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Submitted by George Elliott on Sat, 08/10/2013 - 7:13am.
They’re big, potentially destructive, and more common than you might think. These systems also can produce (though not all do) tornadoes, and sometimes the biggest and most powerful tornadoes that hit. All tornadoes do not form out of these systems, but some that do can be huge. I’m speaking here of something called “Mesoscale Convectve Complexes (MCC’s, or MCC for Mesoscale Convective Complex.)
MCC’s bring some of the heaviest rainfalls of any given year across the Plains, South, and Midwest. Often, they develop along thunderstorm producing systems, including strong upper-air dynamics and low-level inflow of very warm, moist, and unstable air from the south.
The MCC generally first develops in the afternoon hours when instability is at its greatest due to daytime heating. The most dangerous time insofar as the potential to produce tornadoes occurs in the beginning stages and early life of the MCC. This is a time of greatest instability and energy exchange. As the life of the MCC goes on (and sometimes for well over 12 hours), an MCC will be less inclined to produce severe tornadic weather, but still produce torrential rainfall. In general, as an MCC, or cluster of storms becomes more numerous, the severity of the storms will decrease, but the areal coverage of precipitation and storms increases. The greatest areal coverage tends to occur in the late evening hours.
MCCs move by the way of upper-level steering currents (15,000-20,000 feet high), and often expand and spread over great areas as they move generally east in the prevailing westerlies. They are often still occurring past midnight, and you can even still detect the remains of the systems toward sunrise as they dissipate over the eastern U.S. They can dissipate by moving into an environment where the moisture, wind shear, lift and instability are no longer able to sustain the system.
In more technical terms, a mesoscale convective complex has either... 1: an area of cloud top of 38,610 miles squares or greater with temperature less than or equal to -26 degrees F, or 2: an area of cloud top of 19,305 miles squared with temperature less than or equal to -62 °F. Size definitions must be met for 6 hours or greater. Its maximum extent is defined as when cloud shield reaches maximum area..
MCCs commonly develop from the merging of thunderstorms into a squall line, which eventually meet the MCC criteria. The dynamics we talked about to form these systems also include lower and upper-level wind shear (the turning of winds in the atmosphere as you go up), and at least moderate upper-air support in the form of a trough of low pressure in the upper atmosphere.
If you want to be even more precise and technical, the actual structure of the MCC has three layers.
Near the surface, the MCC exhibits high pressure, with an outflow wind boundary, or small-scale cold front, at its leading edge. This is an area of relative high pressure, and is caused by the cooling of the air from the evaporation of rainfall (commonly referred to as a cold pool). In the mid-levels, the MCC exhibits cyclonic (counterclockwise) rotating low pressure which is warm compared to the surrounding environment (referred to as a warm core). This mid-level circulation is referred to as a Mesoscale Convective Vortex. The upper-levels contain an anti-cyclonic (clockwise) rotating high pressure, which is a sign of divergence (the spreading) of air. This air is colder relative to its surrounding environment. The combination of upper-level divergence, low-level convergence, and all the associated thermal and moisture dynamics maintains the MCC along its journey across the country.
By: George Elliott