A listener asked: “When the humidity is high are the chances for rain in the evening better?” Unfortunately, high humidity doesn’t necessarily mean rain! The high humidity levels are generally a product of previous rain events. In addition to the humidity, however, generally some sort of instability in the atmosphere is also required to allow for storm development. Sometimes the most uncomfortable weather can occur when there is a cap in the atmosphere that blocks storm development, but traps hot, humid, stagnant air at the surface. These systems can persist for days, until the cap erodes or a frontal system displaces the air mass.
Mary Knapp, Weather Data Library