Decoding the Terminal Aerodrome Forecast (TAF)

12.taf-decoding. Decoding the Terminal Aerodrome Forecast (TAF)

A Terminal Aerodrome Forecast (TAF) is a concise statement of expected meteorological conditions at an airport during a specified period, normally extending from the surface to 6,000 feet above the airport. TAFs are issued four times daily (0000Z, 0600Z, 1200Z, 1800Z) and are valid for either 24 or 30 hours. Forecasts apply to weather within a 5 statute mile radius of the runway complex. Because the TAF is the pilot's principal forecast for destination weather, decoding it accurately is essential for flight planning, fuel reserves, and alternate selection.

Structure of a TAF. A TAF is built in the same general order every time:

• Report type (TAF, TAF AMD for amended, TAF COR for corrected)
• ICAO station identifier
• Date and time of origin (DDHHMMZ)
• Valid period (DDHH/DDHH)
• Forecast wind, visibility, weather, and sky condition
• Expected changes (FM, BECMG, TEMPO, PROB)

Example: TAF KOKC 051130Z 0512/0618 14008KT 5SM BR BKN030 TEMPO 0513/0516 1 1/2SM BR FM051600 16010KT P6SM SKC BECMG 0522/0524 20013G20KT 4SM SHRA OVC020 PROB30 0604/0607 1SM TSRA OVC008CB

Decoded: TAF for Oklahoma City (KOKC), issued the 5th at 1130Z, valid from the 5th at 1200Z through the 6th at 1800Z. Initial conditions: wind 140° at 8 knots, visibility 5 statute miles in mist, ceiling broken at 3,000 feet AGL.

Wind. Encoded as dddffGggKT. Direction is true (three digits), speed is in knots (two or three digits), and gusts follow G. VRB indicates variable direction at speeds of 6 knots or less. Calm is 00000KT.

Visibility. Always given in statute miles, e.g., 5SM, 1 1/2SM. P6SM means greater than 6 statute miles (the maximum forecast value).

Weather phenomena. Standard METAR/TAF contractions are used:

• Intensity: - light, no symbol moderate, + heavy, VC in vicinity
• Descriptor: MI shallow, BC patches, BL blowing, SH showers, TS thunderstorm, FZ freezing
• Precipitation: RA rain, SN snow, DZ drizzle, PL ice pellets, GR hail, GS small hail/snow pellets
• Obscuration: BR mist (5/8–6 SM), FG fog (<5/8 SM), HZ haze, FU smoke

Sky condition. Reported as coverage and height in hundreds of feet AGL: SKC or CLR clear, FEW 1–2/8, SCT 3–4/8, BKN 5–7/8, OVC 8/8. Cumulonimbus is appended as CB (e.g., OVC008CB). TAFs do not use VV (vertical visibility) the way METARs sometimes do — they use VVhhh only when sky is obscured.

Wind shear. Non-convective low-level wind shear is forecast as WShhh/dddffKT, e.g., WS020/24050KT means wind shear at 2,000 feet AGL with wind from 240° at 50 knots.

Change groups. This is where many students lose the plot:

• FM (From): A rapid change, generally within an hour. FM051600 means "from the 5th at 1600Z." Everything after FM replaces the previous forecast line entirely.
• BECMG (Becoming): A gradual change over the indicated period (typically 2 hours). BECMG 0522/0524 means the change occurs sometime between 2200Z and 2400Z on the 5th.
• TEMPO (Temporary): Temporary fluctuations expected to last less than 1 hour at a time and, in aggregate, less than half the indicated period. Conditions outside TEMPO remain in force.
• PROB30 / PROB40: A 30% or 40% probability of the stated conditions occurring during the period. Probabilities of 50% or higher are stated as a primary forecast or BECMG group; PROB is not used in the first 9 hours of a forecast in U.S. TAFs.

Amendments. A TAF is amended (TAF AMD) when conditions change or are forecast to change to such an extent that the existing TAF no longer adequately describes ongoing or expected weather — typically when ceiling or visibility crosses the 1,000 ft / 3 SM IFR threshold, or when significant convective activity develops.

Practical use. Pilots cross-reference the TAF with the current METAR, area forecast discussion, and convective outlooks. Per 14 CFR 91.103, preflight action requires checking weather reports and forecasts for any flight not in the vicinity of the airport, and 14 CFR 91.169 uses TAF conditions at the ETA to determine whether an alternate is required for IFR flights (the classic 1-2-3 rule: ±1 hour of ETA, ceiling at least 2,000 ft, visibility at least 3 SM).