Decoding TAFs
~12 min read · Read TAF validity periods and FM/TEMPO/PROB change groups for go/no-go calls.
A TAF is the weather that WILL BE — a 24–30 hour airport forecast. The exam's favorite move is hiding the answer inside a change group: what will conditions be at 2100Z given FM2000 and a TEMPO window? Master FM, TEMPO, BECMG, and PROB and the questions unlock.
Structure and validity
`TAF KOKC 051730Z 0518/0624 14012KT P6SM SCT040 ...` — issued day 05 at 1730Z, valid from day 05 hour 18 to day 06 hour 24. TAFs are issued four times daily and cover a 5-SM radius of the airport. `P6SM` means visibility better than 6 statute miles. The base line states prevailing conditions until a change group modifies them.
- Validity: DDhh/DDhh (start/end day-hour, Zulu)
- Coverage: 5 SM around the station only
- P6SM = 'plus' — better than 6 SM
Change groups
FM (from): a rapid, complete change — everything after FM252000 replaces the whole forecast from 2000Z. BECMG: a gradual change over the stated window (BECMG 2224: transition between 22Z and 24Z; after the window, the new condition holds). TEMPO: temporary fluctuations lasting under an hour at a time, covering less than half the period — plan for the worse of base and TEMPO conditions. PROB30/PROB40: 30–40% probability of the stated conditions in the window.
- FM = clean break, full new forecast
- BECMG = gradual transition across the window
- TEMPO = brief excursions; treat as operationally binding
- PROB = probability qualifier (30% or 40%)
Using a TAF for drone windows
Drone crews fly short windows, so the skill is slicing the TAF at mission time: find the last FM before your window, then overlay any TEMPO/PROB group covering it. If a TEMPO drops visibility below 3 SM or a ceiling under ~900 ft into your window, schedule around it — a 30-minute inspection is exactly the kind of operation a TEMPO excursion can shut down mid-flight.
Worked example
TAF excerpt: `1416/1512 20008KT P6SM SCT035 FM142000 23015G25KT 5SM -SHRA BKN010 TEMPO 1420/1424 2SM SHRA OVC006`. Your inspection window is 2115Z on day 14. Legal to plan?
Slice at 2115Z: the governing base is the FM142000 line — wind 230/15G25, 5 SM, light rain showers, ceiling 1,000 AGL. That alone is legal (5 ≥ 3 SM; 1,000 − 500 = 500 ft AGL practical cap — already tight for a 400-ft job). Now the TEMPO 1420/1424 covers 2115Z: temporary 2 SM and OVC006. During those excursions visibility drops below the 3-SM minimum and a 600-ft ceiling leaves only 100 ft of legal cloud clearance headroom. Exam and real-world answer: do not plan the window on the base line alone — the TEMPO makes 2115Z unflyable planning-wise; move before 2000Z or after 2400Z.
Common exam pitfalls
Averaging TEMPO with the base forecast.
TEMPO conditions can occur at any moment in the window — plan to the WORSE of base and TEMPO.
Letting an early FM group govern past a later one.
Each FM completely resets the forecast. Use the last FM before your mission time.
Applying a TAF 20 miles from the airport.
TAF coverage is a 5-SM radius. Beyond it, use area forecasts, nearby TAFs, and graphical products.
FM flips the page, BECMG turns it slowly, TEMPO scribbles in the margin, PROB writes in pencil.
Recap
- TAF: 24–30 hr forecast, 5-SM radius, times in Zulu
- Base line governs until a change group modifies it
- FM = complete replacement from that time
- BECMG = gradual change across a window; TEMPO = sub-hour excursions
- Plan to the worst condition touching your mission window
- PROB30/40 = probability of the bracketed conditions
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