A real terminal aerodrome forecast decoded line by line — then the part most tutorials skip: how to actually use it for a go/no-go decision.
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A TAF (Terminal Aerodrome Forecast) is a 24- or 30-hour forecast for the area within 5 statute miles of an airport, issued four times a day. Read it left to right as a timeline: the first line is the starting conditions, and each FM, TEMPO, BECMG, or PROB30 group changes the picture from that time forward. The line that matters is the one covering your departure or arrival window — not the headline.
Here is a plausible TAF for Portland International (KPDX) on a wet Pacific Northwest day:
TAF KPDX 171730Z 1718/1818 19008KT P6SM FEW035 SCT250
FM180000 17012G20KT P6SM -RA BKN025 OVC040
TEMPO 1802/1806 4SM RA BR BKN015
PROB30 1806/1810 2SM RA BR OVC008
FM181200 20010KT P6SM -SHRA SCT020 BKN035
BECMG 1814/1816 25008KT P6SM SCT035
TAF KPDX 171730Z 1718/1818 — a TAF for KPDX, issued on the 17th at 1730Z. The valid period uses the DDHH/DDHH format: from the 17th at 1800Z to the 18th at 1800Z, a 24-hour forecast. Some larger airports get 30-hour TAFs, so the end day can be two calendar days after the start. If the header reads TAF AMD, this is an amended TAF that replaces the scheduled one — the forecaster has already decided the original was wrong.
19008KT — wind from 190° true at 8 knots. Gusts get a G: 17012G20KT is 170° at 12 gusting 20. VRB03KT means variable direction at 3 knots — light and drifting. Two things to remember: TAF winds are in degrees true (runway numbers are magnetic), and the gust number is the one to run through a crosswind calculator, because the gust is what arrives in the flare.
P6SM — visibility "plus 6 statute miles," i.e., greater than 6 SM. U.S. TAFs cap the forecast at 6; anything better is simply P6SM. Lower values are stated directly: 4SM, 2SM, 1/2SM. Outside the U.S., visibility is in meters, and 9999 is the equivalent of P6SM — 10 kilometers or more.
The weather group uses two-letter codes with intensity prefixes: - light, no prefix moderate, + heavy. The ones you will see most:
| Code | Meaning |
|---|---|
-RA / RA / +RA | Light / moderate / heavy rain |
SHRA | Rain showers — convective, on-and-off |
TS / TSRA | Thunderstorm / thunderstorm with rain |
BR | Mist — visibility 5/8 SM to 6 SM |
FG | Fog — visibility below 5/8 SM |
FZRA | Freezing rain — a no-go item for most GA aircraft |
Cloud cover is reported in coverage bands with heights in hundreds of feet AGL: FEW (1–2 oktas), SCT (3–4), BKN (5–7), OVC (8/8). BKN025 OVC040 means broken at 2,500 feet, overcast at 4,000. The ceiling is the lowest broken or overcast layer — FEW and SCT don't count. CB appended to a layer (SCT030CB) flags cumulonimbus: convection near the field even if no TS is forecast yet.
FM180000 17012G20KT P6SM -RA BKN025 OVC040 — "from the 18th at 0000Z." FM marks a rapid change, expected to happen within about an hour of the stated time, and what follows is a complete new forecast. Nothing carries over from the previous line. Here, at 0000Z the picture becomes: gusty southerly wind, light rain, 2,500-foot ceiling.
TEMPO 1802/1806 4SM RA BR BKN015 — between 0200Z and 0600Z on the 18th, expect temporary fluctuations to 4 SM in rain and mist with a 1,500-foot ceiling. TEMPO conditions last less than an hour at a time and, in aggregate, less than half the TEMPO period. Only the listed elements change; the wind from the prevailing FM line still applies. The planning posture: assume the TEMPO conditions can be there when you arrive, because you can't schedule around them.
PROB30 1806/1810 2SM RA BR OVC008 — a roughly 30% probability of 2 SM and an 800-foot overcast between 0600Z and 1000Z. Many pilots and most professional operators treat a PROB30 of below-minimums weather as operative for planning: a 30% chance of not getting in is far too high to bet a fuel plan on, so file the alternate and carry the fuel as if it will verify. U.S. National Weather Service TAFs use PROB30 only; PROB40 appears in some international and military TAFs and means what it says — about 40%.
BECMG 1814/1816 25008KT P6SM SCT035 — between 1400Z and 1600Z, conditions gradually become westerly at 8 with scattered clouds, and then stay that way. Unlike FM, only the listed elements change, and the change completes sometime within the window. The conservative read: assume the old conditions persist until the end of the BECMG window.
Decoding is the easy half. The decision errors come from how the TAF gets read:
If you want a structure to plug the TAF into, our go/no-go decision framework walks through weighing weather alongside NOTAMs, performance, and pilot factors.
SKC or high FEW at the terminal doesn't clear the route. Terrain-induced cloud, valley fog, and marine layers live between TAF circles.Most TAFs are valid for 24 hours; a set of larger airports get 30-hour TAFs. New TAFs come out four times a day (around 0000, 0600, 1200, and 1800 UTC), and an amended TAF (TAF AMD) can replace one at any time. Check the DDHH/DDHH valid period in the header rather than assuming.
Temporary fluctuations lasting less than an hour at a time and, in total, less than half the TEMPO period. Only the listed elements change; the rest carries over from the prevailing forecast. Plan as though the TEMPO conditions can occur at any moment inside the window.
FM is a rapid change: at the stated time a complete new forecast begins and replaces every element of the previous line. BECMG is a gradual change: it completes sometime within the window (typically two hours) and only the listed elements change. Conservatively, assume pre-BECMG conditions persist until the end of the BECMG window.
Roughly a 30% probability of the listed conditions during the window. Many pilots and most professional operators treat a PROB30 of below-minimums weather as operative for planning — carry the fuel and the alternate as if it will happen. U.S. NWS TAFs use PROB30 only; PROB40 shows up in some international and military TAFs.
No — a TAF covers only the area within 5 statute miles of the airport. For the enroute picture, use the Graphical Forecasts for Aviation on aviationweather.gov, AIRMETs/SIGMETs, and TAFs from airports along the route as sample points.
A repeatable structure for turning a decoded TAF into an actual decision.
The other half of a legal preflight briefing — triaged so the TFR doesn't hide in the noise.
Run the TAF's wind line (use the gust value) against your runway and personal max.
FlightDecide pulls the TAFs and METARs for your route, reads the change group that covers your actual departure window, and scores ceilings, visibility, and winds against your personal minimums — with the raw text one tap away so you can verify everything.
Get FlightDecide on the App StoreEducational guide for flight-planning practice. It is advisory only and not a substitute for an official weather briefing or your own judgment as pilot in command (14 CFR 91.3). Sources: FAA Aviation Weather Handbook (FAA-H-8083-28A); AIM §7-1-28; aviationweather.gov. Last reviewed: July 17, 2026.