FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesMedium
A pilot reviewing a TAF for the destination notes the entry 'PROB30 1518/1520 1SM TSRA'. What does this group indicate to the pilot?
- AA 30% probability of thunderstorms with rain reducing visibility to 1 statute mile between 1800Z and 2000Z on the 15th
- BThere is a 30% chance the TAF itself will be amended before the period begins
- CVisibility will definitely be 1 statute mile during the entire forecast period
- DThunderstorms are expected to begin exactly 30 minutes into the forecast period
Show answer & explanationAnswer & explanation
Correct answer: A. A 30% probability of thunderstorms with rain reducing visibility to 1 statute mile between 1800Z and 2000Z on the 15th
The PROB30 group indicates a 30 percent probability that the specified conditions (here, 1SM visibility in thunderstorms with rain) will occur during the stated time period, in this case 1800Z to 2000Z on the 15th.
Why the other options are wrong
- B. PROB groups forecast probable weather conditions, not the likelihood of an amendment.
- C. PROB30 explicitly conveys uncertainty (30% chance), not a guaranteed condition.
- D. The time group '1518/1520' defines the validity window, not a delay before onset.
TAF PROB Group
A TAF entry indicating the probability (in percent, typically 30 or 40) that a specified set of weather conditions will occur during a given time period.
- Format: PROBnn followed by a time period (ddhh/ddhh)
- Used only for conditions like thunderstorms or precipitation, not routine forecasts
- Not used within the first 6 hours of the TAF for most conditions
Memory trick: PROB = probability dice roll for iffy weather like thunderstorms