depend on the character and interaction of buoyancy (CAPE) and vertical wind shear (VWS).
The strength of the MCS as well as the degree of organization increases with higher magnitudes of vertical wind shear.
bow echoes are typically associated with shallow vertical wind shear profiles, typically 2-3 km above ground level (AGL)
supercells have deeper wind shear profiles of 4 - 6 km AGL
the surface cold pool is the most significant unifying form among the many individual cells of the MCS
occur in the United States during the spring and summer months
area of cloud top with temperature less than or equal to -32 degrees C is 100,000 sq km or greater - satellite view
(area of the state of Iowa is 144,701 sq km)
and, area of cloud top with temperature less than or equal to -52 degrees C is 50,000 sq km or greater
size definitions met for at least 6 hours
can have life-times of twelve hours or more
slow moving
large area of stratiform precipitation
different convective storm structures occur under the enormous anvil shield
most often observed at night, in environments that include a stable boundary layer (decreases surface effect on the nocturnal LLJ)
more dependent than other MCS on interactions with large scale forcing
the heavy rainfall from MCCs account for a significant portion of the precipitation during the warm season in the United States
Genesis and Evolution
four life-time stages
genesis
genesis is in conjunction with a weak, large-scale frontal zone and an eastward progression of a weak high pressure ridge
nocturnal LLJ plays a role in the genesis as well
formation typically begins in the afternoon as scattered thunderstorms that organize, usually overnight, in the presence of wind shear
development
commonly develop from the merging of isolated or clusters of thunderstorms into a squall line
strong warm air advection into the formation environment by a southerly nocturnal low-level jet
strong moisture advection which increases the relative humidity of the formation environment
convergence of air near the surface and divergence of air aloft
the probability for severe weather is highest in the early stages of formation, during the afternoon.
mature
its mature and strongest stage occurs overnight and into the early morning in which the rainfall is characterized as stratiform, the MCC has reached its largest extent
dissipating
dissipation commonly occurs around sunrise, as the LLJ diminishes
The structure of a mature MCC can be separated into three layers
in the boundary layer, the MCC exhibits high pressure associated with the cold pool, with an outflow boundary, or mesoscale cold front, at its leading edge.
In the mid-levels (mid-troposphere), the MCC exhibits a cyclonic (anti-clockwise) 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 Mid-level Cyclonic Vortex (MCV).
The upper-levels contain an anti-cyclonic (clockwise) rotating high pressure which is a sign of divergence of air. This high pressure is colder relative to its surrounding environment. This divergence at upper-levels, and convergence of air at the surface along the cool pool's outflow boundary, results in rising motion which thus aids maintenance of the MCC.
the MCV can survive in weak ambient atmospheric conditions and cause subsequent initiation of convection 12 hours later.
July 1977 in western Pennsylvania, an MCC resulted in heavy rainfall which led to the disastrous flooding of Johnstown, Pennsylvania due to the foot of rain that fell from the large system. The complex was tracked 96 hours back to South Dakota