A multicell cluster is a group of thunderstorm cells at different stages of their lifecycle interacting within a shared area. New cells typically form along the leading edge of the shared cold pool produced by the older cells.
How the cluster sustains itself
The AMS Glossary describes a multicell convective storm as a system usually composed of a cluster of ordinary convective cells at various stages of their life cycle. New cells form either along preexisting low-level boundaries or where the system-scale cold pool from previous cells is lifting warm, moist inflow.
This self-generating behavior lets the overall system persist for many hours even though any single cell within it lives only 30 to 60 minutes. As older cells rain out and decay, fresh cells develop and take over the active precipitation and lightning production.
How to recognize it
On radar the cluster is neither a discrete cell nor a clean line. Multiple reflectivity cores of different intensities are grouped together, often with a common outflow arc visible near the surface. Storm motion is a mix of individual cell motion and propagation along the gust front.
Visually a cluster looks messy from the ground. Multiple updraft towers rise at different heights, older tops are collapsing into rain shafts, and outflow winds can be arriving from multiple directions as different cells vent.
How forecasters interpret it
The main threats from a multicell cluster are heavy rain and localized flash flooding, especially when new cells repeatedly form and move over the same area in a training-echoes pattern. Damaging outflow winds along the leading gust front are common, and hail can occur in the strongest cells.
Brief, weak tornadoes are possible if any individual cell within the cluster develops enough low-level rotation, and occasionally a discrete cell embedded in the cluster can organize into a short-lived supercell. Warning strategy focuses on identifying which cell within the mess is intensifying and where the next cell is likely to develop.