THEWEATHERMOGIL:: Just what is an MCS, anyway?
By official National Weather Service definition, “An MCS is, “a complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.”
This now begs the question, “What is an MCC?”
According to the American Meteorological Society’s glossary, a “Mesoscale Convective Complex (MCC) is a large Mesoscale Convective System (MCS), 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 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. *Duration: Size criteria must be met for at least 6 hours. *Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.”
For practical purposes, let’s not utilize such exacting scientific distinctions. Instead, let’s just realize than an MCS is a large, organized, cluster of thunderstorms in which severe weather, but most likely excessive rainfall, is likely.
While the satellite imagery clearly shows the character of an MCS, radar data (Fig. 2) also shows its structure. The leading edge is marked by the most intense convection or thunderstorms. That is followed by a region of lighter rainfall, which is followed by another zone of heavier rainfall.
Surface weather maps (Fig. 3 and Fig. 4) clearly show the outflow boundary on the east and south sides of the MCS and also the diverging surface wind flow associated with a well-formed MCS.
The outflow boundary is expected to become stationary across Kansas and Missouri as the day unfolds. That boundary will become the focus of new thunderstorm development and allow for multiple thunderstorm transits across the region. Hence, flash flood watches have been posted (Fig. 5) for rainfall of 4 to 6 inches (with locally heavier amounts) during the next 24 to 48 hours.
© 2016 H. Michael Mogil
Originally posted 7/2/16