FAA Instrument Flight Instructor (FII)WeatherMedium
A TAF for the destination includes the group 'PROB30 1618 1SM TSRA.' How should a CFII explain the operational significance of this group to an instrument student?
- AThunderstorms with 1SM visibility are certain to occur between 1600Z and 1800Z
- BThere is a 30% probability of thunderstorms reducing visibility to 1SM between 1600Z and 1800Z, and this should be treated as a contingency, not a certainty
- CThere is a 30% chance conditions will improve to 1SM by 1800Z
- DThe airport will close for arrivals between 1600Z and 1800Z due to thunderstorms
Show answer & explanationAnswer & explanation
Correct answer: B. There is a 30% probability of thunderstorms reducing visibility to 1SM between 1600Z and 1800Z, and this should be treated as a contingency, not a certainty
PROB30 indicates a 30% probability that the specified conditions (here, thunderstorms with 1 statute mile visibility) will occur during the given time period. It represents possible, not certain or guaranteed, conditions and should be factored into contingency planning such as fuel reserves and alternates.
Why the other options are wrong
- A. PROB30 explicitly indicates uncertainty (30% chance), not certainty.
- C. PROB30 indicates a probability of the stated adverse condition, not of improvement.
- D. TAFs do not directly state airport closures; this overstates the certainty implied by PROB30.
TAF PROB30 Group
A TAF probability forecast group indicating a 30% chance of specified meteorological conditions occurring during a given time period.
- PROB30 and PROB40 are the only probability values used
- Not used within the first 6 hours of the TAF (per forecasting conventions)
- Represents a lower-confidence contingency, not a change group like BECMG or TEMPO
Memory trick: PROB30 is a coin flip minus—only 3 in 10 chances, plan but don't panic.