Sudden showers can occur in strong (or even weak) convection, and might intensify with surprising quickness. They may be forecast, but often are not. Unusually early cumulus development warns the perceptive pilot to anticipate eventual over-development. Big clouds growing elsewhere, especially upwind or above higher terrain, generally suggest a probability of more to come where you are. Another sign might be the combination of high humidity and rising temperature. During summer in some climates, each successive day after a cold front typically brings greater humidity and further reduced visibility. Therefore, if yesterday fit the above description and it hasn’t rained overnight, there is reason to expect more of the same today. Until the next front comes, even heavy thundershowers may only bring still higher humidity by depositing moisture on the ground and then leaving it in the sun.
Showers may plummet unseen from fair-weather cumulus directly overhead, instantly reducing visibility to zero. Some high performance sailplanes fly very poorly with wet wings, and others do okay, but precipitation always degrades all aspects of aircraft performance. Hail you should always run from: JUST GET AWAY!