Interesting Weather Information

Wednesday, March 20, 2013

Thunderstorm Primer - Part 5 - Mesoscale Convective Systems - The Mesoscale Convective Complex (MCC)

In this post I am writing about the Mesoscale Convective Complex or MCC. In Parts 2 - 4 of  The Thunderstorm Primer I covered linear mesoscale convective features, the squall line, the bow echo and the line echo wave pattern. Now  we talk about a cluster, a glob, a blob or a collection but not a random hodgepodge of thunderstorms.

The image below is an MCC from June 20, 2007 at 6:15 UTC (2:15 AM EDT) over the southern Great Plains. It shows the typical shape and characteristics of an MCC.

The definition of a mesoscale convective complex stipulates that the cloud top area with temperatures <= -32°C (-25°F) is 100,000 km²  (38,610 mi²) or greater. Alternately the area of cloud tops with temperatures <= -52 °C is 50,000 km² (19,305mi²).  Either of these conditions must persist for 6 hours or longer.  Its eccentricity (the ratio of the length of the minor axis to the major axis) is greater than or equal to 0.7 at maximum extent. If the eccentricity = 1 the area is a circle. This prevents overlap between MCCs and Linear MCSs.

MCCs most often form north of an west-to-east or northwest-to-southeast oriented warm front and follow the winds aloft along the front heading generally eastward. Near the end of the MCC's life many abruptly turn right (southward) and weaken quickly. This makes for a very difficult forecast situation because there is no warning that the turn will happen until it starts. 

In the High Plains "orogenic" MCCs often form late in the day along the slopes of the Rocky Mountains. When a strong low-level jet carries moist air northward during the heat of the day thunderstorms grow above the east slopes of the Rockies.  As they grow then begin to work cooperatively and become a unified system.

The thunderstorms become a cluster of cooperating cells and as the sun sets the MCC moves east, away from the mountains. The MCC moves all night and usually dissipates around dawn.  The remaining cool pool of air and associated left-over circulation from the MCC often serve to generate new thunderstorms later in the day. The cold pool is formed by evaporative cooling of the some of the heavy rain.

MCC May 12, 2010 13:15 UTC (9:15 AM EDT) Below is a radar loop for the same MCC

Surface map with a warm front from a low in the Panhandle of Texas 
to the Atlantic Ocean,  May 12, 2010 13z. The warm front acts as 
"railroad tracks"for the MCC. Plotted and analyzed with 
Digital Atmosphere,

Strong, broad flow of tropical moisture laden air northward to the warm front 
which is feeding the MCC. This is the 850 hPa level, roughly a mile up 
May 12, 2010, 12Z.

Next stop in The Thunderstorm Primer - the nadir of thunderstorm development - Supercells.

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