People often associate humid conditions with severe thunderstorms. There is a reason for this: humidity provides the explosive energy needed to make storms grow rapidly. This energy is known as latent heat, and it appears when lots of water vapor is suddenly cooled to the point of condensation at the top of a column of rising air. However, humidity alone does not guarantee bad storms.
Instability is the tendency for air to rise and is based on the difference in temperature and humidity between the lower and upper atmosphere. The other key ingredient is wind shear, the differences in wind speed and/or wind direction between upper and lower atmosphere. The presence of humidity can be sensed by just stepping outside. The other two require knowledge of measurements made thousands of feet up in the air by weather balloons and the movements of these sets of conditions by computer atmospheric models.