Tuesdays severe weather a Mesoscale storm complex 

Posted: 5:59 am Wednesday, March 22nd, 2017

By Kirk Mellish

Last evenings MCC or mesoscale convective complex (a unique thunderstorm system) originated in West Tennessee and points west as you’ll see below. It headed southeast at  50 mph into Georgia and the metro Atlanta area, as expected hitting hardest north of the perimeter and weakening rapidly as it moved past I-20 and fell apart in the south suburbs. 60+mph wind gusts brought hit and miss damage and scattered hail.

MCC stands for Mesoscale Convective Complex. An MCC is a grouping of storms that is defined by characteristics on infrared satellite imagery. They bring a significant bulk of precipitation events that occur across the eastern United States, especially over the plains, southeast U.S. and Midwest. They are very important precipitation events for irrigating crops. They often begin as a thunderstorm outbreak along a lifting mechanism such as upper level dynamics from a low or a low level convergence boundary. The MCC generally first develops in the afternoon hours when instability is at its greatest due to daytime heating. The greatest severe weather that occurs with an MCC tends to occur in the developing stages since each individual storm has more energy and instability. As the cluster of storms becomes more numerous the severity of the storms will often decrease but the areal coverage of precipitation and storms increases. The greatest areal coverage tends to occur in the late evening hours. MCCs are carried with the upper level winds generally toward the east. They can remain organized and travel over great distances before the storm complex dissipates. It is often after midnight before MCCs will dissipate. They can dissipate by moving into an environment where the moisture, wind shear, lift and instability are no longer able to sustain the system.

There are three characteristics that are used to define whether of storm complex is an MCC. On of the characteristics is the cloud top temperatures on infrared satellite imagery. Cloud top temperatures of -32 C or less need to cover an area of 100,000 km^2 or greater along with the coldest cloud top temperatures that are less than or equal to -52 C covering an area of at least 50,000 km^2. The second characteristic is that the shape of the cold cloud tops of the complex needs to be circular or nearly circular. This is to differentiate MCCs from squall lines and other storm complexes. The length (shortest length) of the MCC must to at least 70% of the width (longest length perpendicular to shorter length). Thus, an MCC will have an elliptical or circular type shape. Third, the MCC needs to last at least 6 hours.

Related to the MCC is the Derecho which this system brought a weaker version of to the area, but looks like it came up short of the classic definition because the strongest winds were not continuous nor as widespread.

Thanks to the fast movement the rain amounts were not excessive in most of the metro averaging half to one inch.




You can see how the storm complex had its origin West of Tennessee and in West Tennessee before dropping southeast following the upper-air flow and feeding into the record warm and humid air over Georgia.  The high in Atlanta was 86, tying the old record of 86 set in 1907.





I’ve often called the MCS\MCC the “in-land equivalent” of a tropical storm or hurricane, although they have no eye and are cold core systems not warm aloft like their tropical counterpart.




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Read more about this and other storm types here.

A Scientific forecasting look here.