly associated with shelf clouds.
Pannus clouds may also form when an updraft ingests precipitation-cooled air from the downdraft. Scud forming in this region of the storm, if moving laterally, will tend to move inward towards the dominant updraft. Rising scud may condense and organize into a wall cloud.
Pannus clouds can often be mistaken for a developing tornado, landspout, or waterspout. The difference is determinable by observing the presence or absence of rotation (not just movement) of the scud clouds. If rotation is present, then a tornado, landspout, or waterspout is possible, and the more intense the rotation, the more likely.