Multi-Cell Convective Clusters (MCC)
Although there are times when a thunderstorm consists of just one ordinary cell that transitions through its life cycle and dissipates without additional new cell formation, thunderstorms often form in clusters with numerous cells in various stages of development merging together.
While each individual thunderstorms cell in a multi-cell cluster behaves as a single cell, the prevailing atmospheric conditions are such that, as the first cell matures, it is carried downstream by the upper level winds and a new cell forms upwind of the previous cell to take its place.
The speed at which the entire cluster of thunderstorms move downstream can make a huge difference in the amount of rain any one place receives. There are many times where the individual cell moves downstream but additional cells form on the upwind side of the cluster and move directly over the path of the previous cell. The term for this type of pattern when viewed by radar is “training echoes”.
Multicell thunderstorms are also called “back building” thunderstorms and, with careful observation, you can count the number of individual thunderstorm cells in the cluster that pass your location. Take note of the direction from which you first hear thunder. The thunder’s volume will increase as the cell approaches your location. Then, after it passes and the volume decreases, you will hear more thunder from the next cell, increasing again, coming from the same direction as the previous cell.
Often, these storms will appear on radar to be stationary. However, if the new development is vigorous, then the thunderstorm cluster appears to move upwind.
Training thunderstorms produce tremendous rainfall over relatively small areas leading to flash flooding.
Next we’ll discuss another type of multi-cell thunderstorm, the Multi-Cell Convective System (MCS), commonly known as a “Squall Line.”