What is a Terminal Aerodrome Forecast (TAF) and what does it cover?

A Terminal Aerodrome Forecast, or TAF, is an aviation weather forecast issued for conditions at a specific airport and its immediate vicinity, typically within a 5 statute mile radius of the airport. TAFs are issued four times daily and are valid for up to 30 hours, making them a primary planning tool for IFR and VFR pilots. They are prepared by National Weather Service forecast offices and follow a standardized coded format recognized internationally.

How do you read the change indicators in a TAF, such as FM, TEMPO, and BECMG?

Change indicators in a TAF signal when and how forecast conditions are expected to shift during the valid period. FM (from) indicates a rapid, permanent change to new conditions beginning at the specified time, BECMG (becoming) describes a gradual change expected to occur within a defined time window, and TEMPO (temporary) indicates fluctuating conditions lasting less than one hour at a time and covering less than half of the indicated period. Understanding these indicators is essential for accurately interpreting how weather will evolve throughout your planned flight, as covered in the Aviation Weather Handbook (FAA-H-8083-28).

What's the difference between a METAR and a TAF?

A METAR (Aviation Routine Weather Report) is an observation of actual current weather conditions recorded at an airport at a specific point in time, while a TAF is a forecast of expected weather conditions at that airport for a future period of up to 30 hours. METARs tell you what the weather is right now, whereas TAFs tell you what meteorologists predict the weather will be, making both products complementary tools for preflight planning. Pilots should consult both documents together to compare observed conditions with what has been forecast, as emphasized in the Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25).

Terminal Aerodrome Forecast (TAF) Interpretation

When you pick up a weather briefing before a flight, one of the most important documents in your hands is the Terminal Aerodrome Forecast, or TAF. Issued by the National Weather Service, a TAF is a coded forecast of expected meteorological conditions at a specific airport, typically covering a radius of about five statute miles around the airfield. Unlike a METAR, which reports what the weather is at a specific moment, a TAF tells you what the weather will be over the coming hours — making it the cornerstone of preflight weather planning. Understanding how to read and interpret a TAF is not just an FAA knowledge-test requirement; it is a genuine safety skill that every pilot needs before leaving the ground.

TAFs are issued by certified weather forecasters four times daily, at 0000Z, 0600Z, 1200Z, and 1800Z. Standard TAFs cover a 24-hour period, while amended or extended TAFs can cover up to 30 hours. They are generally available for airports with an instrument approach procedure and sufficient weather observation infrastructure. If your destination airport does not have a TAF, you will need to rely on area forecasts and nearby METARs to piece together a forecast picture.

Breaking Down the TAF Structure

Every TAF follows a rigid format. Knowing the sequence of each group allows you to decode even an unfamiliar TAF quickly and confidently. Here is a walk-through of a realistic example:

KORD 121720Z 1218/1318 27012KT 9999 FEW040 SCT080 BKN250 TEMPO 1220/1222 27025G38KT 6000 TSRA BKN025CB FM130000 29008KT 9999 SKC

Station and Time Groups

The TAF opens with the station identifier (KORD for Chicago O'Hare), followed by the issuance time in the format DDHHMMz — the 12th day of the month, issued at 1720Z. Next is the valid period: 1218/1318 means the forecast is valid from the 12th at 1800Z through the 13th at 1800Z. Always check the valid period first — a TAF more than 30 hours old is not useful for planning purposes, and you should seek an updated briefing.

Wind

Wind is expressed as direction in degrees true followed by speed in knots. In our example, 27012KT indicates winds from 270° true at 12 knots. If gusts are present, a G and peak gust speed follow the sustained wind speed (e.g., 27025G38KT = 270° at 25 knots, gusting to 38). A variable wind direction under 3 knots is coded VRB03KT. Notice that wind direction in aviation is always reported in degrees true — not magnetic — in both METARs and TAFs. When you use the wind at the airport, remember to apply magnetic variation for runway alignment purposes.

Visibility

Visibility in a TAF is reported in statute miles in the United States. The code 9999 (as seen in international and many domestic formatted TAFs) means visibility of 10 statute miles or greater. Values less than that are written as whole or fractional numbers followed by SM (e.g., 1 1/2SM). Visibility is critical for VFR flight — if the TAF shows less than 3 statute miles, VFR conditions may not exist, and you need to plan accordingly or hold an instrument rating.

Weather Phenomena

Present weather codes appear between visibility and sky condition. These follow the METAR/TAF standard abbreviations: TS for thunderstorm, RA for rain, SN for snow, BR for mist, FG for fog, FZRA for freezing rain, DZ for drizzle, and so on. Intensity qualifiers precede the phenomenon: a minus sign (−) means light, no qualifier means moderate, and a plus sign (+) means heavy. In our example, TSRA means a thunderstorm with moderate rain — a significant hazard you would not want to fly into.

Sky Condition

Sky condition is reported in layers, listed from lowest to highest. Each layer shows coverage and height: FEW (1–2 eighths), SCT (scattered, 3–4 eighths), BKN (broken, 5–7 eighths), or OVC (overcast, 8 eighths). Height is expressed in hundreds of feet AGL — so BKN025CB means a broken ceiling at 2,500 feet AGL composed of cumulonimbus clouds. The CB suffix is a major red flag: cumulonimbus clouds are associated with severe turbulence, icing, hail, and lightning. SKC (sky clear) or CLR (clear below 12,000 feet, used by automated stations) indicate no clouds.

Change Groups: The Heart of TAF Interpretation

A TAF is only as useful as your ability to read its change indicators. These tell you when and how conditions are expected to shift.

FM (From): A rapid change in conditions. FM is always followed by a six-digit time (DDHHMM) and means all previous conditions are replaced starting at that time. In our example, FM130000 means that from the 13th at 0000Z onward, winds shift to 29008KT with visibility 9999 and sky clear — a marked improvement.
TEMPO (Temporary): Conditions that fluctuate temporarily and are expected to last less than one hour at a time, occurring within the stated period. TEMPO 1220/1222 means between 2000Z and 2200Z on the 12th, you can expect temporary conditions of strong gusty winds, reduced visibility, thunderstorms, and a broken ceiling at 2,500 feet with CBs. These conditions do not replace the base forecast but are superimposed on it temporarily.
BECMG (Becoming): A gradual change expected to complete within the stated time window. Unlike TEMPO, once conditions reach BECMG values, they stay at those values.
PROB (Probability): Used to indicate the probability of a condition occurring. PROB30 means a 30% probability; PROB40 means a 40% probability. The FAA does not use PROB20 — if the probability is under 30%, forecasters simply omit it. A common exam question asks what PROB30 TEMPO means: it is a 30% chance of temporary conditions during the period listed.

Why TAF Interpretation Matters

Misreading a TAF can lead to a pilot departing into conditions that do not support VFR flight, or arriving at a destination where the ceiling has dropped below approach minimums. The FAA's risk management framework (as described in the Risk Management Handbook, FAA-H-8083-2) emphasizes that hazardous weather is one of the most frequently cited factors in general aviation accidents. A pilot who understands that a TEMPO group calls for visibility below VFR minimums may choose to delay departure by an hour, wait for the system to pass, or file IFR — decisions that directly protect lives.

TAFs also feed into fuel planning. If a destination shows marginal VFR conditions within the TAF valid period, regulations require VFR pilots to carry enough fuel to reach an alternate. IFR pilots must select an alternate when the TAF for the destination shows, within one hour before or after planned arrival, a ceiling below 2,000 feet or visibility below 3 statute miles — a rule sometimes called the 1-2-3 rule (1 hour before/after, ceiling 2,000 feet, visibility 3 SM).

Key Numbers and Rules

TAFs are issued four times daily (0000Z, 0600Z, 1200Z, 1800Z) and are valid for 24 to 30 hours.
9999 = visibility 10 SM or greater; 0000 = visibility less than 1/4 SM.
Cloud heights in TAFs are always AGL, in hundreds of feet.
PROB30 = 30% probability of conditions (lowest probability TAFs use); PROB20 is not used.
The 1-2-3 rule: IFR alternate required when destination TAF shows ceiling below 2,000 ft or visibility below 3 SM within 1 hour of ETA.
CB or TCU suffixes on a cloud layer indicate cumulonimbus or towering cumulus — always a significant hazard.
Wind direction is in degrees true in all TAFs and METARs.

Common Test Traps

Confusing TEMPO with BECMG: TEMPO conditions are temporary and can repeat within the period; BECMG conditions become and remain the new base forecast once the change completes. The FAA loves testing whether you know that BECMG replaces the forecast permanently.
Forgetting that FM replaces everything: After an FM group, all prior conditions (wind, visibility, weather, sky) are erased and replaced entirely. Students sometimes carry forward the old ceiling into the FM period — do not.
Misreading cloud height units: Heights are in hundreds of feet AGL, not thousands. BKN025 is 2,500 feet, not 25,000 feet.
Wind direction is true, not magnetic: On the written test, do not confuse TAF/METAR wind reports (true north reference) with ATIS or tower communications (magnetic). They are different.
PROB30 vs. omission: If the probability of a phenomenon is below 30%, it does not appear in the TAF at all. Seeing PROB30 means there IS a 30% chance — not that the event is unlikely enough to ignore, especially if it involves thunderstorms or icing.