FAA Aircraft Dispatcher (ADX)MeteorologyMedium
A TAF for a destination airport contains the group 'PROB30 1518 1SM TSRA'. How should a dispatcher interpret this forecast element?
- AThere is a 30 percent probability that thunderstorms with rain and 1SM visibility will occur between 1500Z and 1800Z
- BThunderstorms are certain to occur, with visibility restricted to 1 statute mile
- CA 30-knot wind shift accompanied by thunderstorms is forecast at 1500Z
- DA cloud ceiling of 3,000 feet is expected with a 30 percent chance of light rain showers
Show answer & explanationAnswer & explanation
Correct answer: A. There is a 30 percent probability that thunderstorms with rain and 1SM visibility will occur between 1500Z and 1800Z
PROB30 indicates a 30 percent probability of the specified conditions occurring during the stated time period; here, thunderstorms with heavy rain (TSRA) and visibility of 1 statute mile between 1500Z and 1800Z. PROB groups express probabilistic, not certain, forecast conditions.
Why the other options are wrong
- B. PROB30 expresses only a 30% likelihood, not certainty.
- C. The numbers represent time (1500-1800Z), not a wind shift value.
- D. Misreads the group; there is no ceiling value given and TSRA is thunderstorms with rain, not showers.
TAF PROB Group
A TAF element expressing the probability (30% or 40%) that a specified set of conditions will occur during a given time period.
- PROB30 or PROB40 followed by a 4-digit time period (start-end in Zulu)
- Used typically for thunderstorms or other conditions with lower confidence
- Not used to describe certain or 'temporary' conditions—that role belongs to TEMPO/BECMG
Memory trick: PROB = Probability, always a percentage of a possible storm