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.
Plain English
A note in a forecast saying there is roughly a one-in-three chance of certain weather happening during a given time window.
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.
Why Pilots Care
It helps a pilot decide whether to plan an alternate airport, add extra fuel, or delay departure based on the level of risk.
Intuition Check
PROB30 does not mean the weather will last for 30 percent of the time period. It means there is a 30 percent chance the listed weather will occur during that period.
Example Sentence 1
The TAF included PROB30 2200/2400 TSRA, so the pilot carried extra fuel in case thunderstorms developed near the destination after 2200Z.
Example Sentence 2
Even with PROB30 for low ceilings, the actual conditions stayed above minimums for the entire approach.