Mesoscale Convective Complex

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MCC moving through New England: August 2, 2006 0600 UTC

A Mesoscale Convective Complex (MCC) is a large Mesoscale Convective System, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

Size: Area of cloud top -32°C or less: 100,000 km² or more, and area of cloud top -52°C or less: 50,000 km² or more
Duration: Size criteria must be met for at least 6 hours
Eccentricity: Minor/major axis at least 0.7^[1]

The amount of moisture needed in the Precipitable water column tends to equal or exceed 1.40"/35.5 mm. ^[2]

MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms which expand in scale due to extremely divergent flow aloft, such as near the split in an upper level jet stream. Early on in its life cycle, the potential for severe weather is greatest. As the system matures, a stratiform cloud layer with embedded heavy rain forms behind the leading thunderstorms. During peak intensity, the primary threat shifts toward heavy rain and flooding.

In surface weather analysis, MCCs are depicted as a large outflow boundary at leading edge of a strong mesoscale high pressure system, with large three hour pressure rises and falls noted throughout the region.