What is a ‘mesoscale convective system’ (MCS)?

Dear Tom,

What is a mesoscale convective system? It was mentioned in a recent graphic in the Tribune as a “potential wild card.” Why was that?
— Joy Hajduk-DeGraff
Dear Joy,
A mesoscale convective system (MCS) is a large, organized cluster of thunderstorms that tend to become prolific rain, wind and lightning producers. They often form in the central U.S. after sunset, reaching their peak intensity overnight. Movement is frequently east at the onset, trending southeast as the cluster matures with overall weakening after sunrise. An MCS tends to form on the north edge of a hot air mass in an area of strong low-level winds, referred to as the “low-level jet.” It is often dubbed a “wild card” because the remnant cloudiness after the storm cluster dissipates can be a “forecast buster,” holding temperatures in the 70s and 80s when 90-degree-plus readings were expected.