Plain English Meaning
A note in a forecast saying there is roughly a one-in-three chance of certain weather happening during a given time window.
Full Definition
A TAF group used to indicate a 30 to less than 50 percent probability of occurrence of thunderstorms or other precipitation events, along with the associated weather conditions, during a specified time period in the forecast.
Context Anchor
Seen in Terminal Aerodrome Forecasts before a time period and the weather conditions that may occur.
Derivation
PROB is short for probability, from Latin probabilis meaning likely or provable. The 30 indicates the percentage chance. The format keeps TAFs short while still flagging weather that is possible but not likely enough to forecast outright.